What are the design patterns for anonymous inner classes in Java?

Anonymous inner classes are special inner classes in Java that have no explicit name and are created through the new expression. They are mainly used to implement specific interfaces or extend abstract classes and are used immediately after creation. Common anonymous inner class design patterns include: Adapter pattern: converts one interface into another interface. Strategy Pattern: Defining and Replacement Algorithms. Observer pattern: Register observers and handle events. It is very useful in practical applications, such as sorting a TreeSet by string length, creating anonymous threads, etc.

Design pattern of Java anonymous inner class

Anonymous inner class is a special type of inner class in Java that does not explicit name, instead create it directly using a new expression. It is mainly used to implement a specific interface or extend a specific abstract class, and is usually created within the method body and used immediately.

Main design patterns

The following are common design patterns for using anonymous inner classes:

1. Adapter pattern

Adapter pattern Use anonymous inner classes to convert one interface into another. For example, the following code creates an anonymous inner class that adapts the Adaptee class to the Target interface:

Target target = new Target() {
    @Override
    public void request() {
        adaptee.specificRequest();
    }
};

2. Strategy Pattern

The Strategy pattern uses anonymous inner classes to define families of algorithms and make them interchangeable. For example, the following code creates an anonymous inner class that implements the Comparator interface to compare the lengths of two strings:

Comparator<String> comparator = new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return s1.length() - s2.length();
    }
};

3. Observer pattern

Observer pattern uses anonymous inner classes to register observers and handle events emitted by the topic. For example, the following code creates an anonymous inner class that implements the Observer interface to observe the Subject object:

Subject subject = new Subject();
Observer observer = new Observer() {
    @Override
    public void update(Subject subject) {
        // 处理主题更新
    }
};
subject.addObserver(observer);

Practical case

Example: Create a TreeSet with a Comparator

TreeSet requires its elements to be compared. The following code uses an anonymous inner class to define a custom comparator to sort strings based on their length:

Set<String> treeSet = new TreeSet<>(new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return s1.length() - s2.length();
    }
});

Example: Create an anonymous thread

The following code uses anonymous The inner class creates and starts a thread in which a message is printed:

new Thread() {
    @Override
    public void run() {
        System.out.println("Hello from thread!");
    }
}.start();

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