Detailed explanation of Java internal classes

Ordinary internal classes

There are two main points to master for ordinary internal classes

Holding the parent class reference, so You can access variables and methods with any access rights of the parent class

How to create internal classes outside the parent class

public class Main{
    public static void main(String args[]){
        Outer outer = new Outer();
        //外部类外部创建内部类
        Outer.Inner inner = outer.new Inner();
        inner.method();
    }
}
class Outer{
    private String value = "OuterValue";
    class Inner{
        public void method(){
            //持有父类引用,可以访问父类private变量value
            System.out.println(value);
        }
        //利用.this关键字,可以返回父类对象引用
        public Outer outerObject(){
            return Outer.this;
        }
    }
}

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Method inner class

The inner class directly defined in the method

enjoys the same permissions as the method and access other methods and member variables of the outer class.

You can access the parameters of this method. After 1.8, the parameters do not need to be set to final

Have the same permissions as the local variables of the method, and can access the local variables defined before it

The method inner class can only be accessed in the method scope

public class Main{
    public static void main(String args[]){
        Outer outer = new Outer();
        outer.outerMethod("param");
    }
}
class Outer{
    public void outerMethod(String value){
        class Inner {
            public void innerMethod(){
                //可以访问方法参数
                //1.8以后，内部类访问的方法参数不必设置为final
                System.out.println(value);
            }
        }
        new Inner().innerMethod();
    }
}

Anonymous inner class

Anonymous inner class is actually a key point, and it is also a knowledge point we often use .

For example, creating a new thread

public class Main{
    public static void main(String args[]){
        Thread a = new Thread(new Runnable(){
            @Override
            public void run(){
                System.out.println("thread a is running;");
            }
        });
        a.start();
    }
}

The above code means to create an anonymous class object that implements the Runnable interface, which is equivalent to the following code. Due to the convenience of writing, we often use the second one

public class Main{
    public static void main(String args[]){
        Thread a = new Thread(new RunnableImp());
        a.start();
    }
    static class RunnableImp implements Runnable{
        @Override
        public void run(){
            System.out.println("thread a is running;");
        }
    }
}

Static inner class

Remember the following two points

To create a static inner class object, you do not need an object of the outer class (ordinary inner classes require )

Cannot access non-static external class methods or member variables from static inner classes

public class Main{
    Outer.Inner inner = new Outer.Inner();
}

class Outer{
    private String ordValue;
    private static String staValue;
    static class Inner{
        {
            System.out.println(staValue);

            //这句编译通不过
            //System.out.println(ordValue);
        }
    }
}

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