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Common API classes and exception systems

2022-07-06 00:22:00 【Wang Xiaoya】

1. Commonly used API

1.1 Math（ application ）

Application program interface （ English ：Application Programming Interface, abbreviation ：API）, Also known as application programming interface , It is the agreement of linking different components of the software system .
API The main purpose is to provide the ability for applications and developers to access a set of routines , Without access to the source code , Or understand the details of the internal working mechanism . Provide API The software of the defined function is called this API The implementation of the .API It's an interface , Therefore, it is an abstraction .

  • 1、Math Class Overview

    • Math Contains methods for performing basic number operations

  • 2、Math How to call methods in

    • Math No construction method in class , But the internal methods are static , You can use the Class name . To call

  • 3、Math Common methods of class

    Method name Method name explain
    public static int abs(int a) Returns the absolute value of the parameter
    public static double ceil(double a) Returns the smallest... That is greater than or equal to the parameter double value , It's a whole number
    public static double floor(double a) Returns the maximum... That is less than or equal to the parameter double value , It's a whole number
    public static int round(float a) Return the nearest parameter to the round int
    public static int max(int a,int b) Return to two int The larger of the values
    public static int min(int a,int b) Return to two int The smaller of the values
    public static double pow (double a,double b) return a Of b The value of the power
    public static double random() The return value is double Positive value of ,[0.0,1.0)

example ：

package com.com.object_11.MathTest_01;
//Math Class common methods 
public class MathDemo {
    
    public static void main(String[] args) {
    
//public static int abs(int a) Returns the absolute value of the parameter 
        System.out.println(Math.abs(66));   //66
        System.out.println(Math.abs(-66));   //66
        System.out.println("---------------");

// public static double ceil(double a) Returns the smallest... That is greater than or equal to the parameter double value , It's a whole number 
        System.out.println(Math.ceil(6.18));   //7.0
        System.out.println(Math.ceil(6.65));   //7.0
        System.out.println("---------------");

// public static double floor(double a) Returns the maximum... That is less than or equal to the parameter double value , It's a whole number 
        System.out.println(Math.floor(6.18));  //6.0
        System.out.println(Math.floor(6.65));  //6.0
        System.out.println("---------------");

// public static int round(float a) Return the nearest parameter to the round int
        System.out.println(Math.round(6.18));  //6
        System.out.println(Math.round(6.65));  //7
        System.out.println("---------------");


// public static int max(int a,int b) Return to two int The larger of the values 
        System.out.println(Math.max(6,9));   //9
        System.out.println("---------------");



// public static int min(int a,int b) Return to two int The smaller of the values 
        System.out.println(Math.min(6,9));   //6
        System.out.println("---------------");


// public static double pow (double a,double b) return a Of b The value of the power 
        System.out.println(Math.pow(2,7));   // 128 2^7
        System.out.println("---------------");


// public static double random() The return value is double Positive value of ,[0.0,1.0)
        System.out.println(Math.random());        //0.14007993398889917 [0.0,1.0) Can't get at random 1
        System.out.println(Math.random()*100);    //55.15963738381813 [0.0,100.0) Can't get at random 100
        System.out.println((int)(Math.random()*100));    //7  Forced conversion to int type  0-99 Random 
        System.out.println((int)(Math.random()*100)+1);  //27  Forced conversion to int type   After one 0-100 Random 

    }
}

1.2 System（ application ）

System Represents the system of the program , Some corresponding system attribute information is provided , And system operation .System class You cannot create objects manually , Because the construction method is private modification , Prevent the outside world from creating objects .System All of the classes are static Method , Just access the class name .

  • System Common methods of class
Method name explain
public static void exit(int status) Terminate the currently running Java virtual machine , Non zero means abnormal termination
public static long currentTimeMillis() Return current time ( In Milliseconds ) Get the current system time and 1970 year 01 month 01 Japan 00:00 The millisecond difference between the points
  • Sample code 
package com.com.object_11.APITest_01;

public class SystemDemo {
    
    public static void main(String[] args) {
    
// System.out.println(" Start ");
public static void exit(int status): Terminate the currently running Java virtual machine , Non zero means abnormal termination 
// System.exit(0); // It should have output  “ Start   end ”,exit Terminate it , So it only outputs “ Start ”
// System.out.println(" end ");

