9 common classes

1. Inner class

Java Classification of inner classes in ：

• Member inner class ： Define a class in another class , As a member of another class
• Static inner class ：static
• Local inner classes ： Define a class inside a method
• Anonymous inner class ：

The concept of inner classes ： Define a complete class inside a class

( A class will generate its own class file , An inner class will also )

characteristic ：

• After compilation, an independent bytecode file can be generated
• The inner class has direct access to the private members of the outer class , Without destroying encapsulation
• It can provide necessary internal functional components for external classes

1.1 Member inner class

(1) Define inside the class , And instance variables 、 Class at the same level as instance method

(2) An inner class is an instance part of an outer class , When creating inner class objects , Must rely on external class objects .

• Outer out = new Outer();
• Inner in = out.new Inner();

Simple use of member inner classes ：

package com.song.demo01;

// External class 
public class Outer {
    
    // Instance variables 
    private String name="zhangsan";
    private  int age=20;

    // Inner class 
    class Inner{
    
        private String address="Beijing";
        private String phone="110";

        // Method 
        public void show(){
    
            // Print the properties of the external class 
            System.out.println(name);
            System.out.println(age);
            // Print the properties of the inner class 
            System.out.println(address);
            System.out.println(phone);
        }
    }
}

package com.song.demo01;
import com.song.demo01.Outer.Inner;
public class TestOuter {
    
    public static void main(String[] args) {
    
        //1. Create an external class object 
        Outer outer = new Outer();
        //2. Create inner class objects 
        Inner inner = outer.new Inner();
        //Outer.Inner inner = outer.new Inner();

        inner.show();


        //1 And 2 Merge into one step , One step in place 
        Inner inner1=new Outer().new Inner();
        inner1.show();
    }
}

(3) When the inner class 、 When an external class has an attribute with the same name , Will access the properties of the inner class first .

• If you want to access external class properties , Add external class name .this. Property name
• Internal class properties , The default is this. Property name

package com.song.demo01;

// External class 
public class Outer {
    
    // Instance variables 
    private String name = "zhangsan";
    private int age = 20;

    // Inner class 
    class Inner {
    
        private String address = "Beijing";
        private String phone = "110";
        // If the inner class attribute name is exactly the same as the outer class name , If you want to access external class properties , You need to add the external class name .this
        private String name = "lisi";

        // Method 
        public void show() {
    
            // Print the properties of the external class 
            System.out.println(Outer.this.name);
            System.out.println(Outer.this.age);
            // Print the properties of the inner class ,this It refers to the inner class itself 
            System.out.println(this.name);
            System.out.println(this.address);
            System.out.println(this.phone);
        }
    }
}

(4) Member inner classes cannot define static members

package com.song.demo01;

// External class 
public class Outer {
    
    // Instance variables 
    private String name = "zhangsan";
    private int age = 20;

    // Inner class 
    class Inner {
    
        private String address = "Beijing";
        private String phone = "110";
        // If the inner class attribute name is exactly the same as the outer class name , If you want to access external class properties , You need to add the external class name .this
        private String name = "lisi";

        //private static String country = "china";// Static members cannot be included in an inner class 
        private static final String country = "china";// But it can contain static constants 

        // Method 
        public void show() {
    
            // Print the properties of the external class 
            System.out.println(Outer.this.name);
            System.out.println(Outer.this.age);
            // Print the properties of the inner class ,this It refers to the inner class itself 
            System.out.println(this.name);
            System.out.println(this.address);
            System.out.println(this.phone);
        }
    }
}

1.2 Static inner class

Static keywords ：static

Tips ： Only static inner classes can be used static modification , Normal classes cannot be used static Embellished ！

• Static inner classes do not depend on outer class objects , You can create it directly or access it through the class name , You can declare static members

package com.song.demo02;

// External class 
public class Outer {
    
    private String name = "songsong";
    private int age = 18;

    // Static inner class , The level is the same as the external class ; Different from the member inner class, the member variable level is the same 
    static class Inner {
    
        private String address = "shanghai";
        private String phone = "111";
        private static int count = 100;

        public void show() {
    
            // Calling properties of external classes ？( Consider how an outside class uses another class ）
            //1. First, create an external class object 
            Outer outer = new Outer();
            //2. Calling properties of an external class object 
            System.out.println(outer.name);
            System.out.println(outer.age);

            // Call static inner class properties 
            System.out.println(address);
            System.out.println(phone);
            // Call the static internal class static attribute ( Static properties are accessed through the class name ）
            System.out.println(Inner.count);

        }
    }

}

package com.song.demo02;

public class TestOuter {
    
    public static void main(String[] args) {
    
        //1. Create static inner class objects directly 
        // Ahead Outer. It represents an inclusive relationship , did not new Outer(), There are no brackets , Not created Outer object 
        Outer.Inner inner = new Outer.Inner();
        //2. Calling method 
        inner.show();
    }
}

3. Local inner classes

Local ： Means in a method

• Local inner classes are defined in outer class methods , Scope and create objects are limited to the current method .
• When a local inner class accesses a local variable in the current method of an outer class , Because the life cycle of variable cannot be guaranteed to be the same as itself , Variables must be decorated final, Become constant .
• The use of local inner classes is limited , Can only be used in the current method .

