回调函数callback，是指通过函数传递参数传递到其他代码，某一块可执行的代码引用。

废话不多说先看一段代码实例。

/*** @author keying* @date 2022-12-05 20:07:05*/
public class Boss221205 implements CallbackInterface {private Employee221205 employee221205;public Boss221205(Employee221205 employee221205){this.employee221205 = employee221205;}public void question(){employee221205.question(this);}@Overridepublic void callbackMethod() {}@Overridepublic void tellBoss(String answer) {System.out.println("回答boss:" + answer);}
}/*** @author keying* @date 2022-12-05 19:53:35*/
public interface CallbackInterface {public void callbackMethod();public void tellBoss(String answer);
}/*** @author keying* @date 2022-12-05 20:08:14*/
public interface Employee221205 {public void question(Boss221205 boss221205);
}/*** @author keying* @date 2022-12-05 20:11:43*/
public class EmployeeA221205 implements Employee221205{@Overridepublic void question(Boss221205 boss221205) {try{Thread.sleep(3000);}catch (Exception e){}// 回调boss221205.tellBoss("三秒后回调answer");}
}/*** @author keying* @date 2022-12-05 19:55:02*/
public class Main20221205 {public static void main(String[] args) {Employee221205 employeeA221205 = new EmployeeA221205();Boss221205 boss221205 = new Boss221205(employeeA221205);boss221205.question();}
}

除了一些函数存在固定代码以外，还有一部分代码根据实际业务来写不同的代码，而这种业务场景最适合用回调函数来替代。回调函数如同占位符号，代码由调用者传递执行，回调函数提升函数重用率。

/*** @author keying* @date 2022-12-05 21:31:47*/
public interface Callback1205 {void append(FileWriter fileWriter);
}/*** @author keying* @date 2022-12-05 21:33:10*/
public class Write20221205 {public void write(String filename, Callback1205 callback1205) {FileWriter fileWriter;try{fileWriter = new FileWriter(filename,true);callback1205.append(fileWriter);fileWriter.flush();fileWriter.close();}catch (Exception e){}}
}/*** @author keying* @date 2022-12-05 21:35:22*/
public class WriteMain20221205 {public static void main(String[] args) {Write20221205 write20221205 = new Write20221205();write20221205.write("D:\\fw.txt", fileWriter -> {try{fileWriter.write("write 20221205");}catch (Exception e){}});/*   Write20221205 write20221205 = new Write20221205();write20221205.write("D:\\fw.txt", new Callback1205() {@Overridepublic void append(FileWriter fileWriter) {try{fileWriter.write("回调");}catch (Exception e){}}});*/}
}

先来一段lambda简写形式：上面代码可以看到，我们可以通过回调函数来实现自己需要write什么内容，这样写增加了代码重用率，在A函数里通过书写函数b的函数名来调用。按照定义好的接口规范，来供其他函数调用。使用技巧则是定义一个接口，在接口中定义我们想回调的方法。

在写个监听按钮的回调函数加深印象：

1、我们先定义一个监听按钮接口。

2、定义点击类，然后main方法调用点击。

/*** @author keying* @date 2022-12-05 22:28:54*/
public interface ButtonListener {void listener();
}/*** @author keying* @date 2022-12-05 22:29:50*/
public class ButtonMain1205 {public static void main(String[] args) {ClickButton clickButton = new ClickButton();clickButton.click();}
}/*** @author keying* @date 2022-12-05 22:29:19*/
public class ClickButton {public void click(){System.out.println("点击成功");}}

这时候已经点击成功了，那么如何通过回调函数监听这个点击动作呢。

  public static void main(String[] args) {ClickButton clickButton = new ClickButton();clickButton.setButtonListener(new ButtonListener() {@Overridepublic void listener() {System.out.println("触发点击动作");}});clickButton.click();}private ButtonListener buttonListener;public void setButtonListener(ButtonListener buttonListener){this.buttonListener = buttonListener;}public void click(){if(buttonListener!=null){buttonListener.listener();}System.out.println("点击成功");}