先來簡單小結一下equals的用法:
1.首先如果只是基本數據類型,那麼判斷兩個變量是否相等用 == 即可
2.比較對象,有兩種情況:
一種就是使用java已經給我們提供好的類。如包裝器類型,如Boolean、Double、Integer,這些只要直接使用equals比較即可,因爲他們內部已經重寫了equals和hashCode方法,所以我們可以直接使用。
二種當然也是比較經常使用的自定義類型。如果要比較自定義的的兩個類的對象是否相等,就需要我們重寫equals和hashCode方法。如下:
@Override
public boolean equals(Object otherObject) {
if (this == otherObject) {
return true;
}
if (otherObject == null) {
return false;
}
if (getClass() != otherObject.getClass()) {
return false;
}
Employee other = (Employee)otherObject;
boolean flag = name.equals(other.getName()) && salary.equals(other.getSalary()) && hireDay.equals(other.getHireDay());
return flag;
}
@Override
public int hashCode() {
return name.hashCode() + hireDay.hashCode() + salary.hashCode();
}
這兩者是對應的,必須都要重寫。
==================================代碼示例======================
本代碼摘抄至‘Java核心技術’,因爲版本的問題,修改的些許代碼。以下直接貼出代碼。。
1.僱員類:
package com.lin.model;
import java.time.LocalDate;
/**
* 僱員類
*
* @author linrui
* @date 2019/03/27
* */
public class Employee {
private String name;
private Double salary;
private LocalDate hireDay;
public Employee(String name, Double salary, int year, int month, int day) {
this.name = name;
this.salary = salary;
this.hireDay = LocalDate.of(year, month, day);
}
public String getName() {
return name;
}
public Double getSalary() {
return salary;
}
public LocalDate getHireDay() {
return hireDay;
}
public void raiseSalary(Double byPercent) {
double raise = salary * byPercent/100;
salary += raise;
}
@Override
public String toString() {
return "Employee{" +
"name='" + name + '\'' +
", salary=" + salary +
", hireDay=" + hireDay +
'}';
}
@Override
public boolean equals(Object otherObject) {
if (this == otherObject) {
return true;
}
if (otherObject == null) {
return false;
}
if (getClass() != otherObject.getClass()) {
return false;
}
Employee other = (Employee)otherObject;
boolean flag = name.equals(other.getName()) && salary.equals(other.getSalary()) && hireDay.equals(other.getHireDay());
return flag;
}
@Override
public int hashCode() {
return name.hashCode() + hireDay.hashCode() + salary.hashCode();
}
}
2.經理類(僱員類的子類)
package com.lin.model;
import java.time.LocalDate;
/**
* 經理類
*
* @author linrui
* @date 2019/03/27
* */
public class Manager extends Employee{
private Double bonus;
public Manager(String name, Double salary, int year, int month, int day) {
super(name, salary, year, month, day);
this.bonus = new Double(0);
}
@Override
public Double getSalary() {
Double baseSalary = super.getSalary();
return baseSalary + bonus;
}
public void setBonus(Double bonus) {
this.bonus = bonus;
}
@Override
public boolean equals(Object otherObject) {
if (!super.equals(otherObject)) {
return false;
}
Manager other = (Manager)otherObject;
return bonus.equals(other.bonus);
}
@Override
public int hashCode() {
return super.hashCode() + 17 * bonus.hashCode();
}
@Override
public String toString() {
return super.toString() + "[bonus=" +bonus + "]";
}
}
3.測試代碼
package com.lin.test;
import com.lin.model.Employee;
import com.lin.model.Manager;
/**
* 測試類
*
* @author linrui
* @date 2019/03/27
* */
public class EqualsTest {
public static void main(String[] args) {
// Employee alice1 = new Employee("Alice Adams", new Double(7500), 1987, 12, 15);
// Employee alice2 = alice1;
// Employee alice3 = new Employee("Alice Adams", new Double(7500), 1987, 12, 15);
// Employee bob = new Employee("Bob Brandson", new Double(5000), 1989, 10, 1);
//
// System.out.println("alice1 == alice2:" + (alice1 == alice2));
//
// System.out.println("alice1 == alice3:" + (alice1 == alice3));
//
// System.out.println("alice1.equals(alice3)):" + alice1.equals(alice3));
//
// System.out.println("alice1.equals(bob)):" + alice1.equals(bob));
//
// System.out.println("bob.toString():" + bob);
//
Manager carl = new Manager("Carl Cracker", new Double(8000), 1987, 12, 15);
Manager boss = new Manager("Carl Cracker", new Double(8000), 1987, 12, 15);
boss.setBonus(new Double(5000));
carl.setBonus(new Double(5000));
// System.out.println("boss.toString()" + boss);
System.out.println("carl.equals(boss):" + carl.equals(boss));
// System.out.println("alice1.hashCode():" + alice1.hashCode());
// System.out.println("alice3.hashCode():" + alice3.hashCode());
// System.out.println("bob.hashCode():" + bob.hashCode());
// System.out.println("carl.hashCode():" + carl.hashCode());
}
}
4.使用小結:
直接看上面的測試類,註釋掉的地方都是很好理解的,就不在累贅。
因爲Manager中的equals方法調用了父類Employee的equals方法
所以,直接將斷點打到Employee的equals()方法中,然後debug跑起來,
雖然otherObject被強制轉換成了Employee,但是other並不是一個對象,而是指向otherObject對象的一個引用,而實際上otherObject是一個Manager對象。
所以,在調用otherObject.getSalary()方法的時候,實際上執行的是Manager中的方法
所以最終的輸出結果是flase。
看明白的話自然還是好理解的,但是一開始我看的時候發現怎麼想都不太對,於是就去跟蹤源碼,發現了是這裏的問題。其實要是想簡單明白,就直接將Manager中的equals方法重寫爲以下這個就好理解了。
@Override
public boolean equals(Object otherObject) {
if (this == otherObject) {
return true;
}
if (otherObject == null) {
return false;
}
if (getClass() != otherObject.getClass()) {
return false;
}
Manager other = (Manager)otherObject;
return bonus.equals(other.bonus);
}