java回调机制

最简单的回调机制如下：

public class CallbackDemo { public static void main(String[] args) { new A().aa(); } } class A{ public void aa(){ new B().b(); System.out.println("This is aa!"); } public void ab(){ System.out.println("This is ab!"); } } class B{ public void b(){ new A().ab(); System.out.println("This is b!"); } }

执行的结果如下：

This is ab! This is b! This is aa!

上面的例子看上去没有什么实际的应用，现在我们想象这样的一个场景：每个人参加工作之后都会面临交税的事，但是除了税务财务人员，其他人并不关心具体的算法，他们只关心交了多少税（2019年的税法复杂了点，采用）；

1.应纳税所得额 = 税前工资收入金额 - 五险一金（个人缴纳部分） -费用减除额

2.应纳税额 = 应纳税所得额 x 税率 - 速算扣除数

级数全年应纳税所得额税率（%）速算扣除数

1

不超过36000元的

3

0

2

超过36000元至144000元的部分

10

2520

3

超过144000元至300000元的部分

20

16920

4

超过300000元至420000元的部分

25

31920

5

超过420000元至660000元的部分

30

52920

6

超过660000元至960000元的部分

35

85920

7

超过960000元的部分

45

181920

最原始的代码：

public class CallbackDemo2 { public static void main(String[] args) { Student xiaomiStudent = new Student("小米", 200000, 6000, 5000); xiaomiStudent.say(); } } class Student{ private String name; private int wages; private int five; private int jian; Student(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name) { this.name = name; } // 应纳税所得额 = 税前工资收入金额 - 五险一金（个人缴纳部分） -费用减除额 // 应纳税额 = 应纳税所得额 x 税率 - 速算扣除数 public int getTax(int wages,int five,int jian){ int cost; int level = wages<36000?1:wages<144000?2:wages<300000?3:wages<420000?4:wages<660000?5:wages<960000?6:7; switch(level){ case 1: cost= (wages-five-jian)*3/100; break; case 2: cost= (wages-five-jian)*10/100-2520; break; case 3: cost= (wages-five-jian)*20/100-16920; break; case 4: cost= (wages-five-jian)*25/100-31920; break; case 5: cost= (wages-five-jian)*30/100-52920; break; case 6: cost= (wages-five-jian)*35/100-85920; break; case 7: cost= (wages-five-jian)*45/100-181920; break; default: cost = 0; } return cost; } public void say() { System.out.println(this.name + ":需要交税：" + getTax(this.wages,this.five,this.jian)); } }

运行结果：

小米:需要交税：20880

现在有一个老师需要完成交税的任务，将上面的类在写一遍？如果又有销售需要交税。。。现在我们将公共的方法提取出来。

public class CallbackDemo2 { public static void main(String[] args) { Student xiaomiStudent = new Student("小米同学", 200000, 6000, 5000); xiaomiStudent.say(); Tercher laoshiTercher = new Tercher("大米老师", 400000, 8000, 8000); laoshiTercher.say(); } } class Student{ private String name; private int wages; private int five; private int jian; Student(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name) { this.name = name; } public void say() { System.out.println(this.name + ":需要交税：" + new Common().getTax(this.wages,this.five,this.jian)); } } class Common{ // 应纳税所得额 = 税前工资收入金额 - 五险一金（个人缴纳部分） -费用减除额 // 应纳税额 = 应纳税所得额 x 税率 - 速算扣除数 public int getTax(int wages,int five,int jian){ int cost; int level = wages<36000?1:wages<144000?2:wages<300000?3:wages<420000?4:wages<660000?5:wages<960000?6:7; switch(level){ case 1: cost= (wages-five-jian)*3/100; break; case 2: cost= (wages-five-jian)*10/100-2520; break; case 3: cost= (wages-five-jian)*20/100-16920; break; case 4: cost= (wages-five-jian)*25/100-31920; break; case 5: cost= (wages-five-jian)*30/100-52920; break; case 6: cost= (wages-five-jian)*35/100-85920; break; case 7: cost= (wages-five-jian)*45/100-181920; break; default: cost = 0; } return cost; } } class Tercher{ private String name; private int wages; private int five; private int jian; Tercher(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name) { this.name = name; } public void say() { System.out.println(this.name + ":需要交税：" + new Common().getTax(this.wages,this.five,this.jian)); } }

运行结果：

小米同学:需要交税：20880 大米老师:需要交税：64080

    这里我们调用了一个公共类替我们完成了税的计算，但是与回调显然是有区别的。想想这样的一个场景，现在公司要求统计每个人全年的交税金额，财务方面更希望是能得到银行给的数据，也就是说员工查询后，公共类将结果也返回了财务部门；使用回调功能需要将自身的一部分方法暴露出来；

