java 中的equals hashCode

简介

hashCode 与 equals 应该有等同性，如果A equals B 那么 A 和 B 的hashCode 应该一样
还有toString方法可以在后期调试的过程中起到很好的作用

code

EXAMPLE

package com;

public class EqualsTest {
	public static void main(String[] args){
		Employee alice1 = new Employee("Alice Adams", 75000, 1987, 12, 15);
		Employee alice2 = alice1;
		Employee alice3 = new Employee("Alice Adams", 75000, 1987, 12, 15);
		Employee bob = new Employee("Bob Brandson", 50000, 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", 80000, 1987, 12, 15);
		Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
		boss.setBonus(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());
	}
}

package com;

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);
		bonus = 0;
	}
	
	public double getSalary() {
		double baseSalary = super.getSalary();
		return baseSalary + bonus;
	}
	
	public void setBonus(double bonus) {
		this.bonus = bonus;
	}
	
	public boolean equals(Object otherObject){
		if (!super.equals(otherObject)) return false;
		Manager other = (Manager) otherObject;
		return bonus == other.bonus;
	}
	
	public int hashCode(){
		return super.hashCode() + 17 * new Double(bonus).hashCode();
	}
	
	public String toString() {
		return super.toString() + "[bonus=" + bonus + "]";
	}
}

package com;

import java.time.*;
import java.util.Objects;
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;
		hireDay = LocalDate.of(year, month, day);
	}
	
	public String getName() {
		return name;
	}
	
	public double getSalary() {
		return salary;
	}
	
	public LocalDate getHireDay() {
		return hireDay;
	}
	
	public String getDescription() {
		return String.format("an emplyee with a salary of $%.2f", salary);
	}
	
	public void raiseSalary(double byPercent) {
		double raise = salary * byPercent / 100;
		salary += raise;
	}
	
	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;
		return Objects.equals(name, other.name) && salary == other.salary
				&& Objects.equals(hireDay,  other.hireDay);
	}
	
	public int hashCode(){
		return Objects.hash(name, salary, hireDay);
	}
	
	public String toString() {
		return getClass().getName() + "[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]";
	}
}

Answer

alice1 == alice2: true
alice1 == alice3: false
alice1.equals(alice3): true
alice1.equals(bob): false
bob.toString(): com.Employee[name=Bob Brandson,salary=50000.0,hireDay=1989-10-01]
boss.toString(): com.Manager[name=Carl Cracker,salary=80000.0,hireDay=1987-12-15][bonus=5000.0]
carl.equals(boss): false
alice1.hashCode(): -808853550
alice3.hashCode(): -808853550
bob.hashCode(): -624019882
carl.hashCode(): -341762419