Interview knowledge points

1、 The difference between abstract class and interface

        1.1、 The difference between members ：

          abstract class ：
                Construction method ： There's a way to structure , Used for subclass instantiation .
                Member variables ： It could be a variable , It can also be a constant .
                Member method ： It can be abstract , It can also be non abstract .

          Interface ：
                 Construction method ： There is no way to construct
                 Member variables ： It can only be constant . Default modifier ：public static final
                 Member method ：jdk1.7 It can only be abstract . Default modifier ：public abstract ( recommend ： Please always give the default modifier manually ),jdk1.8 It can be written as default and static The specific method at the beginning

        1.2、 The relationship between class and interface is different ：

         Class and class ：
                 Inheritance relationships , Only inheritance . Multiple levels of inheritance .

         Classes and interfaces ：
                 Realization relationship , It can be realized by itself , It can also be realized more .
                 Class can also inherit a class and implement multiple interfaces at the same time .

         Interface and interface ：
                 Inheritance relationships , Can inherit alone , You can also inherit more .

        1.3、 Reflect different ideas :

         What is defined in an abstract class is the common content in an inheritance system .
         An interface is a collection of functions , Is an additional function of the system , It's the exposed rules

         When you focus on the nature of a thing , Use abstract classes ; When you focus on an operation , Use interface .

2、 What is the inner class , Use scenarios

        2.1、 What is an inner class

        Define a class in a class or a method

        2.2、 Types of inner classes

                Member inner class 、 Static inner class 、 Anonymous inner class 、 Local inner classes

        2.2.1 Member inner class

        Member inner class is the most common inner class , The inner class of a member can access all properties and methods of the outer class , But the external class accesses the properties and methods of the internal class , You must instantiate the inner class , Member inner classes cannot contain static properties and methods .

public class InnerClassTest {

    public int outField1 = 1;
    protected int outField2 = 2;
    int outField3 = 3;
    private int outField4 = 4;

    public InnerClassTest() {
        //  Inside the external class object , Directly through  new InnerClass();  Create inner class objects 
        InnerClassA innerObj = new InnerClassA();
        System.out.println(" establish  " + this.getClass().getSimpleName() + "  object ");
        System.out.println(" Of its inner class  field1  The value of the field is : " + innerObj.field1);
        System.out.println(" Of its inner class  field2  The value of the field is : " + innerObj.field2);
        System.out.println(" Of its inner class  field3  The value of the field is : " + innerObj.field3);
        System.out.println(" Of its inner class  field4  The value of the field is : " + innerObj.field4);
    }

    public class InnerClassA {
        public int field1 = 5;
        protected int field2 = 6;
        int field3 = 7;
        private int field4 = 8;
//        static int field5 = 5; //  Compile error ！ You cannot define  static  attribute 

        public InnerClassA() {
            System.out.println(" establish  " + this.getClass().getSimpleName() + "  object ");
            System.out.println(" Of its external class  outField1  The value of the field is : " + outField1);
            System.out.println(" Of its external class  outField2  The value of the field is : " + outField2);
            System.out.println(" Of its external class  outField3  The value of the field is : " + outField3);
            System.out.println(" Of its external class  outField4  The value of the field is : " + outField4);
        }
    }

    public static void main(String[] args) {
        InnerClassTest outerObj = new InnerClassTest();
        //  Not inside the outer class , Use ： External class objects . new  Inner class constructors ();  To create an internal class object 
//        InnerClassA innerObj = outerObj.new InnerClassA();
    }
}

        2.2.2 Static inner class

         Static inner class is to add one more in the inner class of the member  static  keyword . Static inner class Can only Access static member variables and methods of external classes （ Including private static ）

public class InnerClassTest {
	public int field1 = 1;
    
	public InnerClassTest() {
		System.out.println(" establish  " + this.getClass().getSimpleName() + "  object ");
        //  Create static inner class objects 
        StaticClass innerObj = new StaticClass();
        System.out.println(" Of its inner class  field1  The value of the field is : " + innerObj.field1);
        System.out.println(" Of its inner class  field2  The value of the field is : " + innerObj.field2);
        System.out.println(" Of its inner class  field3  The value of the field is : " + innerObj.field3);
        System.out.println(" Of its inner class  field4  The value of the field is : " + innerObj.field4);
    }
	
    static class StaticClass {

        public int field1 = 1;
        protected int field2 = 2;
        int field3 = 3;
        private int field4 = 4;
        //  Static inner classes can define  static  attribute 
        static int field5 = 5;

        public StaticClass() {
            System.out.println(" establish  " + StaticClass.class.getSimpleName() + "  object ");
//            System.out.println(" Of its external class  field1  The value of the field is : " + field1); //  Compile error ！！
        }
    }

    public static void main(String[] args) {
	    //  No need to rely on external class objects , Create internal class objects directly 
//        InnerClassTest.StaticClass staticClassObj = new InnerClassTest.StaticClass();
		InnerClassTest outerObj = new InnerClassTest();
    }
}