Static methods cannot directly access non static properties （ Static methods –> Class method ; Non static properties –> Object properties ）

package com.song.demo03;

public class Outer {
    
    private String name = "Lisa";
    private int age = 20;

    public void show() {
    
        // Defining local variables （ Default final）
        // Why it has to be final？ Local variables will disappear after execution , however inner The object does not immediately disappear ,Inner Classes will not disappear , A method in a class cannot reference a variable that has disappeared , So we need to add final, Make it a constant （ Don't disappear ）
        String address = "Shandong";
        //final String address = "Shandong";

        // Define local inner classes , Same level as local variable .( Note that no access modifiers can be added )
        class Inner {
    
            // Local inner class properties 
            private String phone = "1230";
            private String email = "[email protected]";

            //private static int count=100;// Local inner classes cannot define static members 
            private static final int count = 100;// But you can define static constants 

            public void show2() {
    
                // Access the properties of the external class , Same level as local variable . For non static methods , You can directly access （ It is also equivalent to omitting Outer.this.
                System.out.println(name);
                //System.out.println(Outer.this.name);
                System.out.println(age);

                // Access the properties of the inner class ( Equivalent to omitting this.)
                System.out.println(phone);
                System.out.println(email);

                // Accessing local variables ,jdk1.7 The variable must be a constant final;jdk1.8 Automatically added by default final
                // Constant final It means that he cannot be modified later 
                System.out.println(address);
            }
        }
        // Create local inner class objects 
        Inner inner = new Inner();
        inner.show2();
    }
}

package com.song.demo03;

public class TestOuter {
    
    public static void main(String[] args) {
    
        Outer outer = new Outer();
        outer.show();// Methods of external classes 

    }
}

4. Anonymous inner class

• Local inner classes without class names （ All features are the same as local inner classes ）.
• Must inherit a Parent class ( It can be an abstract class ) Or implement a Interface
• Anonymous inner classes are actually defining classes 、 Implementation class 、 Create a syntax merge of objects , Only one object of this class can be created , Suitable for classes that are used only once .
• advantage ： Reduce the amount of code
• shortcoming ： Poor readability
• Anonymous inner classes actually have names , It's just not a name , It is assigned by the program .

package com.song.demo06;
// Interface 
public interface Usb {
    
    // Method 
    void service();
}

package com.song.demo06;

// Implementation class of interface 
public class Mouse implements Usb {
    
    @Override
    public void service() {
    
        System.out.println(" The connection to the computer is successful , The mouse is working ");
    }
}

package com.song.demo06;

public class TestUsb {
    
    public static void main(String[] args) {
    
        // Create variables of interface type 
        /*Usb usb = new Mouse();// Create a variable with an interface ---> polymorphic  usb.service();*/

        // Local inner classes 
        class Fan implements Usb {
    
            @Override
            public void service() {
    
                System.out.println(" The connection to the computer is successful , The fan starts to work ");
            }
        }
        // Use local inner classes to create objects 
        Usb usb = new Fan();
        usb.service();

        // For classes that are used only once , Use anonymous inner classes （ It's equivalent to creating a local inner class ）
        Usb usb2 = new Usb() {
    
            @Override
            public void service() {
    
                System.out.println(" The connection to the computer is successful , Fan 2 start-up ");
            }
        };
        usb2.service();
    }
}

2.Object class

• Superclass 、 Base class , The direct or indirect parent of all classes , At the top of the inheritance tree .
• Any class , If there is no writing extends Show inheriting a class , They all default to direct inheritance Object class , Otherwise, it is indirect inheritance .
• Object Methods defined in class , It's a method that all objects have
• Object Type can store any object
  • As a parameter , Any object can be accepted
  • As return value , You can return any object

2.1 getClass Method

• public final Class<?> getClass() {}
• Returns the actual object stored in the reference type
• application ： It is usually used to determine whether the actual storage object types in two references are consistent .

package com.song.demo07;

public class Student {
    
    private String name;
    private int age;

    public Student(String name, int age) {
    
        this.name = name;
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public int getAge() {
    
        return age;
    }

    public void setAge(int age) {
    
        this.age = age;
    }
}

package com.song.demo07;

public class TestStudent {
    
    public static void main(String[] args) {
    
        Student s1 = new Student("zhangsan", 18);
        Student s2 = new Student("songsong", 20);
        // Judge s1 and s2 Is it the same type 
        Class class1 = s1.getClass();
        Class class2 = s2.getClass();
        if (class1 == class2) {
    
            System.out.println("s1 and s2 In the same category ");
        } else {
    
            System.out.println("s1 and s2 Not of the same type ");
        }
    }
}

2.2 hasCode() Method

• public int hasCode() {}
• Returns the hash code value of the object
• The hash value is based on Address of the object or character string or Numbers Use hash The algorithm works out int Type value .
• In general Same object Returns the same hash code .— It can be used to determine whether it is the same object

package com.song.demo07;

public class TestStudent {
    
    public static void main(String[] args) {
    
        Student s1 = new Student("zhangsan", 18);
        Student s2 = new Student("songsong", 20);

        //getClass() Method 
        // Judge s1 and s2 Is it the same type 
        Class class1 = s1.getClass();
        Class class2 = s2.getClass();
        if (class1 == class2) {
    
            System.out.println("s1 and s2 In the same category ");
        } else {
    
            System.out.println("s1 and s2 Not of the same type ");
        }

        //hasCode() Method 
        //s1,s2 It's two different objects , Different spaces are created in the heap of memory 
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
        Student s3 = s1;
        System.out.println(s3.hashCode());
    }
}