// public static long currentTimeMillis(): Return current time ( In Milliseconds )
//  Get the current system time and 1970 year 01 month 01 Japan 00:00 The millisecond difference between the points 
        System.out.println(System.currentTimeMillis()); // 2022/6028 17:48 1656409731638
        System.out.println(System.currentTimeMillis() * 1.0 / 1000 / 60 / 60 / 24 / 365 + " year ");
//  It's converted into 52.524408030124306 year  1000ms = 1 s
}
}
  • demand ： Output at console 1-10000, Calculate how many milliseconds this code executed 
public class SystemDemo {
    
    public static void main(String[] args) {
    
        //  Get the start time node 
        long start = System.currentTimeMillis();
        for (int i = 1; i <= 10000; i++) {
    
            System.out.println(i);
        }
        //  Get the time node after the code runs 
        long end = System.currentTimeMillis();
        System.out.println(" Total time consuming ：" + (end - start) + " millisecond ");
    }
}

1.3 Object Class toString Method （ application ）

  • Object Class Overview

    • Object Is the root of the class hierarchy , Each class can put Object As a superclass . All classes inherit directly or indirectly from this class , let me put it another way , The methods of this class , All classes will have one

  • How to view the method source code

    • Selection method , Press down Ctrl + B

  • rewrite toString Method of method

      1. Alt + Insert choice toString
      1. In the blank area of the class , Right click -> Generate -> choice toString

  • toString The role of methods ：

    • In a good format , More convenient display of attribute values in objects

  • Sample code ：

  package com.com.object_11.APITest_01;

// Definition Student class 
public class ObjectStudent extends Object {
    
    // Because the test class uses Object Class toString Method , So it's equivalent to inheriting Object
        //  Yes 4 Attributes ：
        private String name;
        private int age;
        private String school;
        private String major;

        //  Space parameter structure 
        public ObjectStudent(){
    }

        //  There are parametric structures 
        public ObjectStudent(String name,int age){
    
            this.name = name;
            this.age = age;
        }

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

        public ObjectStudent(String name,int age,String school,String major){
    
            this.name = name;
            this.age = age;
            this.school = school;
            this.major = major;
        }

    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;
    }

    public String getSchool() {
    
        return school;
    }

    public void setSchool(String school) {
    
        this.school = school;
    }

    public String getMajor() {
    
        return major;
    }

    public void setMajor(String major) {
    
        this.major = major;
    }

    // Print 
        public void show(){
    
            System.out.println(" Student "+name+" Age "+age+" year , Study in "+school+major+" major ");
        }

    @Override    //toString Method rewriting Alt+Insert,toString Method 
    public String toString() {
    
        return "ObjectStudent{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", school='" + school + '\'' +
                ", major='" + major + '\'' +
                '}';
    }
}

package com.com.object_11.APITest_01;

public class ObjectDemo {
    
    public static void main(String[] args) {
    
        ObjectStudent s = new ObjectStudent();
        s.setName(" Lixiaolang ");
        s.setAge(10);
        s.setSchool(" Youzhi middle school ");
        s.setMajor(" Magic major ");
        System.out.println(s);              //[email protected]
        System.out.println(s.toString());   //[email protected]
// ObjectStudent{name=' Lixiaolang ', age=10, school=' Youzhi middle school ', major=' Magic major '}
// ObjectStudent{name=' Lixiaolang ', age=10, school=' Youzhi middle school ', major=' Magic major '}

        /* public void println(Object x) { String s = String.valueOf(x); synchronized(this) { this.print(s); this.newLine(); } } */

/* public static String valueOf(Object obj) { return obj == null ? "null" : obj.toString(); }*/

/* public String toString() { return this.getClass().getName() + "@" + Integer.toHexString(this.hashCode()); }*/

    }
    }
  • Running results ：
ObjectStudent{
    name=' Lixiaolang ', age=10, school=' Youzhi middle school ', major=' Magic major '}
ObjectStudent{
    name=' Lixiaolang ', age=10, school=' Youzhi middle school ', major=' Magic major '}

1.4 Object Class equals Method （ application ）

  • equals The role of methods

    • For comparison between objects , return true and false Result
    • give an example ：s1.equals(s2); s1 and s2 Two objects

  • rewrite equals Method scenario

    • You don't want to compare the address values of objects , When you want to compare with object properties .

