equals,hashCode

一、 equals()、hashCode()、 toString()

如果重新定义equals方法，就必须重新定义hashCode方法。

• Class getClass()返回包含对象信息的类对象
• boolean equals(Object otherObject)比较两个对象是否相等，如果两个对象指向同一块存储区域，方法返回true；否则方法返回false。在自定义类中，应该覆盖这个方法。
• String toString() 返回描述该对象的字符串。在自定义类中，应该覆盖这个方法。
• Class getSuperclass() 一Class对象的形式返回这个类的超类信息。

1. EqualsTest.java

package equals;
/**
 * This program demonstrates the equals method
 * @version 1.80 2017-11-03
 * @author Mr.Ding
 *
 */
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 == alice2: " + (alice1 == alice2));
        System.out.println("alice1 == alice3: " + (alice1 == 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());
    }
}

2. Employee.java
   java.util.Objects;
static boolean equals(Object a, Object b)
Objects.equals(this.name, other.name)
   如果a和b都为null，返回true；如果只有其中之一为null，则返回false；否则返回a.equals(b)。

package equals;

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

public class Employee {
    private String name = "";
    private double salary = 0.0;
    private Date hireDay;
    
    public Employee(String name, double salary, int year, int month, int day){
        this.name = name;
        this.salary = salary;
        GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
        hireDay = calendar.getTime();
    }
    
    public String getName(){
        return this.name;
    }

    public double getSalary() {
        return this.salary;
    }
    //better returns hireDay's clone
    public Date getHireDay() {
        return this.hireDay;
    }
    
    public void raiseSalary(int byPercent){
        double raise = this.salary * byPercent / 100;
        this.salary += raise;
    }
    
    public boolean equals(Object otherObject){
        //a quick test to see if the objects are identical
        if(this == otherObject){
            return true;
        }
        
        //must return false if the explicit parameter is null
        if(otherObject == null){
            return false;
        }
        
        //if the classes don't match,they can't be equal
        if(getClass() != otherObject){
            return false;
        }
        
        //now we know otherObject is a non-null Employee
        Employee other = (Employee) otherObject;
        
        //test whether the fields have identical values
        return Objects.equals(this.name, other.name) && salary == other.salary && Objects.equals(this.hireDay, other.hireDay);
    }
    
    public int hashCode(){
        return Objects.hash(this.name, this.salary, this.hireDay);
    }
    
    //better choice is getClass() instead of class_name
    public String toString(){
        return getClass().getName() + "[name= " + this.name + ", salary=" + this.salary + ", hireDay=" + this.hireDay + "]";
    }
}

3. Manager.java

public boolean equals(Object otherObject){
        //super.equals checked that this and other belong to the same class
        if(!super.equals(otherObject)){
            return false;
        }
        Manager other = (Manager) otherObject;
        return this.bonus == other.bonus;
    }

在子类中定义equals方法时，首先调用超类的equals。如果检测失败，对象就不可能相等。如果超类中的域都相等，就需要比较子类中的实力域。

package equals;

public class Manager extends Employee{
    private double bonus = 0.0;
    
    public Manager(String name, double salary, int year, int month, int day){
        super(name, salary, year, month, day);
        bonus = 0.0;
    }
    
    public double getSalary(){
        double baseSalary = super.getSalary();
        return baseSalary + this.bonus;
    }
    
    public void setBonus(double bonus){
        this.bonus = bonus;
    }
    
    public boolean equals(Object otherObject){
        //super.equals checked that this and other belong to the same class
        if(!super.equals(otherObject)){
            return false;
        }
        Manager other = (Manager) otherObject;
        return this.bonus == other.bonus;
    }
    
    public int hashCode(){
        return super.hashCode() + 17 * new Double(bonus).hashCode();
    }
    
    public String toString(){
        return super.toString() + "[bonus=" + bonus + "]";
    }
}

4. 运行结果

   
   

5. 散列码（hash code）是由对象导出的一个整型值。散列码是没有规律的。如果x和y是两个不同的对象，x.hashCode()与y.hashCode()基本上不会相同。字符串的散列码是由内容导出的。
     equals()比较的是两个对象的值是否相等；==表示的是两个对象的地址是否相等。==比较堆内存的值是否相等（对象地址存放在堆内存），equals()比较栈内存的值（对象的值存在于栈内存中）。但是String是个例外，String有个”常量池“。String a = "abc";String b = "abc";a==b是返回true的。String a = "abc";String b = new String("abc");a==b是返回false的，a和b不是一个对象，地址不相等。
   *****可以用javac编译得到class文件，然后javap -verbose class查看字节码，如果是常量就是一样，如果不是，是通过StringBuilder实现的“+”，就是新对象。

   
   
   
   

   字节码里显示是“//String abc” ，证明a,b是同一个对象，a==b返回的是true。