2.3 toString() Method

• public String toString() {}
• Returns the string representation of the object （ form ）
• Can be covered according to program requirements ( rewrite ) The method , Such as ： Display each attribute value of the object .– Methods inherited from the parent class cannot meet the requirements , Can be rewritten .

package com.song.demo07;

public class Student {
    
    private String name;
    private int age;

    public Student(String name, int age) {
    
        this.name = name;
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public int getAge() {
    
        return age;
    }

    public void setAge(int age) {
    
        this.age = age;
    }

    @Override
    public String toString() {
    
        return "Student--[name=" + name + ", age=" + age + "]";
    }

}

package com.song.demo07;

public class TestStudent {
    
    public static void main(String[] args) {
    
        Student s1 = new Student("zhangsan", 18);
        Student s2 = new Student("songsong", 20);

        //getClass() Method 
        // Judge s1 and s2 Is it the same type 
        Class class1 = s1.getClass();
        Class class2 = s2.getClass();
        if (class1 == class2) {
    
            System.out.println("s1 and s2 In the same category ");
        } else {
    
            System.out.println("s1 and s2 Not of the same type ");
        }

        //hasCode() Method 
        //s1,s2 It's two different objects , Different spaces are created in the heap of memory 
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
        Student s3 = s1;
        System.out.println(s3.hashCode());

        //toString() Method 
        System.out.println(s1.toString());// Before rewriting ：[email protected]（@ Later, it was also Hashimoto , Same as above 
        System.out.println(s2.toString());// Before rewriting ：[email protected]
        //toString() Method 
        System.out.println(s1.toString());// After rewriting ：Student--[name=zhangsan, age=18]
        System.out.println(s2.toString());// After rewriting ：Student--[name=songsong, age=20]
    }
}

2.4 equals() Method

• public boolean equals(Object obj) {}
• The default implementation is （this==obj), Compare the two Object address Are they the same? .
• It can be covered （ rewrite ）, Compare the contents of two objects .

package com.song.demo07;

public class TestStudent {
    
    public static void main(String[] args) {
    
        Student s1 = new Student("zhangsan", 18);
        Student s2 = new Student("songsong", 20);

        //getClass() Method 
        // Judge s1 and s2 Is it the same type 
        Class class1 = s1.getClass();
        Class class2 = s2.getClass();
        if (class1 == class2) {
    
            System.out.println("s1 and s2 In the same category ");
        } else {
    
            System.out.println("s1 and s2 Not of the same type ");
        }

        //hasCode() Method 
        //s1,s2 It's two different objects , Different spaces are created in the heap of memory 
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
        Student s3 = s1;
        System.out.println(s3.hashCode());

        //toString() Method 
        System.out.println(s1.toString());
        System.out.println(s2.toString());

        //equals() Method -- Before rewriting , Compare object addresses 
        System.out.println(s1.equals(s2));// Before rewriting , Compare addresses --false
        System.out.println(s1.equals(s3));//true

        Student s4 = new Student("lisa", 24);
        Student s5 = new Student("lisa", 24);
        System.out.println(s4.equals(s5));//fasle--s4 and s5 These are two objects in the heap 
    }
}

equals() Method override step ：

• Compare whether two references point to the same object
• Judge obj Is it null
• Determine whether the actual object types pointed to by two references are consistent
• Cast
• Compare the values of each attribute in turn

package com.song.demo07;

public class Student {
    
    private String name;
    private int age;

    public Student(String name, int age) {
    
        this.name = name;
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public int getAge() {
    
        return age;
    }

    public void setAge(int age) {
    
        this.age = age;
    }

    @Override
    public String toString() {
    
        return "Student--[name=" + name + ", age=" + age + "]";
    }

    @Override
    public boolean equals(Object obj) {
    
        //1. Determine whether two object types are the same reference 
        if (this == obj) {
    
            return true;
        }
        //2. Judge obj Whether it is null
        if (obj == null) {
    
            return false;
        }
        //3. Determine whether it is the same type 
        //if (this.getClass() == obj.getClass()) {
    
        //instanceof You can determine whether an object is of a certain type 
        if (obj instanceof Student) {
    
            //4. Cast 
            Student s = (Student) obj;
            // String comparison with equals
            if (this.name.equals(s.getName()) && this.age == s.getAge())
                return true;
        }
        return false;
    }
}

package com.song.demo07;

public class TestStudent {
    
    public static void main(String[] args) {
    
        Student s1 = new Student("zhangsan", 18);
        Student s2 = new Student("songsong", 20);

        //getClass() Method 
        // Judge s1 and s2 Is it the same type 
        Class class1 = s1.getClass();
        Class class2 = s2.getClass();
        if (class1 == class2) {
    
            System.out.println("s1 and s2 In the same category ");
        } else {
    
            System.out.println("s1 and s2 Not of the same type ");
        }

        //hasCode() Method 
        //s1,s2 It's two different objects , Different spaces are created in the heap of memory 
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
        Student s3 = s1;
        System.out.println(s3.hashCode());

        //toString() Method 
        System.out.println(s1.toString());
        System.out.println(s2.toString());

        //equals() Method -- Before rewriting , Compare object addresses 
        System.out.println(s1.equals(s2));// Before rewriting , Compare addresses --false
        System.out.println(s1.equals(s3));//true