  • rewrite equals Method of method

      1. alt + insert choice equals() and hashCode(),IntelliJ Default, All the way next,finish that will do
      1. In the blank area of the class , Right click -> Generate -> choice equals() and hashCode(), The following is the same as above .

  • Sample code ：

class Student {
    
      private String name;
      private int age;
  
      public Student() {
    
      }
  
      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 boolean equals(Object o) {
    
          //this -- s1
          //o -- s2
          Compare the addresses , If the same , Go straight back to true
        if (this == o) return true;
```java
 // Judge whether the parameter is null
// Determine whether two objects come from the same class 
        if (o == null || getClass() != o.getClass()) return false;
//  Move down 
        Student student = (Student) o; //student -- s2
// Compare ages 
        if (age != student.age) return false;
// Compare names for equality 
          return name != null ? name.equals(student.name) : student.name == null;
      }
  }

  public class ObjectDemo {
    
      public static void main(String[] args) {
    
          Student s1 = new Student();
          s1.setName(" Brigitte Lin ");
          s1.setAge(30);
  
          Student s2 = new Student();
          s2.setName(" Brigitte Lin ");
          s2.setAge(30);
  
          // demand ： Compare the contents of two objects 
          System.out.println(s1.equals(s2));
      }
  }

1.5 Bubble sorting principle （ understand ）

  • Bubble sort Overview
    • A way of sorting , Compare the adjacent data in the data to be sorted , Put larger data behind , Operate all data in turn , Until all data are sorted as required
  • If there is n Data to sort , In total, we need to compare n-1 Time
  • After each comparison , The next comparison will involve less data

1.6 Bubble sort code implementation （ understand ）

  • Code implementation 
/*  Bubble sort ：  A way of sorting , Compare the adjacent data in the data to be sorted , Put larger data behind ,  Operate all data in turn , Until all data are sorted as required  */
public class ArrayDemo {
    
    public static void main(String[] args) {
    
        // Define an array 
        int[] arr = {
    24, 69, 80, 57, 13};
        System.out.println(" Before ordering ：" + arrayToString(arr));

        //  Here minus 1, Is to control the number of comparisons per round 
        for (int x = 0; x < arr.length - 1; x++) {
    
            // -1 To avoid index overruns ,-x It is to improve the comparative efficiency 
            for (int i = 0; i < arr.length - 1 - x; i++) {
    
                if (arr[i] > arr[i + 1]) {
    
                    int temp = arr[i];
                    arr[i] = arr[i + 1];
                    arr[i + 1] = temp;
                }
            }
        }
        System.out.println(" After ordering ：" + arrayToString(arr));

    }

    // The elements in the array form a string according to the specified rules ：[ Elements 1,  Elements 2, ...]
    public static String arrayToString(int[] arr) {
    
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        for (int i = 0; i < arr.length; i++) {
    
            if (i == arr.length - 1) {
    
                sb.append(arr[i]);
            } else {
    
                sb.append(arr[i]).append(", ");
            }
        }
        sb.append("]");
        String s = sb.toString();
        return s;
    }
}

1.7 Arrays（ application ）

  • Arrays The common method of

    Method name explain
    public static String toString(int[] a) Returns a string representation of the contents of the specified array
    public static void sort(int[] a) Arranges the specified array in numerical order

  • Tool design ideas

    1、 Construction method private modification
    In order to prevent the outside world from creating objects

    2、 For members public static modification
    In order to use the class name to access the member method 

package com.com.object_11.APITest_01.Arrays;

import java.util.Arrays;
//public static String toString(int[] a)  Returns a string representation of the contents of the specified array  |
//public static void sort(int[] a)  Arranges the specified array in numerical order  |
public class ArraysDemo {
    
    public static void main( String[] args) {
    
        // Define an array 
        int[] arr = {
    24,69,80,57,13};
        System.out.println(" Before ordering :" + Arrays.toString(arr));
// Before ordering :[24, 69, 80, 57, 13]
        Arrays.sort(arr) ;
        System.out.println(" After ordering :" + Arrays.toString(arr));
// After ordering :[13, 24, 57, 69, 80]
    }
}

2. Date class

2.1 Date class （ application ）

  • Date Class Overview

    ​ Date Represents a specific time , Accurate to milliseconds

  • Date Class constructor

    Method name explain
