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此文档为java的Object类学习笔记

一、介绍Object类的几个方法

Object类是java所有类的始祖，在java中每个类都是由它扩展而来。可以用Object类型变量引用任何类型的对象

Object obj = new Employee("bob", 40000);
Employee e = (Employee)obj;

toString方法

toString返回表示对象值的字符串

hash与hashCode方法

Objects.hash(Object… objs)产生并返回一个散列码。Objects.hash(name, age, hireDay)根据参数产生一个散列码

Object.hashCode()查看返回对象已经存在的散列码，每个对象都有一个默认的散列码，其值为对象的存储地址

equals方法

如果重新定义equals方法，就必须重新定义hashCode方法(e.g.String类重新定义hashCode方法，使得相同内容的字符串都有相同的hashCode)，以便用户可以将对象插入到散列表中

equals与hashCode的定义必须一致：如果x.equals(y)返回true，那么x.hashCode()就必须与y.hashCode()具有相同的值；再比如，如果用定义的Employee.equals方法比较雇员的ID，那么hashCode方法就只需要散列ID，而不是雇员的姓名或存储地址

二、示例代码

import java.util.*;

public class Test
{
    public static void main(String args[])
    {
        Employee e1 = new Employee("Alice", 75000, 1987, 12, 15);        
        Employee e2 = e1;
        Employee e3 = new Employee("Alice", 75000, 1987, 12, 15);

        Employee b1 = new Employee("Bob", 50000, 1989, 10, 1);

        System.out.println("e1 == e2: " + (e1==e2));
        System.out.println("e1 == e3: " + (e1==e3));
        System.out.println("e1.equals(e2): " + e1.equals(e2));
        System.out.println("e1.equals(e3): " + e1.equals(e3));
        System.out.println("e1.hashCode(): " + e1.hashCode());
        System.out.println("e3.hashCode(): " + e3.hashCode());
        System.out.println("e1.equals(b1): " + e1.equals(b1));
        System.out.println("b1.toString(): " + b1.toString());
        System.out.println("b1.hashCode(): " + b1.hashCode());

        Manager carl = new Manager("carl", 80000, 1987, 12, 15);
        Manager boss = new Manager("carl", 80000, 1987, 12, 15);
        boss.setBonus(5000);
//        carl.setBonus(5000);

        System.out.println("boss.toString(): " + boss.toString());
        System.out.println("carl.equals(boss): " + carl.equals(boss));
        System.out.println("carl.hashCode(): " + carl.hashCode());
        System.out.println("boss.hashCode(): " + boss.hashCode());

    }
}

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 cal = new GregorianCalendar(year, month-1, day);
        hireDay = cal.getTime();
    }

    public String getName()
    {
        return name;
    }

    public double getSalary()
    {
        return salary;
    }

    public Date getHireDay()
    {
        return hireDay;
    }

    public void raiseSalary(double percent)
    {
        salary *= (1 + percent/100);
    }

    public boolean equals(Object obj)
    {
        if(this == obj)
            return true;
        if(this == null)
            return false;
        if(this.getClass() != obj.getClass())
            return false;

        Employee tmp = (Employee)obj;
        return Objects.equals(name, tmp.name) && this.salary==tmp.salary && hireDay.equals(tmp.hireDay);
    }

    public int hashCode()
    {
        return Objects.hash(name, salary, hireDay);
    }

    public String toString()
    {
        return getClass().getName()+"[name="+name+",salary="+salary+",hireDay="+hireDay+"]";
    }
}

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

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

    public boolean equals(Object obj)
    {
        if(!super.equals(obj))
            return false;
        Manager m = (Manager)obj;
        return bonus == m.bonus;
    }

    public int hashCode()
    {
        return super.hashCode() + 17*new Double(bonus).hashCode();
    }

    public String toString()
    {
        return super.toString() + "[bonus=" + bonus + "]";
    }
}

output:

e1 == e2: true
e1 == e3: false
e1.equals(e2): true
e1.equals(e3): true
e1.hashCode(): -270951486
e3.hashCode(): -270951486
e1.equals(b1): false
b1.toString(): test.Employee[name=Bob,salary=50000.0,hireDay=Sun Oct 01 00:00:00 CST 1989]
b1.hashCode(): -133040315
boss.toString(): test.Manager[name=carl,salary=80000.0,hireDay=Tue Dec 15 00:00:00 CST 1987][bonus=5000.0]
carl.equals(boss): false
carl.hashCode(): 1908734458
boss.hashCode(): -1112473094

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