Anonymous internal classes are internal classes without names
Because they have no names, anonymous internal classes can only be used once. They are usually used to simplify code writing
But use anonymous internals The class also has a prerequisite: it must inherit a 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 an instance of Child and transform it up into a reference to the Person class
However, if the Child here A class is only used once, so wouldn't it be troublesome to write it as an independent class?
At this time, anonymous inner classes were introduced
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();
}
}
Running result: eat something
As you can see, we directly implement the methods in the abstract class Person in braces
This way we can omit the writing of a class
And anonymous inner classes can also be used On the interface
Example 3: Using anonymous inner class on the interface
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();
}
}
Running result: 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 to implement multi-threading, you must inherit the Thread class or the Runnable interface
Example 4: Anonymous inner class implementation of the Thread class
public class Demo {
public static void main(String[] args) {
new Thread() {
public void run() {
for (int i = 1; i <= 5; i++) {
System.out.print(i + " ");
}
}
}.start();
}
}
Running results: 1 2 3 4 5
Example 5: Anonymous inner class implementation of 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();
}
}
In fact, it can be written more concisely
public static void main(String[] args) {
new Thread(new Runnable() {
public void run() {
for (int i = 1; i <= 5; i++) {
System.out.print(i + " ");
}
}
}).start();
}
Run results: 1 2 3 4 5
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