    public Date() Allocate one Date object , And initialization , So that it represents the time it is allocated , Accurate to milliseconds
    public Date(long date) Allocate one Date object , And initialize it to represent the specified number of milliseconds from the standard reference time

  • Sample code 

package com.com.object_11.APITest_01.Date;

import java.util.Date;

public class DateDemo001 {
    
    public static void main(String[] args) {
    
// public Date() Allocate one  Date object , And initialization , So that it represents the time it is allocated , Accurate to milliseconds 
   Date d1 = new Date();   // It's using java.util Created Date
        System.out.println(d1);

//public Date(long date) Allocate one  Date object ,
//  And initialize it to represent the specified number of milliseconds from the standard reference time 
//  If the given millisecond value contains time information , Then the driver will set the time component to correspond to zero GMT Default time zone for （ Running the application Java The time zone of the virtual machine ）.
//date - 1970 year 1 month 1 Milliseconds in days ,00:00:00 GMT No more than 8099 The millisecond of a year indicates . Negative said 1970 year 1 month 1 Japan 00:00:00 GMT The number of milliseconds before .
long date = 1000*60*60;  // Hours , So it's the whole hour ,1970 year 1 month 1 Japan 1:00, With time zone 
Date d2 = new Date(date);
        System.out.println(d2);
    }
}
//Wed Jun 29 09:35:17 CST 2022
//Thu Jan 01 09:00:00 CST 1970

Insert picture description here

2.2Date Class common methods （ application ）

  • Common methods

    Method name explain
    public long getTime() Get the date object from 1970 year 1 month 1 Japan 00:00:00 The millisecond value up to now
    public void setTime(long time) Setup time , Given is the millisecond value

public void setTime(long time) If the given millisecond value contains time information , Then the driver will set the time component to correspond to zero GMT Default time zone for （ Running the application Java The time zone of the virtual machine ）.
date - 1970 year 1 month 1 Milliseconds in days ,00:00:00 GMT No more than 8099 The millisecond of a year indicates . Negative said 1970 year 1 month 1 Japan 00:00:00 GMT The number of milliseconds before .

  • Sample code 
 package com.com.object_11.APITest_01.Date;

import java.util.Date;

public class DateDemo002 {
    
    public static void main(String[] args) {
    
        // Create date object 
        Date d = new Date();

        //public long getTime(): Get the date object from 1970 year 1 month 1 Japan  00:00:00 The millisecond value up to now 
        System.out.println(d.getTime());
        System.out.println(d.getTime() * 1.0 / 1000 / 60 / 60 / 24 / 365 + " year ");
   //1656469559780
  //52.5263051680619 year 

// public void setTime(long time);  Setup time , Given is the millisecond value 
// long time = 1000 * 60 * 60;
        long time = System.currentTimeMillis();
        d.setTime(time);
        System.out.println(d);
// Wed Jun 29 10:21:40 CST 2022
    }
}

2.3SimpleDateFormat class （ application ）

  • SimpleDateFormat Class Overview

    ​ SimpleDateFormat It's a concrete class , Used to format and parse dates in locale sensitive ways .

    ​ We focus on date formatting and parsing

  • SimpleDateFormat Class constructor

    Method name explain
    public SimpleDateFormat() Construct a SimpleDateFormat, Use default mode and date format
    public SimpleDateFormat(String pattern) Construct a SimpleDateFormat Use the given pattern and default date format

  • SimpleDateFormat Common methods of class

    • format ( from Date To String)
      • public final String format(Date date)： Format the date as a date / Time string
    • analysis ( from String To Date)
      • public Date parse(String source)： Parse the text from the beginning of the given string to generate the date

  • Code 

package com.com.object_11.APITest_01.Date;
// Toxic poison 
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/*SimpleDateFormat Class constructor  public SimpleDateFormat()  Construct a SimpleDateFormat, Use default mode and date format  public SimpleDateFormat(String pattern)  Construct a SimpleDateFormat Use the given pattern and default date format  - SimpleDateFormat Common methods of class  -  format ( from Date To String) - public final String format(Date date)： Format the date as a date / Time string  -  analysis ( from String To Date) - public Date parse(String source)： Parse the text from the beginning of the given string to generate the date */
public class SimpleDateFormatDemo {
    
    public static void main(String[] args) throws ParseException {
    

//  format ( from Date To String)
        //  Parameterless construction creates objects 
        Date d = new Date();
        SimpleDateFormat sdf = new SimpleDateFormat();   // No arguments structure 
        // How to create  sdf.format(d); then Ctrl+Alt+v Automatic generation 
