Anonymous inner classes are inner classes without names
Because they have no names, anonymous inner classes can only be used once. They are usually used to simplify code writing
But there is a prerequisite for using anonymous inner classes: you must inherit one Parent class or implement an interface
Example 1: Do not use anonymous inner classes to implement abstract methods
abstract class Person { public abstract void eat(); } class Child extends Person { public void eat() { System.out.println("eat something"); } } public class Demo { public static void main(String[] args) { Person p = new Child(); p.eat(); } }
Running results: eat something
As you can see, we use Child to inherit the Person class, and then implement a Child instance, upcast it to a reference to the Person class
However, if the Child class here is only used once, wouldn't it be troublesome to write it as an independent class?
Anonymous inner classes are introduced at this time
Example 2: Basic implementation of anonymous inner classes
abstract class Person { public abstract void eat(); } public class Demo { public static void main(String[] args) { Person p = new Person() { public void eat() { System.out.println("eat something"); } }; p.eat(); } }
Run result: eat something
As you can see, we directly put the methods in the abstract class Person in curly brackets Implemented
This way you can omit the writing of a class
Moreover, anonymous inner classes can also be used on interfaces
Example 3: Using anonymous inner classes on interfaces
interface Person { public void eat(); } public class Demo { public static void main(String[] args) { Person p = new Person() { public void eat() { System.out.println("eat something"); } }; p.eat(); } }
Run results: eat something
By As can be seen from the above example, as long as a class is abstract or an interface, then the methods in its subclasses can be implemented using anonymous inner classes
The most common situation is in multi-threaded implementation, because it needs to be implemented Multi-threading must inherit the Thread class or inherit the Runnable interface
Example 4: Anonymous inner class implementation of the Thread class
public class Demo { public static void main(String[] args) { Thread t = new Thread() { public void run() { for (int i = 1; i <= 5; i++) { System.out.print(i + " "); } } }; t.start(); } }
Running results: 1 2 3 4 5
Example 5: Anonymous inner class implementation of the Runnable interface
public class Demo { public static void main(String[] args) { Runnable r = new Runnable() { public void run() { for (int i = 1; i <= 5; i++) { System.out.print(i + " "); } } }; Thread t = new Thread(r); t.start(); } }
Running results: 1 2 3 4 5