        2.2.3 Anonymous inner class

         Anonymous inner classes take many forms , One of the most common forms is to create an interface object in a method parameter / Class object , And implement the interface declaration / Class ：

Thread pool execution method execute Inside new Runnable,Runnable It's the anonymous inner class

public class InnerClassTest {

    public int field1 = 1;
    protected int field2 = 2;
    int field3 = 3;
    private int field4 = 4;

    public InnerClassTest() {
        System.out.println(" establish  " + this.getClass().getSimpleName() + "  object ");
    }
    //  Custom interface 
	interface OnClickListener {
        void onClick(Object obj);
    }

    private void anonymousClassTest() {
        //  In this process, an anonymous inner class object will be created ,
        //  This anonymous inner class implements  OnClickListener  Interface and override  onClick  Method 
        OnClickListener clickListener = new OnClickListener() {
	        //  You can define properties in inner classes , But it can only be used in the current inner class ,
	        //  Cannot use... In an external class , Because the external class cannot get the class name of the current anonymous internal class ,
	        //  You can't create objects of anonymous inner classes 
	        int field = 1;
	        
            @Override
            public void onClick(Object obj) {
                System.out.println(" object  " + obj + "  Clicked ");
                System.out.println(" Of its external class  field1  The value of the field is : " + field1);
                System.out.println(" Of its external class  field2  The value of the field is : " + field2);
                System.out.println(" Of its external class  field3  The value of the field is : " + field3);
                System.out.println(" Of its external class  field4  The value of the field is : " + field4);
            }
        };
        // new Object()  The procedure creates an anonymous inner class , Inherited from  Object  class ,
        //  Rewrite the  toString()  Method 
        clickListener.onClick(new Object() {
            @Override
            public String toString() {
                return "obj1";
            }
        });
    }

    public static void main(String[] args) {
        InnerClassTest outObj = new InnerClassTest();
        outObj.anonymousClassTest();
    }
}

        2.2.4 Local inner classes

         Local inner classes are rarely used , It is declared in a method body / Inside a code block , And it cannot be used if it is not within the definition domain of the definition class , The functions it provides can be implemented using anonymous inner classes , The anonymous inner class itself can be written more succinctly , Therefore, local inner classes are rarely used . Take a look at a small example of a local inner class ：

public class InnerClassTest {

    public int field1 = 1;
    protected int field2 = 2;
    int field3 = 3;
    private int field4 = 4;

    public InnerClassTest() {
        System.out.println(" establish  " + this.getClass().getSimpleName() + "  object ");
    }
    
    private void localInnerClassTest() {
	    //  Local inner classes  A, Can only be used in the current method 
        class A {
	        // static int field = 1; //  Compile error ！ Cannot be defined in a local inner class  static  Field 
            public A() {
	            System.out.println(" establish  " + A.class.getSimpleName() + "  object ");
                System.out.println(" Of its external class  field1  The value of the field is : " + field1);
                System.out.println(" Of its external class  field2  The value of the field is : " + field2);
                System.out.println(" Of its external class  field3  The value of the field is : " + field3);
                System.out.println(" Of its external class  field4  The value of the field is : " + field4);
            }
        }
        A a = new A();
        if (true) {
	        //  Local inner classes  B, Can only be used in the current code block 
            class B {
                public B() {
	                System.out.println(" establish  " + B.class.getSimpleName() + "  object ");
                    System.out.println(" Of its external class  field1  The value of the field is : " + field1);
                    System.out.println(" Of its external class  field2  The value of the field is : " + field2);
                    System.out.println(" Of its external class  field3  The value of the field is : " + field3);
                    System.out.println(" Of its external class  field4  The value of the field is : " + field4);
                }
            }
            B b = new B();
        }
//        B b1 = new B(); //  Compile error ！ Not in class  B  In the domain of , Class not found  B,
    }

    public static void main(String[] args) {
        InnerClassTest outObj = new InnerClassTest();
        outObj.localInnerClassTest();
    }
}

3、 Internal class function

        3.1、 Inner classes can be well hidden .
Non inner classes cannot be used  private and  protected Embellished , But inner classes can , So as to achieve the function of hiding . At the same time, you can also organize classes with certain logical relationships , enhance readability .

        3.2、 Indirect implementation of multiple inheritance .
Each inner class can inherit independently from a ( Interface ) Realization , So whether or not the outer class has inherited a certain ( Interface ) Realization , No effect on inner classes . If there is no internal class to provide the ability to inherit multiple concrete or abstract classes , Some design and programming problems are hard to solve . So the inner class indirectly implements multiple inheritance .