        Student s4 = new Student("lisa", 24);
        Student s5 = new Student("lisa", 24);
        System.out.println(s4.equals(s5));//fasle--s4 and s5 These are two objects in the heap 

        //equals() Method -- After rewriting , Compare object addresses , Object value, etc 
        System.out.println(s1.equals(s2));//false
        System.out.println(s1.equals(s3));//true
        System.out.println(s4.equals(s5));//true
    }
}

2.5 finalize() Method

• When an object is determined to be a garbage object , from JVM Call this method automatically , To mark junk objects , Enter the recycle queue
• Garbage object ： When no valid reference points to this object , For the garbage
• Garbage collection ： from GC Destroy garbage objects , Free up data storage space
• Automatic recycling mechanism ：JVM Run out of memory , Recycle all junk objects at once
• Manual recycling mechanism ： Use System.gc(); notice JVM Carry out garbage collection .– Whether to implement recycling , It still depends on the program's own judgment , It is not necessary to recycle after notification

package com.song.demo07;

public class Student {
    
    private String name;
    private int age;

    public Student(String name, int age) {
    
        this.name = name;
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public int getAge() {
    
        return age;
    }

    public void setAge(int age) {
    
        this.age = age;
    }

    @Override
    public String toString() {
    
        return "Student--[name=" + name + ", age=" + age + "]";
    }

    @Override
    public boolean equals(Object obj) {
    
        //1. Determine whether two object types are the same reference 
        if (this == obj) {
    
            return true;
        }
        //2. Judge obj Whether it is null
        if (obj == null) {
    
            return false;
        }
        //3. Determine whether it is the same type 
        //if (this.getClass() == obj.getClass()) {
    
        //instanceof You can determine whether an object is of a certain type 
        if (obj instanceof Student) {
    
            //4. Cast 
            Student s = (Student) obj;
            // String comparison with equals
            if (this.name.equals(s.getName()) && this.age == s.getAge())
                return true;
        }
        return false;
    }

    @Override
    protected void finalize() throws Throwable {
    
        System.out.println(this.name+" The object is recycled ");
    }
}

package com.song.demo08;

import com.song.demo07.Student;

public class TestStudent2 {
    
    public static void main(String[] args) {
    
// Student s1 = new Student("aaa",12);
// Student s2 = new Student("bbb",14);
// Student s3 = new Student("ccc",16);
// Student s4 = new Student("ddd",18);
// Student s5 = new Student("eee",20);
        new Student("aaa", 12);
        new Student("bbb", 14);
        new Student("ccc", 16);
        new Student("ddd", 18);
        new Student("eee", 20);
        // Recycling waste 
        System.gc();
        System.out.println(" Recycling waste ");
    }
}

3. Packaging

• Java Eight basic types in ：byte、short、int、long、float、double、char、boolean

  • The basic data types are stored on the stack ; Other objects are stored in the heap , Only the address is stored in the stack .

  • The basic data type itself has no method , The corresponding reference types are designed for the eight data types （ Objects in the heap ）, Provide methods to use

• The reference data type corresponding to the basic data type .

• Object Can unify all data , The default value for wrapper classes is null

• Packaging class corresponds to – stay java The core of the package java.lang in

3.1 Type conversion and Boxing 、 Unpacking

Packing ： Put the objects in the stack into the heap （ Basic type to reference type ）

Unpacking ： Put the objects in the heap on the stack （ Convert reference type to base type ）

8 There are three kinds of packaging classes that provide the conversion between different types ：

• Number Of the parent class 6 Methods for unpacking （ Convert reference type to base type ）– It is byte、int、short、long、float、double Parent class of , You can convert the reference type to the basic type directly by its method
• parseXXX() Provide static methods – It can realize the conversion between string and basic type
• valueOf() Static methods

Be careful ： Type compatibility is required , Otherwise throw NumberFormatException abnormal .

package com.song.demo09;

public class Demo01 {
    
    public static void main(String[] args) {
    

        //jdk1.5 The previous packing and unpacking methods 
        // Type conversion ： Boxing operation , Basic type to reference type 
        // Basic types 
        int num1 = 18;// Basic types , Store in the stack 
        // Use Integer Class creation object 
        Integer integer1 = new Integer(num1);
        Integer integer2 = Integer.valueOf(num1);

        // Type conversion ： Unpacking operation , Convert reference type to base type 
        Integer integer3 = new Integer(100);// The object is in the pile 
        int num = integer3.intValue();


        //jdk1.5 Later packing and unpacking methods ,java It provides automatic packing and unpacking function -- The compiler automatically invokes method conversions 
        int age = 24;
        // Automatic boxing 
        Integer integer4 = age;
        // Automatic dismantling 
        int age2 = integer4;
    }
}

• parseXXX() Provide static methods – It can realize the conversion between string and basic type

package com.song.demo09;

public class Demo02 {
    
    public static void main(String[] args) {
    
        // Conversion between basic types and strings 
        //1. Basic type converted to string 
        int n1 = 15;
        // The way 1： Use + Number 
        String s1 = n1 + "";
        // The way 2： Use Integer Medium toString() Method 
        String s2 = Integer.toString(n1);
        String s3 = Integer.toString(n1, 16);// heavy load , Turn into 16 Base number 
        System.out.println(s1);
        System.out.println(s2);
        System.out.println(s3);

        //2. String to basic type 
        String str = "150";
        // Use Integer.parseXXX()
        int n2 = Integer.parseInt(str);

