Differences and examples between internal classes and static internal classes

1. The difference between inner class and static inner class

1. Static inner class is independent of outer class , Variables in external classes cannot be accessed directly in static inner classes 、 Method . If you want to visit , It has to be new An object of an external class , Use new Come out of the object to access . But you can access static variables directly , Call static methods .

2. A normal inner class exists as a member of an outer class , In a normal inner class, you can directly access the properties of the outer class , Call methods of external classes .

3. If the outer class wants to access the properties of the inner class or call the methods of the inner class , You have to create an internal class object , Use this object to access properties or call methods .

4. If other classes want to access the properties of ordinary inner classes or call the methods of ordinary inner classes , You must create an object of the inner class in the outer class as a property , External classes can call methods of ordinary internal classes or access attributes of ordinary internal classes through attributes .

5. If other classes want to access the properties of static inner classes or call the methods of static inner classes , Just create a static inner class object .

2. Illustrate with examples

2.1 Inner class

public class Father {
    
    private String name;
   
    public Father(String name) {
    
        this.name = name;
    }

    public void say(String dogName) {
    
         Dog dog = new Dog(dogName);
        System.out.println("i am " + name + ", i have a dog " + dog.dogName);
    }
    
    public void feed() {
    
        System.out.println("feed in Father");
    }
    
    public  class Dog {
    
        private String dogName;

        public Dog(String dogName) {
    
            this.dogName = dogName;
        }

        public void bark() {
    
            feed();
            System.out.println("i am " + dogName + ", i am belong to " + name);
        }
        public String getDogName() {
    
            return dogName;
        }
    }
}

For the first 2 strip , Ordinary inner classes as members , You can directly access variables of external classes , Call its methods . You can see Dog Inside bark() Methods can use variables directly name, You can call feed Method .

For the first 3 strip , We are Father Class , Want to use Dog Properties in a class dogName, You must first instantiate a dog object , Then call the method through this object , Get variable .

For the first 4 strip , First create an external class

public class YYTest {
    
    
    public static void main(String[] args)  {
    
        final Father zhangsan = new Father("zhangsan");
        Father.Dog gg = zhangsan.new Dog("GG");
        gg.getDogName();
        gg.bark();
    }
}

You can see , Through object zhangsan.new Dog() Method instantiates an internal class object gg, Variables can be accessed through this internal class object , Calling method .

2.2 Static inner class

public class Father {
    
    private String name;
   private static String sex = "female";
    public Father(String name) {
    
        this.name = name;
    }

    public void say(String dogName) {
    
         Dog dog = new Dog(dogName);
        System.out.println("i am " + name + ", i have a dog " + dog.dogName);
    }
    
    public void feed() {
    
        System.out.println("feed in Father");
    }
    
    public static class Dog {
    
        private String dogName;

        public Dog(String dogName) {
    
            this.dogName = dogName;
        }

        public void bark() {
    
            Father lisi = new Father("lisi");
            lisi.feed();
            System.out.println("i am " + dogName + ", i am belong to " + lisi.name + ", his sex is " + sex);
        }

        public String getDogName() {
    
            return dogName;
        }
    }
}

For the first 1 spot , You can see bark() Method you want to use name attribute , You need to instantiate Father object , Call... By object . Static variables do not need to be accessed by instantiating objects , You can directly access .

For the first 1 spot

public class YYTest {
    
    
    public static void main(String[] args)  {
    
        Father.Dog hh = new Father.Dog("HH");
        hh.bark();
    }
}

Just instantiate the static inner class directly . Pay attention to the instantiation method .
Static classes can only be internal .