如何正确的重写equals() 和 hashCode()方法

比较两个Java对象时, 我们需要覆盖equals和 hashCode。

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public class User{

private String name;

private int age;

private String passport;

//getters and setters, constructor

}

在比较结果时：

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User user1 = new User("mkyong", 35, "111222333");

User user2 = new User("mkyong", 35, "111222333");

System.out.println(user1.equals(user2)); // false

下面我们将介绍几种常用方法：

1.经典方式

这种17和31散列码的想法来自经典的Java书籍——《Effective Java》第九条。下面我们来看看是如何实现的...

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public class User {

private String name;

private int age;

private String passport;

//getters and setters, constructor

@Override

public boolean equals(Object o) {

if (o == this) return true;

if (!(o instanceof User)) {

return false;

}

User user = (User) o;

return user.name.equals(name) &&

user.age == age &&

user.passport.equals(passport);

}

//Idea from effective Java : Item 9

@Override

public int hashCode() {

int result = 17;

result = 31 * result + name.hashCode();

result = 31 * result + age;

result = 31 * result + passport.hashCode();

return result;

}

}

2.JDK 7

对于JDK7及更新版本，你可以是使用

java.util.Objects

来重写 equals 和 hashCode 方法，代码如下

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import java.util.Objects;

public class User {

private String name;

private int age;

private String passport;

//getters and setters, constructor

@Override

public boolean equals(Object o) {

if (o == this) return true;

if (!(o instanceof User)) {

return false;

}

User user = (User) o;

return age == user.age &&

Objects.equals(name, user.name) &&

Objects.equals(passport, user.passport);

}

@Override

public int hashCode() {

return Objects.hash(name, age, passport);

}

}

3.Apache Commons Lang

或者,您可以使用Apache Commons Lang

EqualsBuilder

和

HashCodeBuilder

方法。代码如下

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import org.apache.commons.lang3.builder;

public class User {

private String name;

private int age;

private String passport;

//getters and setters, constructor

@Override

public boolean equals(Object o) {

if (o == this) return true;

if (!(o instanceof User)) {

return false;

}

User user = (User) o;

return new EqualsBuilder()

.append(age, user.age)

.append(name, user.name)

.append(passport, user.passport)

.isEquals();

}

@Override

public int hashCode() {

return new HashCodeBuilder(17, 37)

.append(name)

.append(age)

.append(passport)

.toHashCode();

}

}

最后测试总结：

在使用上述三种任何一种方式都可以到如下结果：

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User user1 = new User("mkyong", 35, "111222333");

User user2 = new User("mkyong", 35, "111222333");

System.out.println(user1.equals(user2)); // true</span>

其实后两种都是对于17和31散列码思想的封装实现。具体请参考《Effective Java》第九条。