equals方法,hashCode方法

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)); //falseSystem.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() */@Overridepublic 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)); //falseSystem.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: 563152583age: 26,name:Lilei,hireDate:Sun Apr 22 09:13:37 CST 2012age: 30,name:Mike,hireDate:Sun Apr 21 12:36:39 CST 2013age: 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: 721796890age: 30,name:Mike,hireDate:Sun Apr 21 12:36:39 CST 2013age: 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/