        String s = sdf.format(d);
        //  Output this string 
        System.out.println(s);
        //2022/6/29  In the morning 11:03 ( Default mode )
        System.out.println("-----------");

        // format ( from Date To String)
//  Parameterless construction creates objects 
        Date d1 = new Date();
// SimpleDateFormat sdf = new SimpleDateFormat();
        //  Remove the nonparametric structure , Write it yourself 
        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy year MM month dd Japan  HH：mm：ss");  // Mm / DD / yyyy HHM / S 
// How to create  sdf.format(d1); then Ctrl+Alt+v Automatic generation 
        String s1 = sdf.format(d1);
//  Output this string 
        System.out.println(s1);
        // ( Default mode )
        System.out.println("-----------");

// ( from String To Date)
        String ss = "2025-06-29 11:20:00";
        //ParseException  Report errors , Parsing exceptions 
// SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy year MM month dd Japan  HH：mm：ss");
        SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date dd = sdf2.parse(ss);
        System.out.println(dd);
    }
}

2.4 Date tool case （ application ）

  • Case needs

    ​ Define a date tool class (DateUtils), There are two methods ： Converts the date to a string in the specified format ; Parse the string into a date in the specified format , Then define a test class (DateDemo), Test the method of the date tool class

  • Code implementation

    • Tool class 
package com.com.object_11.APITest_01.Date;

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

public class DateUtils {
    
    //  Constructor private （ The outside world cannot create objects ）
    private DateUtils() {
    
    }

    //  Member method static （ In the future, call... By class name ）
//  Converts the date to a string in the specified format 
//  return type :String
// Parameters :Date date,String format
    public static String dataToString(Date date, String format) {
    
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        String s = sdf.format(date);
        return s;
    }

    /* Parse the string into a date in the specified format   return type :Date  Parameters :string s, String format*/
    public static Date stringToDate(String s, String format) throws ParseException {
    
        SimpleDateFormat sdf = new SimpleDateFormat(format);
        Date d = sdf.parse(s);
        return d;
    }
}
  • Test class 
package com.com.object_11.APITest_01.Date;

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

// Test class 
public class DateUtilsDemo {
    
    public static void main(String[] args) throws ParseException {
    
//  Create date object 
        Date d = new Date();

        String s1 = DateUtils.dataToString(d, "yyyy year MM month dd Japan  HH:mm:ss");
        System.out.println(s1);

        String s2 = DateUtils.dataToString(d, "yyyy year MM month dd Japan ");
        System.out.println(s2);

        String s3 = DateUtils.dataToString(d, "HH:mm:ss");
        System.out.println(s3);
        System.out.println("------------");

        String s = "2025-06-29 11:20:00";
        Date dd = DateUtils.stringToDate(s, "yyyy-MM-dd HH:mm:ss");
        System.out.println(dd);
    }
}

2.5Calendar class （ application ）

  • Calendar Class Overview

    ​ Calendar Some methods are provided for the conversion between a specific moment and a set of calendar fields , And provides some methods for operating calendar fields

    ​ Calendar Provides a class method getInstance Used to get generally useful objects of this type .

    ​ This method returns a Calendar object .

    ​ Its calendar fields have been initialized with the current date and time ：Calendar rightNow = Calendar.getInstance();

  • Calendar Class common methods

    Method name explain
    public int get(int field) Returns the value of the given calendar field
    public abstract void add(int field, int amount) According to calendar rules , Add or subtract the given calendar field... From the specified amount of time
    public final void set(int year,int month,int date) Set the month, year and day of the current calendar

  • Sample code 

    public class CalendarDemo {
      
    public static void main(String[] args) {
      
        // Get calendar class object 
        Calendar c = Calendar.getInstance();

        //public int get(int field): Returns the value of the given calendar field 
        int year = c.get(Calendar.YEAR);
        int month = c.get(Calendar.MONTH) + 1;   // Because the month is from 0 Start , So calculate the month to +1
        int date = c.get(Calendar.DATE);
        System.out.println(year + " year " + month + " month " + date + " Japan ");

        //public abstract void add(int field, int amount): According to calendar rules , Add or subtract the given calendar field... From the specified amount of time 
        // demand 1:3 Years ago today 
// c.add(Calendar.YEAR,-3);