        //boolean Convert string to basic type ,“true"-->true; Not "true"-->false
        String str2 = "true";
        boolean b1 = Boolean.parseBoolean(str2);
        System.out.println(b1);
    }
}

3.2 Integer buffer

• Java Pre created 256 A commonly used integer wrapper type object ( Range ：-128~127)
• In practical application , Reuse the created objects .

low=-128; high=127

package com.song.demo09;

public class Demo03 {
    
    public static void main(String[] args) {
    
        // Interview questions 
        Integer integer1 = new Integer(100);// Reference type 
        Integer integer2 = new Integer(100);// Reference type 
        System.out.println(integer1 == integer2);//false
        // Reference type , Objects are stored in the heap ; Here we compare the addresses of two objects stored in the stack 

        //Integer integer3 = 100;// Automatic boxing , It's using Integer.valueOf() Method 
        //Integer integer4 = 100;
        Integer integer3 = Integer.valueOf(100);
        Integer integer4 = Integer.valueOf(100);
        System.out.println(integer3 == integer4);//true
        //100 stay -128 To 127 Buffer between , The object returned is the buffer interval 

        Integer integer5 = 200;// Automatic boxing 
        Integer integer6 = 200;
        System.out.println(integer5 == integer6);//false
        //200 be not in -128 To 127 Between , Not a buffer , The actual object address is returned 
    }
}

4.String class

• Strings are constants , After creating It can't be changed
• String literals are stored in String pool in , Sure share

Space ：

• Stack
• Pile up
• Method area

Not to modify the original data , Is to open up a new space .–> Immutability

package com.song.demo09;

public class Demo04 {
    
    public static void main(String[] args) {
    
        String name = "hello";//"hello" Constants are stored in the string pool 
        name = "zhangsan";//“zhangsan” Assign a value to name--- Is a new space in the string pool , It's not about putting “hello” modify 
        String name2 = "zhangsan";
    }
}

Realize the sharing in the string pool

String creation ：

• String s=“Hello”;// Produce an object , String pool
• String s=new String(“Hello”);// Produce two objects , Pile up 、 Each pool stores one .（ Actually run , Objects in the string pool are pointed to in the heap ）

package com.song.demo09;


public class Demo04 {
    
    public static void main(String[] args) {
    
        String name = "hello";//"hello" Constants are stored in the string pool 
        name = "zhangsan";//“zhangsan” Assign a value to name--- Is a new space in the string pool , It's not about putting “hello” modify 
        String name2 = "zhangsan";

        // Another way to create a string 
        String str = new String("Java Is the best language in the world ");// It's a waste of space 
        String str2 = new String("Java Is the best language in the world ");
        System.out.println(str == str2);//false
        System.out.println(str.equals(str2));//true

    }
}

Tips ： String comparison with equals

• == The comparison is the address
• equals The comparison is data （String Rewrite the equals Method ）

4.1 Common methods

• public int length()： Returns the length of the string
• public char charAt(int index)： Get the character from the subscript
• public boolean contains(String str)： Determine if the current string contains str
• public char[] toCharArray()： Convert string to char Array
• public int indexOf(String str)： lookup str First time subscript , There is , The subscript is returned ; non-existent , Then return to -1
• public int lastIndexOf(String str)： lookup str Last occurrence of subscript , There is , The subscript is returned ; non-existent , Then return to -1
• public String trim()： Remove the space before and after the string
• public String toUpperCase()： Convert lowercase to uppercase
• public boolean endsWith(String str)： Determine whether the string is based on str ending
• public String replace(char oldChar,char newChar)： Replace the old string with the new one
• public String[] split(String str)： according to str Make a split
• equals(); More equal
• compareTo(); Compare the order and size of strings in the dictionary table

package com.song.demo09;


import java.util.Arrays;
import java.util.Locale;

public class Demo04 {
    
    public static void main(String[] args) {
    
        String name = "hello";//"hello" Constants are stored in the string pool 
        name = "zhangsan";//“zhangsan” Assign a value to name--- Is a new space in the string pool , It's not about putting “hello” modify 
        String name2 = "zhangsan";

        // Another way to create a string 
        String str = new String("Java Is the best language in the world ");// It's a waste of space 
        String str2 = new String("Java Is the best language in the world ");
        System.out.println(str == str2);//false
        System.out.println(str.equals(str2));//true

        System.out.println("------------------ The use of string methods ---------------");
        // The use of string methods 
        //1.length(); Returns the length of the string 
        //2.charAt(int index); Returns a character in a position 
        //3.contains(String str); Determine whether to include a string 

        String content = "Java Is the best language in the world ,Java It's delicious ,Java splendid ";
        System.out.println(content.length());
        System.out.println(content.charAt(content.length() - 1));
        System.out.println(content.contains("Java"));
        System.out.println(content.contains("PHP"));

        System.out.println("-----------------------");
        //4.toCharArray(); Returns the array corresponding to the string 
        //5.indexOf(); Returns the first occurrence of a string 
        //6.lastIndexOf(); Returns the last occurrence of a string 
        System.out.println(content.toCharArray());
        System.out.println(Arrays.toString(content.toCharArray()));
        System.out.println(content.indexOf("Java"));
        System.out.println(content.indexOf("Java", 4));
        System.out.println(content.lastIndexOf("Java"));