public class CallbackDemo2 { public static void main(String[] args) { Student xiaomiStudent = new Student("小米同学", 200000, 6000, 5000); xiaomiStudent.say(); Tercher laoshiTercher = new Tercher("大米老师", 400000, 8000, 8000); laoshiTercher.say(); } } class Student implements People{ private String name; private int wages; private int five; private int jian; Student(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name){ this.name = name; } public int getTax(int wages, int five, int jian) { System.out.println(name + "询问银行交的税"); return new Common().getTax(wages, five, jian); } public void say() { new Bank().say(wages, five, jian,name,this); } } interface People{ public int getTax(int wages,int five,int jian); } class Bank{ public void say(int wages, int five, int jian,String name, People people){ System.out.println("银行说:"+name+"交税："+ people.getTax(wages, five, jian)); } } class Common{ // 应纳税所得额 = 税前工资收入金额 - 五险一金（个人缴纳部分） -费用减除额 // 应纳税额 = 应纳税所得额 x 税率 - 速算扣除数 public int getTax(int wages,int five,int jian){ int cost; int level = wages<36000?1:wages<144000?2:wages<300000?3:wages<420000?4:wages<660000?5:wages<960000?6:7; switch(level){ case 1: cost= (wages-five-jian)*3/100; break; case 2: cost= (wages-five-jian)*10/100-2520; break; case 3: cost= (wages-five-jian)*20/100-16920; break; case 4: cost= (wages-five-jian)*25/100-31920; break; case 5: cost= (wages-five-jian)*30/100-52920; break; case 6: cost= (wages-five-jian)*35/100-85920; break; case 7: cost= (wages-five-jian)*45/100-181920; break; default: cost = 0; } return cost; } } class Tercher implements People{ private String name; private int wages; private int five; private int jian; Tercher(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name){ this.name = name; } public int getTax(int wages, int five, int jian) { System.out.println(name + "询问银行交的税"); return new Common().getTax(wages, five, jian); } public void say() { new Bank().say(wages, five, jian,name,this); } }

运行结果：

小米同学询问银行交的税 银行说:小米同学交税：20880 大米老师询问银行交的税 银行说:大米老师交税：64080

这里的方法：

public int getTax(int wages, int five, int jian) { System.out.println(name + "询问银行交的税"); return new Common().getTax(wages, five, jian); }

      就是一个回调方法，显然代码看上去有点傻傻的，因为看上去多此一举，为什么要借助Bank类，这是因为我们这里使用的是同步回调，面对一些复杂的业务我们想得到这样的结果：

小米同学询问银行交的税 大米老师询问银行交的税 银行说:小米同学交税：20880 银行说:大米老师交税：64080 public class CallbackDemo2 { public static void main(String[] args) { Student xiaomiStudent = new Student("小米同学", 200000, 6000, 5000); Tercher laoshiTercher = new Tercher("大米老师", 400000, 8000, 8000); new Thread(xiaomiStudent).start(); new Thread(laoshiTercher).start(); } } class Student implements People,Runnable { private String name; private int wages; private int five; private int jian; Student(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name){ this.name = name; } public int getTax(int wages, int five, int jian) { System.out.println(name + "询问银行交的税"); return new Common().getTax(wages, five, jian); } public void say() { new Bank().say(wages, five, jian,name,this); } @Override public void run() { say(); } } interface People{ public int getTax(int wages,int five,int jian); } class Bank{ public void say(int wages, int five, int jian,String name, People people){ int shui = people.getTax(wages, five, jian); try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("银行说:"+name+"交税："+ shui); } } class Common{ // 应纳税所得额 = 税前工资收入金额 - 五险一金（个人缴纳部分） -费用减除额 // 应纳税额 = 应纳税所得额 x 税率 - 速算扣除数 public int getTax(int wages,int five,int jian){ int cost; int level = wages<36000?1:wages<144000?2:wages<300000?3:wages<420000?4:wages<660000?5:wages<960000?6:7; switch(level){ case 1: cost= (wages-five-jian)*3/100; break; case 2: cost= (wages-five-jian)*10/100-2520; break; case 3: cost= (wages-five-jian)*20/100-16920; break; case 4: cost= (wages-five-jian)*25/100-31920; break; case 5: cost= (wages-five-jian)*30/100-52920; break; case 6: cost= (wages-five-jian)*35/100-85920; break; case 7: cost= (wages-five-jian)*45/100-181920; break; default: cost = 0; } return cost; } } class Tercher implements People,Runnable{ private String name; private int wages; private int five; private int jian; Tercher(String name,int wages,int five,int jian){ this.name = name; this.wages = wages; this.five = five; this.jian = jian; } public int getWages() { return wages; } public void setWages(int wages) { this.wages = wages; } public int getFive() { return five; } public void setFive(int five) { this.five = five; } public int getJian() { return jian; } public void setJian(int jian) { this.jian = jian; } public String getName() { return name; } public void setName(String name){ this.name = name; } public int getTax(int wages, int five, int jian) { System.out.println(name + "询问银行交的税"); return new Common().getTax(wages, five, jian); } public void say() { new Bank().say(wages, five, jian,name,this); } @Override public void run() { say(); } }

执行结果：

小米同学询问银行交的税 大米老师询问银行交的税 银行说:大米老师交税：64080 银行说:小米同学交税：20880

这里采用了多线程，实际上本质是采用了阻塞，异步阻塞。小米同学在询问银行后，她的线程是在阻塞中的，直到银行处理完成后，她的线程才继续执行。关于异步与同步，阻塞与非阻塞的内容 会在另外的博文介绍。