// year = c.get(Calendar.YEAR);
// month = c.get(Calendar.MONTH) + 1;
// date = c.get(Calendar.DATE);
// System.out.println(year + " year " + month + " month " + date + " Japan ");

        // demand 2:10 Years later, 10 Days ago, 
// c.add(Calendar.YEAR,10);
// c.add(Calendar.DATE,-10);
// year = c.get(Calendar.YEAR);
// month = c.get(Calendar.MONTH) + 1;
// date = c.get(Calendar.DATE);
// System.out.println(year + " year " + month + " month " + date + " Japan ");

        //public final void set(int year,int month,int date): Set the month, year and day of the current calendar 
        c.set(2050,10,10);
        year = c.get(Calendar.YEAR);
        month = c.get(Calendar.MONTH) + 1;
        date = c.get(Calendar.DATE);
        System.out.println(year + " year " + month + " month " + date + " Japan ");

    }
}

2.6 February day case （ application ）

  • Case needs

    ​ Get the number of days in February of any year

  • Code implementation 

    public class CalendarTest {
      
    public static void main(String[] args) {
      
        // Keyboard entry any year 
        Scanner sc = new Scanner(System.in);
        System.out.println(" Please enter year ：");
        int year = sc.nextInt();

        // Sets the year of the calendar object 、 month 、 Japan 
        Calendar c = Calendar.getInstance();
        c.set(year, 2, 1);

        //3 month 1 Push forward one day , Namely 2 Last day of the month 
        c.add(Calendar.DATE, -1);

        // Just get the output of this day 
        int date = c.get(Calendar.DATE);
        System.out.println(year + " Year of 2 Month has " + date + " God ");
    }
}

3. abnormal

3.1 abnormal （ memory ）

  • Overview of exceptions

    ​ An exception is an abnormal condition in the program

  • Abnormal architecture
    Please add a picture description

3.2JVM The default way to handle exceptions （ understand ）

  • If something goes wrong with the program , We didn't do anything , Final JVM Will do the default processing , The processing method has the following two steps ：

  • Name the exception , The error reason and the location of the exception are output on the console

  • Program stop

3.3try-catch Handle exceptions in different ways （ application ）

  • Define format 

    try {
      
	 Code with possible exception ;
} catch( Exception class name   Variable name ) {
      
	 Exception handling code ;
}

  • Execute the process

    • The program from try The code inside starts to execute
    • Something unusual happened , Will jump to the corresponding catch Go inside and do it
    • After execution , The program can continue to execute

  • Sample code 

    public class ExceptionDemo01 {
      
    public static void main(String[] args) {
      
        System.out.println(" Start ");
        method();
        System.out.println(" end ");
    }

    public static void method() {
      
        try {
      
            int[] arr = {
      1, 2, 3};
            System.out.println(arr[3]);
            System.out.println(" Can you access it here ");
        } catch (ArrayIndexOutOfBoundsException e) {
      
// System.out.println(" The array index you accessed does not exist , Please go back and modify it to the correct index ");
            e.printStackTrace();
        }
    }
}

3.4Throwable Member method （ application ）

  • Common methods

    Method name explain
    public String getMessage() Back here throwable The detailed message string for
    public String toString() Returns a short description of this throw
    public void printStackTrace() Output the abnormal error message to the console

  • Sample code 

    public class ExceptionDemo02 {
      
    public static void main(String[] args) {
      
        System.out.println(" Start ");
        method();
        System.out.println(" end ");
    }

    public static void method() {
      
        try {
      
            int[] arr = {
      1, 2, 3};
            System.out.println(arr[3]); //new ArrayIndexOutOfBoundsException();
            System.out.println(" Can you access it here ");
        } catch (ArrayIndexOutOfBoundsException e) {
       //new ArrayIndexOutOfBoundsException();
// e.printStackTrace();

            //public String getMessage(): Back here  throwable  The detailed message string for 
// System.out.println(e.getMessage());
            //Index 3 out of bounds for length 3

            //public String toString(): Returns a short description of this throw 
// System.out.println(e.toString());
            //java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3

            //public void printStackTrace(): Output the abnormal error message to the console 
            e.printStackTrace();
// java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
// at com.itheima_02.ExceptionDemo02.method(ExceptionDemo02.java:18)
// at com.itheima_02.ExceptionDemo02.main(ExceptionDemo02.java:11)

        }
    }
}

3.5 The difference between compile time and run time exceptions （ memory ）

