Basic syntax and application of callback functions in Java

Basic writing and usage of Java callback function

Introduction:
In Java programming, callback function is a common programming pattern. Through callback function , you can pass a method as a parameter to another method to achieve indirect calling of the method. The use of callback functions is very common in scenarios such as event-driven, asynchronous programming and interface implementation. This article will introduce the basic writing and usage of Java callback functions, and provide specific code examples.

1. Definition of callback function
The callback function is a special function that can be passed as a parameter to other methods and called within the method. Callback functions are usually used for event processing and completion notification of asynchronous operations. In Java, callback functions are usually defined through interfaces.

The following is a simple callback function interface definition example:

public interface Callback {
    void onCallback();
}

In this example, the Callback interface defines a onCallback method, which Methods have no parameters and return values.

2. How to use the callback function
When using the callback function, you first need to define an interface (callback function interface), then implement the interface and rewrite its method. When you need to use the callback function method , pass the callback function as a parameter and call the callback function when needed.

The following is a simple example to illustrate the use of callback functions. Suppose there is a download tool class Downloader, which is used to download files. When the file download is completed, some specific operations need to be performed.

First, define the callback function interface DownloadCallback:

public interface DownloadCallback {
    void onDownloadComplete();
}

Next, implement the DownloadCallback interface:

public class SimpleCallback implements DownloadCallback {
    @Override
    public void onDownloadComplete() {
        System.out.println("下载完成，执行回调函数中的操作");
    }
}

Then, write Downloader Class:

public class Downloader {
    public void downloadFile(DownloadCallback callback) {
        // 模拟下载过程
        System.out.println("开始下载文件");
        try {
            Thread.sleep(3000); // 等待3秒，模拟下载时间
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("下载完成");
        // 执行回调函数
        callback.onDownloadComplete();
    }
}

Finally, use the Downloader class for file downloading:

public class Main {
    public static void main(String[] args) {
        Downloader downloader = new Downloader();
        DownloadCallback callback = new SimpleCallback();
        downloader.downloadFile(callback);
    }
}

In this example, Downloader The downloadFile method of the class receives a DownloadCallback type parameter, which represents the callback function executed after the file download is completed.

3. Summary
Through the callback function, a method can be passed as a parameter to other methods, and the callback is executed when needed. This programming model is very useful in scenarios such as event-driven, asynchronous programming and interface implementation. In Java, callback functions are generally implemented by defining a callback function interface and a class that implements the interface.

This article introduces the basic writing and usage of Java callback functions, and provides specific code examples. I hope this article can help readers understand and use the callback function programming pattern.

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