        System.out.println("----------------------");
        //7.trim(); Remove the space before and after the string 
        //8.toUpperCase(); Change lowercase to uppercase  toLowerCase(); Turn upper case into lower case 
        //9.endsWith(str); Judge whether str ending  startsWith(str); Judge whether str start 
        String content2 = " Hello World ";
        System.out.println(content2.trim());
        System.out.println(content2.toUpperCase());
        System.out.println(content2.toLowerCase());
        String filename = "Hello.java";
        System.out.println(filename.endsWith(".java"));
        System.out.println(filename.startsWith("Hello"));

        System.out.println("----------------------");
        //10.replace(old,new); Replace old with new characters or strings 
        //11.split(); Split the string 
        System.out.println(content.replace("Java", "PHP"));
        String say = "Java is the best programming language";
        String[] arr = say.split(" ");
        System.out.println(arr.length);
        for (String string : arr) {
    
            System.out.println(string);
        }
        String say2 = "Java is the best programming language,java is good";
        String[] arr2 = say2.split("[ ,]");//[] Express choice ,
        System.out.println(Arrays.toString(arr2));

        String say3 = "Java is the best programming language,java is good";
        String[] arr3 = say2.split("[ ,]+");//+ Indicates that the preceding space and comma can appear one or more consecutively 
        System.out.println(Arrays.toString(arr3));

        // Supplementary methods 
        //equals(); More equal 
        //compareTo(); Compare the order and size of strings in the dictionary table 
        String s1 = "hello";
        String s2 = "HELLO";
        System.out.println(s1.equals(s2));
        System.out.println(s1.equalsIgnoreCase(s2));// Ignore size comparisons 

        String s3 = "abc";//a--> 97
        String s4 = "xyz";//x--> 120
        String s5 = "acb";
        System.out.println(s3.compareTo(s4));//-23  The first character is different , Just compare the first , Difference between -23, There will be no comparison later 
        System.out.println(s3.compareTo(s5));//-1  The first character is the same , Compare the second , Difference between -1, The latter is no longer difficult 

        String s6 = "abc";
        String s7 = "abcxyz";
        System.out.println(s6.compareTo(s7));//-3  The previous part is exactly the same , Compare the length 

    }
}

4.2 example

package com.song.demo09;

public class Test {
    
    public static void main(String[] args) {
    
        String str = "this is a text";
        //1. Put the word alone 
        String[] arr = str.split(" ");
        for (String a : arr) {
    
            System.out.println(a);
        }
        //2. Replace 
        System.out.println(str.replace("text", "practice"));
        //3. Insert 
        System.out.println(str.replace("text", "easy text"));// Workarounds ！！

        //4. title case 
        for (int i = 0; i < arr.length; i++) {
    
            char first = arr[i].charAt(0);// Word initials 
            // Capitalize the first letter 
            char upperfirst = Character.toUpperCase(first);
            String newstr = upperfirst + arr[i].substring(1);//subs Intercepting string 
            System.out.println(newstr);

        }
    }
}

4.3 Variable string

• StringBuffer： Variable length string ,JDK1.0 Provide , Slow operation efficiency 、 Thread safety .
• StringBuilder： Variable length string ,JDK5.0 Provide , It's very efficient , Thread unsafe

package com.song.demo09;

/* StringBuffer and StringBuilder Use ：  and String The difference between ：1） Than String Efficient ;2） Than String Save memory  */
public class Demo05 {
    
    public static void main(String[] args) {
    
        StringBuffer sb = new StringBuffer();
        //StringBuilder sb =new StringBuilder();//StringBuilder And StringBuffer The method is the same 
        //1.append(); Additional 
        sb.append("java is best");
        System.out.println(sb.toString());
        sb.append(" Java  It's delicious ");
        System.out.println(sb.toString());
        sb.append(" Java  splendid ");
        System.out.println(sb.toString());

        //2.insert(); Add insert 
        sb.insert(0, "first");
        System.out.println(sb.toString());

        //3.replace(); Replace 
        sb.replace(0, 5, "hello");
        System.out.println(sb.toString());

        //4.delete(); Delete 
        sb.delete(0, 5);
        System.out.println(sb.toString());

        //5. Empty 
        sb.delete(0, sb.length());
        System.out.println(sb.length());
    }
}

Efficiency comparison ：StringBuilder>StringBuffer>String

package com.song.demo09;

// verification StringBuilder Efficiency is higher than String
public class demo06 {
    
    public static void main(String[] args) {
    
        /*long start = System.currentTimeMillis(); String str = ""; for (int i = 0; i < 99999; i++) { str += i; } System.out.println(str); long end = System.currentTimeMillis(); System.out.println(" when ：" + (end - start));//38879 */
        long start = System.currentTimeMillis();
        StringBuilder str = new StringBuilder();
        for (int i = 0; i < 99999; i++) {
    
            str.append(i);
        }
        System.out.println(str);
        long end = System.currentTimeMillis();
        System.out.println(" when ：" + (end - start));//49
    }

}

5.BigDecimal class

package com.song.demo10;

public class TestBigDecimal {
    
    public static void main(String[] args) {
    
        double d1 = 1.0;
        double d2 = 0.9;
        System.out.println(d1 - d2);//0.0999999999

        // Interview questions 
        double result=(1.4-0.5)/0.9;
        System.out.println(result);//0.9999999

        // The above questions are answered by double The approximate value storage method of the results in , To see is 1 Maybe what's actually stored is 0.9999999
    }
}

Many practical applications require precise operations , and double yes Approximate value storage , Unqualified , You need to use an exact class ——BigDecimal.

