方法在继承过程中可能出现的问题

首先构造两个类,Employee类和Manager,前者是父类,后者是子类

public class Employee implements Comparable<Employee> {
    private String name;
    private int age;
    private double salary;
    @Override
    public int compareTo(Employee o) {
        return Double.compare(this.salary,o.salary);
    }
}

public class Manager extends Employee {
    private double bonus;

    public double getBonus() {
        return bonus;
    }

    public void setBonus(double bonus) {
        this.bonus = bonus;
    }

}

观察上面看到Employee类实现了Comparable 接口,里面的compareTo(x,y) 方法,这个方法是比较两个数的大小,如果x<y,则返回-1,如果x=y,则返回0,如果x>y,则返回一个正数.

上面的代码看似没有问题,但是在继承的过程中有可能会出现问题(这里Manager类是故意没写实现compareTo方法的,忽略这个问题).这是因为Manager扩展了Employee,而Employee实现的是Comparable,而不是Comparable.如果Manager覆盖了CompareTo,就必须要有经理与雇员进行比较的思想准备,决不能仅仅将雇员转换为Manager.

public class Manager extends Employee {
    private double bonus;

    public double getBonus() {
        return bonus;
    }

    public void setBonus(double bonus) {
        this.bonus = bonus;
    }

    @Override
    public int compareTo(Employee other) {
        Manager otherManger = (Manager) other;   //这样绝对不行
        ...
    }
}

这不符合"反对称"的规则.如果x是一个Employee对象,y是一个Employee对象,调用x.compareTo(y)不会抛出异常,它只是将x和y都作为雇员进行比较,因为每一个经理也都是雇员,这是可以进行比较的,但是反过来,y.compareTo(x)将会抛出一个异常ClassCastException.这是因为不是所有的雇员都是经理,不能把所有员工当成经理进行比较.

 @Test
    public void testCompareTo(){
        Employee employee = new Employee("张三", 23, 6598.12);
        Manager manager = new Manager("李四",24,5648.32,1.2);
        int i = employee.compareTo(manager);
        System.out.println(i);  //这样是可以运行的,结果是1,因为是把manager当成employee比较
    }

@Test
    public void testCompareTo(){
        Employee employee = new Employee("张三", 23, 6598.12);
        Manager manager = new Manager("李四",24,5648.32,1.2);
        int i = manager.compareTo(employee);
        System.out.println(i);  //这样是不能运行的,因为Employee不能当成Manager处理  java.lang.ClassCastException: domain.Employee cannot be cast to domain.Manager
    }

解决方法:可以在Manager中重写方法,直接调用父类的方法,进行比较

 @Override
    public int compareTo(Employee other) {
        return super.compareTo(other);
}