java核心技术 equals方法 EqualTest.java

最新推荐文章于 2024-12-07 09:00:00 发布

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最新推荐文章于 2024-12-07 09:00:00 发布

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本文通过一个Java示例程序展示了如何比较自定义类的对象，并解释了equals方法和hashCode方法的实现细节及其重要性。

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public class EqualTest {

    public static void main(String[] args) {

        Employee alice1 = new Employee("Alice Adams", 75000, 1989, 12, 15);
        Employee alice2 = alice1;
        Employee alice3 = new Employee("Alice Adams", 75000, 1989, 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 car1 = 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): "+car1.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("car1.hashCode(): "+car1.hashCode());

    }

}

import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Objects;

public class Employee {

    private String name;
    private double salary;
    private Date hireDay;

    public Employee(String n,double s,int year,int month,int day){
        name = n;
        salary = s;
        GregorianCalendar calendar = new GregorianCalendar(year,month-1,day);
        hireDay = calendar.getTime();
    }

    public String getName(){
        return name;
    }
    public double getSalary(){
        return salary;
    }
    public Date getHireDay(){
        return hireDay;
    }

    public void raiseSalary(double byPercent){
        double raise = salary * byPercent / 100;
        salary += raise;
    }
    public boolean equals(Object otherObject){
    //这里获得一个对象参数，第一个if语句判断两个引用是否是同一个，如果是那么这两个对象肯定相等
        if(this == otherObject)
            return true;
//这里判断这个参数是否引用空值
        if(otherObject == null)
            return false;
//getClass()方法是得到对象的类，这里就是如果两个对象的类不一样，那么就不相等
        if(getClass()!=otherObject.getClass())
            return false;
//在以上判断完成，再将得到的参数对象强制转换为该对象，考虑到父类引用子类的对象的出现，然后再判断对象的属性是否相同      
        Employee other = (Employee)otherObject;

        return Objects.equals(name, other.name)&&salary ==other.salary&&
                Objects.equals(hireDay, other.hireDay);
    }
    //哈希散列，有自己的计算方法，根据字符串来得到一段数字
    public int hashCode(){
        return Objects.hash(name,salary,hireDay);
    }

//toString()方法，可自动生成
    @Override
    public String toString() {
        return getClass().getName()+ "[name=" + name + ", salary=" + salary + ", hireDay=" + hireDay + "]";
    }


}


public class Manager extends Employee {

    private double bonus;

    public Manager(String n,double s, int year,int month,int day){
        super(n, s, year, month, day);
        bonus = 0;
    }

    public double getSalary(){
        double baseSalary = super.getSalary();
        return baseSalary + bonus;
    }

    public void setBonus(double b){
        bonus = b;
    }

    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+"]";
    }
}

在Manager类中，继承了Employee的方法

运行结果
alice1 == alice2: true
alice1 == alice3: false
alice1.equals(alice3): true
alice1.equals(bob): false
bob.toString(): code58.Employee[name=Bob Brandson, salary=50000.0, hireDay=Sun Oct 01 00:00:00 CST 1989]
boss.toString(): code58.Manager[name=Carl Cracker, salary=80000.0, hireDay=Tue Dec 15 00:00:00 CST 1987][bonus=]5000.0]
carl.equals(boss): false
alice1.hashCode(): 1776251221
alice3.hashCode(): 1776251221
bob.hashCode(): -223951770
car1.hashCode(): -785515519