• Location ：java.math
• effect ： accurate Calculate floating point number
• How it was created ：BigDecimal b2 = new BigDecimal(“0.9”);
• Method ：
  • add() Add
  • subtract() reduce
  • multiply() ride
  • divide() except

package com.song.demo10;

import javax.swing.plaf.basic.BasicButtonUI;
import javax.xml.bind.SchemaOutputResolver;
import java.math.BigDecimal;

public class demo01 {
    
    public static void main(String[] args) {
    
        BigDecimal b1 = new BigDecimal("1.0");// Be sure to use the string construction method ！！ To guarantee accuracy 
        BigDecimal b2 = new BigDecimal("0.9");
        // Subtraction 
        BigDecimal r1 = b1.subtract(b2);
        System.out.println(r1);

        // Add 
        BigDecimal r2 = b1.add(b2);
        System.out.println(r2);

        // Multiplication 
        BigDecimal r3 = b1.multiply(b2);
        System.out.println(r3);

        // division 
        BigDecimal r4 = new BigDecimal("1.4")
                .subtract(new BigDecimal("0.9"))
                .divide(new BigDecimal("0.5"));
        System.out.println(r4);
        // Particular attention , For division , When there is no end to it , You should specify how many decimal places to keep and how to keep them , Otherwise, the report will be wrong 

        BigDecimal r5 = new BigDecimal("10")
                .divide(new BigDecimal("3"), 2, BigDecimal.ROUND_HALF_UP);// Round to two decimal places 
        System.out.println(r5);
    }
}

6. Date related classes

6.1Date class

• Date Represents a specific moment , Accurate to milliseconds .Dtae Most of the methods in the class have been Calendaer Class .
• Location ：java.util.Date
• Time unit
  • 1 second =1000 millisecond
  • 1 millisecond =1000 Microsecond
  • 1 Microsecond =1000 nanosecond

Many methods are out of date , By Calendar Class substitution .

package com.song.demo10;

import java.util.Date;

public class demo02 {
    
    public static void main(String[] args) {
    
        //1. establish Date object 
        Date date1 = new Date();// Today's time 
        System.out.println(date1.toString());
        System.out.println(date1.toLocaleString());// obsolete 

        Date date2 = new Date(date1.getTime() - 60 * 60 * 24 * 1000);// Yesterday's time 
        System.out.println(date2.toString());

        //2. Method  after before
        boolean b1 = date1.after(date2);
        System.out.println(b1);
        boolean b2 = date1.before(date2);
        System.out.println(b2);

        //3. Compare compareTo(); Compare in milliseconds , Big return 1, Small return -1, Equal return 0
        System.out.println(date1.compareTo(date2));
        System.out.println(date1.compareTo(date1));

        //4. Comparison is equal  equals() Method 
        boolean b = date1.equals(date2);
        System.out.println(b);

    }
}

6.2Calendar class

• Calendar Provides methods to get and set various calendar fields

• Construction method

  protected Calander()： Because the modifier is protected, So you can't create the object directly .

• Other methods

package com.song.demo10;

import java.util.Calendar;

public class demo03 {
    
    public static void main(String[] args) {
    
        //1. establish Calendar object 
        Calendar calendar = Calendar.getInstance();
        System.out.println(calendar.getTime().toString());
        System.out.println(calendar.getTimeInMillis());
        //2. Get time information 
        // Year of acquisition 
        int year = calendar.get(Calendar.YEAR);
        // Get the month  0-11, The actual display can +1, It is the month of normal understanding 
        int month = calendar.get(Calendar.MONTH);
        // Acquisition date 
        int day = calendar.get(Calendar.DATE);
        // For hours 
        int hour = calendar.get(Calendar.HOUR_OF_DAY);//HOUR_OF_DAY--24 Hours ;HOUR--12 Hours 
        // Get minutes 
        int minute = calendar.get(Calendar.MINUTE);
        // Get seconds 
        int second = calendar.get(Calendar.SECOND);
        System.out.println(year + " year " + (month + 1) + " month " + day + " Japan " + hour + ":" + minute + ":" + second);

        //3. Modification time 
        Calendar calendar2 = Calendar.getInstance();
        System.out.println(calendar2.getTime().toString());
        calendar2.set(Calendar.DAY_OF_MONTH, 5);// Revision date 
        System.out.println(calendar2.getTime().toLocaleString());
        calendar2.set(Calendar.MONTH, 2);// Modify the month 
        System.out.println(calendar2.getTime().toLocaleString());
        calendar2.set(Calendar.HOUR, 8);// When modifying 
        System.out.println(calendar2.getTime().toLocaleString());

        //4. Modify based on the current time , increase 
        Calendar calendar3 = Calendar.getInstance();
        System.out.println(calendar3.getTime().toLocaleString());
        calendar3.add(Calendar.HOUR, 1);// Based on the current time , Plus an hour 
        System.out.println(calendar3.getTime().toLocaleString());
        calendar3.add(Calendar.HOUR, -2);// Based on the current time , reduce 2 Hours 
        System.out.println(calendar3.getTime().toLocaleString());

        //5. Supplementary methods 
        // Get the maximum value of the month in which the current time is located 
        int max = calendar3.getActualMaximum(Calendar.DAY_OF_MONTH);
        int min = calendar3.getActualMinimum(Calendar.DAY_OF_MONTH);
        System.out.println(max);
        System.out.println(min);
    }
}

