Java internal class details demonstration

1. Java allows a class A to be declared in another class B, then class A is an inner class, and class B is called an outer class
2. Classification of internal classes: member internal classes (static, non-static) and local internal classes (in methods, code blocks, and constructors)
3. Member inner class:

   • On the one hand, as a member of an external class:

     • Call the structure of the external class
     • Can be modified by static
     • Can be modified by 4 different permissions

   • On the other hand, as a class:

     • Properties, methods, constructors, etc. can be defined in the class
     • Can be modified by final, which means that this class cannot be inherited. The implication is that it can be inherited without using final
     • Can be modified by abstract

/**
 * @Author: YuShiwen
 * @Date: 2020/11/19 10:21 AM
 * @Version: 1.0
 */
public class InnerClass {
    
    
    public static void main(String[] args) {
    
    
        //创建静态的成员内部类实例:
        Student.Maths maths = new Student.Maths();
        maths.appraise();

        //创建非静态的成员内部类实例:
        //错误示范：
        //Student.Physics physics = new Student.Physics();
        //需要先创建外部类对象
        Student student = new Student();
        Student.Physics physics = student.new Physics();
        physics.appraise();
        physics.namesake(18);
    }

}
class Student{
    
    
    String name = "Mr.Yu";
    int age = 21;

    public void drink(){
    
    
        System.out.println("口渴了，喝水！");
    }

    //静态成员内部类：
    static class Maths{
    
    
        String teacher = "Mr.Wu";
        int score;

        public void appraise(){
    
    
            System.out.println("该学生成绩优秀，学习刻苦认真！");
            //静态成员中，不能调用非静态属性或方法
            //drink();
            //System.out.println(name);
        }
    }

    //非静态成员内部类
    public final class Physics{
    
    
        String teacher = "Mr.Li";
        int age = 48;
        int score;

        public void appraise(){
    
    
            System.out.println("该学生成绩优秀，学习刻苦认真！");
            drink();
            System.out.println(name);
        }

        //内部类方法的形参、内部类的属性、外部类的属性同名情况
        public void namesake(int age){
    
    
            //内部类方法的形参
            System.out.println(age);
            //内部类的属性
            System.out.println(this.age);
            //外部类的属性
            System.out.println(Student.this.age);
        }
    }


    //以下都为局部内部类演示
    //方法中声明局部内部类
    public void localInnerClassDisplay(){
    
    
        //局部内部类
        class LocalInnerClass{
    
    

        }
    }
    //代码块中声明局部内部类
    {
    
    
        //局部内部类
        class LocalInnerClass{
    
    

        }
    }
    //构造器中声明局部内部类
    public Student(){
    
    
        //局部内部类
        class LocalInnerClass{
    
    

        }
    }


}

Output result:

该学生成绩优秀，学习刻苦认真！
该学生成绩优秀，学习刻苦认真！
口渴了，喝水！
Mr.Yu
18
48
21

Process finished with exit code 0

4. Demonstration of local internal classes:

interface Person{
    
    
    void eat();
    void drink();
}

public class LocalInnerClass {
    
    

    //返回一个实现了Person接口的类的对象
    //方式一：
    public Person getPerson1(){
    
    
        //1.先创建一个实现了Comparable接口的类:局部内部类
        class Student implements Person{
    
    

            @Override
            public void eat() {
    
    
                System.out.println("吃饭！");
            }

            @Override
            public void drink() {
    
    
                System.out.println("喝牛奶！");
            }
        }
        //2.返回局部内部类对象
        return new Student();
    }

    //返回一个实现了Person接口的类的对象
    //方式二：
    public Person getPerson2(){
    
    
        //直接返回一个匿名局部内部类的匿名对象
        return new Person(){
    
    

            @Override
            public void eat() {
    
    
                System.out.println("吃牛肉！");
            }

            @Override
            public void drink() {
    
    
                System.out.println("喝饮料！");
            }
        };
    }
}

class MainTest{
    
    
    public static void main(String[] args) {
    
    
        LocalInnerClass localInnerClass = new LocalInnerClass();

        Person person1 = localInnerClass.getPerson1();
        Person person2 = localInnerClass.getPerson2();
        person1.eat();
        person1.drink();
        person2.eat();
        person2.drink();
    }
}

operation result:

吃饭！
喝牛奶！
吃牛肉！
喝饮料！

Process finished with exit code 0