  • Compile time exception

    • All are Exception Classes and subclasses
    • Processing must be displayed , Otherwise, the program will have an error , Unable to compile

  • Runtime exception

    • All are RuntimeException Classes and subclasses
    • No display processing is required , You can also handle the same as compile time exceptions

3.6throws Handle exceptions in different ways （ application ）

  • Define format 

    public void  Method () throws  Exception class name  {
      
    
}

  • Sample code 

    public class ExceptionDemo {
      
    public static void main(String[] args) {
      
        System.out.println(" Start ");
// method();
        try {
      
            method2();
        }catch (ParseException e) {
      
            e.printStackTrace();
        }
        System.out.println(" end ");
    }

    // Compile time exception 
    public static void method2() throws ParseException {
      
        String s = "2048-08-09";
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Date d = sdf.parse(s);
        System.out.println(d);
    }

    // Runtime exception 
    public static void method() throws ArrayIndexOutOfBoundsException {
      
        int[] arr = {
      1, 2, 3};
        System.out.println(arr[3]);
    }
}

  • matters needing attention

    • This throws The format follows the parentheses of the method
    • Compile time exceptions must be handled , Two treatment schemes ：try…catch … perhaps throws, If the throws This program , Who will call who will handle
    • Run time exceptions can be ignored , When something goes wrong , We need to come back and modify the code

3.7throws and throw The difference between （ memory ）

Please add a picture description
throws

  • Used after method declaration , With the exception class name

  • Means throw an exception , Handled by the caller of the method

  • Represents a possibility of an exception , These exceptions do not necessarily occur

    throw

  • Used in method body , Followed by the exception object name

  • Means throw an exception , Handled by statements inside the method

  • perform throw Must have thrown some kind of exception

3.8 Custom exception （ application ）

If Java The built-in exception type provided does not meet the requirements of programming , At this time, we can design our own Java Class library or framework , This includes exception types . The implementation of custom exception class needs to inherit Exception Class or its subclasses , If the custom runtime exception class needs to inherit RuntimeException Class or its subclasses .

The syntax form of the custom exception is ：
< Custom exception name >
In terms of coding specification , Generally, the class name of the custom exception class is XXXException, among XXX Used to represent the role of the exception .

Custom exception classes generally contain two construction methods ： One is the default construction method without parameters , Another constructor receives a custom exception message as a string , And pass the message to the constructor of the superclass .

for example , The following code creates a file named IntegerRangeException Custom exception class for ：

class IntegerRangeException extends Exception {
    
    public IntegerRangeException() {
    
        super();
    }
    public IntegerRangeException(String s) {
    
        super(s);
    }
}

The custom exception class created by the above code IntegerRangeException Class inherits from Exception class , There are two constructors in this class .

  • Custom exception classes 

    public class ScoreException extends Exception {
      

    public ScoreException() {
      }

    public ScoreException(String message) {
      
        super(message);
    }

}

  • The teacher class 

    public class Teacher {
      
    public void checkScore(int score) throws ScoreException {
      
        if(score<0 || score>100) {
      
// throw new ScoreException();
            throw new ScoreException(" You gave the wrong score , The score should be 0-100 Between ");
        } else {
      
            System.out.println(" Normal performance ");
        }
    }
}

  • Test class 

 
  public class Demo {
    
      public static void main(String[] args) {
    
          Scanner sc = new Scanner(System.in);
          System.out.println(" Please enter the score ：");
  
          int score = sc.nextInt();
  
          Teacher t = new Teacher();
          try {
    
              t.checkScore(score);
          } catch (ScoreException e) {
    
              e.printStackTrace();
// Or is it e.getMessage();
          }
      }
  }

Enter the running results of grades in the range
Insert picture description here

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本文为[Wang Xiaoya]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/187/202207060016518192.html

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