6.3SimpleDateFormat class

• SimpleDateFormat Is a concrete class that formats and parses dates in a locale dependent manner
• format （ date -> Text ）、 analysis （ Text -> date ）
• Common time pattern letters

package com.song.demo10;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class demo04 {
    
    public static void main(String[] args) throws ParseException {
    
        //1. establish SimpleDateFormat object  y  year  M  month ( Custom format ）
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy year MM month dd Japan HH:mm:ss");
        //2. establish Date Time 
        Date date = new Date();

        // format ( Convert date to string ）
        String str = sdf.format(date);
        System.out.println(str);

        // analysis ( Convert string to date )
        Date date2 = sdf.parse("1998 year 01 month 08 Japan 08:12:12");// Must completely follow one of the above custom formats 
        System.out.println(date2);
    }
}

7.System class

• System System class , It is mainly used to obtain system attribute data and other operations , The constructor is private .

package com.song.demo10;

public class Student {
    
    private String name;
    private int age;

    public Student(String name, int age) {
    
        this.name = name;
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public int getAge() {
    
        return age;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public void setAge(int age) {
    
        this.age = age;
    }

    @Override
    public String toString() {
    
        return "Student [name=" + name + ",age=" + age + "]";
    }

    //finalize Is the method that will be executed during garbage collection 
    @Override
    protected void finalize() throws Throwable {
    
        System.out.println(" Recycled "+name);
    }
}

package com.song.demo10;

import java.util.Arrays;

public class demo05 {
    
    public static void main(String[] args) {
    
        //1.arraycopy Implement array copy ,
        //src: Source array 
        //srcpos: Where to start copying 
        //des: Target array 
        //despos: Where to start placing 
        //length: The length of the copy 
        int[] src = {
    20, 18, 16, 14, 12, 88};
        int[] des = new int[6];
        System.arraycopy(src, 0, des, 0, src.length);
        System.out.println(Arrays.toString(des));
        System.arraycopy(src, 2, des, 0, src.length - 2);
        System.out.println(Arrays.toString(des));

        //2.currentTimeMillis() Get the current time , Often used to time 
        System.out.println(System.currentTimeMillis());

        //3.gc();// Garbage collection , Just suggest , It doesn't have to be recycled 
// Student s1 = new Student("aaa", 18);
// Student s2 = new Student("bbb", 18);
// Student s3 = new Student("ccc", 18);
        new Student("aaa", 18);
        new Student("bbb", 18);
        new Student("ccc", 18);
        System.gc();
        // here s1 s2 s3 No longer used , Determined as garbage object , Being recycled 

        //4.exit(); sign out jvm
        System.exit(0);// state 0 Indicates normal exit 
        System.out.println(" Did the program exit ？");// The program has exited and will not execute the statement again 
    }
}

summary

• Inner class ：

  • Define a complete class inside a class
  • Member inner class 、 Static inner class 、 Local inner classes 、 Anonymous inner class

• Object class

  Direct or indirect parent of all classes , Can store any object

• Packaging

  The reference data type corresponding to the basic data type , You can make Object Unify all data

• String class

  Strings are constants , It cannot be modified after creation , Literal values are stored in the string pool , Can be Shared .

• BigDecimal class

  Floating point numbers can be calculated accurately

• Date class – At a certain time

• Calendar class – The calendar

• SimpleDateFormat– Format time

• System– System class

  System.out.println(“ Recycled ”+name);
  }
  }

```java
package com.song.demo10;

import java.util.Arrays;

public class demo05 {
    public static void main(String[] args) {
        //1.arraycopy Implement array copy ,
        //src: Source array 
        //srcpos: Where to start copying 
        //des: Target array 
        //despos: Where to start placing 
        //length: The length of the copy 
        int[] src = {20, 18, 16, 14, 12, 88};
        int[] des = new int[6];
        System.arraycopy(src, 0, des, 0, src.length);
        System.out.println(Arrays.toString(des));
        System.arraycopy(src, 2, des, 0, src.length - 2);
        System.out.println(Arrays.toString(des));

        //2.currentTimeMillis() Get the current time , Often used to time 
        System.out.println(System.currentTimeMillis());

        //3.gc();// Garbage collection , Just suggest , It doesn't have to be recycled 
//        Student s1 = new Student("aaa", 18);
//        Student s2 = new Student("bbb", 18);
//        Student s3 = new Student("ccc", 18);
        new Student("aaa", 18);
        new Student("bbb", 18);
        new Student("ccc", 18);
        System.gc();
        // here s1 s2 s3 No longer used , Determined as garbage object , Being recycled 

        //4.exit(); sign out jvm
        System.exit(0);// state 0 Indicates normal exit 
        System.out.println(" Did the program exit ？");// The program has exited and will not execute the statement again 
    }
}

summary

• Inner class ：

  • Define a complete class inside a class
  • Member inner class 、 Static inner class 、 Local inner classes 、 Anonymous inner class

• Object class

  Direct or indirect parent of all classes , Can store any object

• Packaging

  The reference data type corresponding to the basic data type , You can make Object Unify all data

• String class

  Strings are constants , It cannot be modified after creation , Literal values are stored in the string pool , Can be Shared .

• BigDecimal class

  Floating point numbers can be calculated accurately

• Date class – At a certain time

• Calendar class – The calendar

• SimpleDateFormat– Format time

• System– System class