equals方法,hashCode方法

时间：2021-10-16 16:08:42

需求1： 如果两个empolyee的 age,name,hireDate相等，则认为是同一个empolyee

import java.util.Date;

 public class Empolyee{
private int age;
private String name;
private Date hireDate;

public Empolyee(int age,String name,Date hireDate){
this.setAge(age);
this.setName(name);
this.setHireDate(hireDate);
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public String toString(){
return "age: " + age + ",name:" + name + ",hireDate:" + hireDate;
}

public Date getHireDate() {
return hireDate;
}

public void setHireDate(Date hireDate) {
this.hireDate = hireDate;
}
}

public class EmpolyeeTest {
    public static void main(String[] args) throws ParseException {

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Empolyee e1 = new Empolyee(26, "Lilei",sdf.parse("2012-04-22 09:13:37"));
Empolyee e2 = new Empolyee(30, "Mike", sdf.parse("2013-04-21 12:36:39"));
Empolyee e3 = new Empolyee(30, "Mike", sdf.parse("2013-04-21 12:36:39"));

System.out.println("does e1 equals e2 ? " + e1.equals(e2)); 
System.out.println("does e2 equals e3 ? " + e2.equals(e3));

    }
}


运行EmpolyeeTest.java

System.out.println("does e1 equals e2 ? " + e1.equals(e2)); //false
System.out.println("does e2 equals e3 ? " + e2.equals(e3)); //false

e2.equals(e3) 返回结果为false,与期望的true不一致。

解决方案：重载equals方法

/**
 * 一个完整的override equals方法共有4步,如果有父类，先调用super.equals()
 */
@Override
public boolean equals(Object otherObject){
    //1. 引用是否相同
    if(this == otherObject){
return true;
     };

     //2. 是否为null
     if(otherObject == null){
 return false;
     }
     // 3.是否属于同一个类
     if(this.getClass() !=otherObject.getClass() ){
 return false;
     }

     // 4.将otherObject 转换为相应的类类型变量
     Empolyee other = (Empolyee)otherObject;

    return name.equals(other.name) 
           && age == other.age
           && hireDate.equals(other.hireDate);

}

重新运行EmpolyeeTest.java

System.out.println("does e1 equals e2 ? " + e1.equals(e2)); //false
System.out.println("does e2 equals e3 ? " + e2.equals(e3)); //true

 需求2：将empolyee存储在集合HashSet中

在EmpolyeeTest.java添加下面代码

 

      System.out.println("e2 hashcode: " + e2.hashCode() + ",e3 hashcode: " + e3.hashCode());

   Set<Empolyee> empolyeeSet = new HashSet<Empolyee>();
   empolyeeSet.add(e1);
   empolyeeSet.add(e2);
   empolyeeSet.add(e3);

   for(Empolyee e:empolyeeSet ){
       System.out.println(e.toString());
   }

输出结果为： 

e2 hashcode: 1046495759,e3 hashcode: 563152583
age: 26,name:Lilei,hireDate:Sun Apr 22 09:13:37 CST 2012
age: 30,name:Mike,hireDate:Sun Apr 21 12:36:39 CST 2013
age: 30,name:Mike,hireDate:Sun Apr 21 12:36:39 CST 2013

问题：集合中不允许存在相同元素。按照最初的逻辑：如果两个empolyee的 age,name,hireDate相等，则认为是同一个empolyee，那么e2,e3只保存一个即可。

解决方案：覆盖hashCode方法。在《effective java》有条规则：覆盖equals方法是总要覆盖hashCode方法

在Emoplyee.java中添加

@Override
/**
 * 重载equals方法时，必须重载hashCode方法。因为Object.hashCode的约定：相等的对象，必须具有相同的hashCode。
 * hashCode()方法何时被调用？参考http://www.cnblogs.com/batys/archive/2011/10/25/2223942.html
 * Set中不允许存在相同的元素。当对象要存储到Set集合中时，调用对象的hashCode方法，确定该集合存储的位置。
 */
public int hashCode(){
    return name.hashCode() + hireDate.hashCode() + age;
}

输出结果： 

e2 hashcode: 721796890e3 hashcode: 721796890
age: 30,name:Mike,hireDate:Sun Apr 21 12:36:39 CST 2013
age: 26,name:Lilei,hireDate:Sun Apr 22 09:13:37 CST 2012

关于hashCode方法的详细解释，参考： http://www.ibm.com/developerworks/cn/java/j-jtp05273/


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