equals,hashcode

一、equals方法的作用

1、默认情况&＃xff08;没有覆盖equals方法&＃xff09;下equals方法都是调用Object类的equals方法&＃xff0c;而Object的equals方法主要用于判断对象的内存地址引用是不是同一个地址&＃xff08;是不是同一个对象&＃xff09;。



2 、要是类中覆盖了equals方法&＃xff0c;那么就要根据具体的代码来确定equals方法的作用了&＃xff0c;覆盖后一般都是通过对象的内容是否相等来判断对象是否相等。



没有覆盖equals方法代码如下&＃xff1a;





[java] view plaincopy

//学生类

public class Student {

private int age;

private String name;



public Student() {

}

public Student(int age, String name) {

super();

this.age &＃61; age;

this.name &＃61; name;

}

public int getAge() {

return age;

}

public String getName() {

return name;

}

public void setAge(int age) {

this.age &＃61; age;

}

public void setName(String name) {

this.name &＃61; name;

}

}

测试 代码如下&＃xff1a;



[java] view plaincopy

import java.util.HashSet;

import java.util.LinkedList;

import java.util.Set;





public class EqualsTest {

public static void main(String[] args) {

LinkedList list &＃61; new LinkedList();

Set set &＃61; new HashSet();

Student stu1 &＃61; new Student(3,"张三");

Student stu2 &＃61; new Student(3,"张三");

System.out.println("stu1 &＃61;&＃61; stu2 : "&＃43;(stu1 &＃61;&＃61; stu2));

System.out.println("stu1.equals(stu2) : "&＃43;stu1.equals(stu2));

list.add(stu1);

list.add(stu2);

System.out.println("list size:"&＃43; list.size());



set.add(stu1);

set.add(stu2);

System.out.println("set size:"&＃43; set.size());

}



}





运行结果&＃xff1a;



stu1 &＃61;&＃61; stu2 : false

stu1.equals(stu2) : false

list size:2

set size:2

结果分析&＃xff1a;Student类没有覆盖equals方法&＃xff0c;stu1调用equals方法实际上调用的是Object的equals方法。所以采用对象内存地址是否相等来判断对象是否相等。因为是两个新对象所以对象的内存地址不相等&＃xff0c;所以stu1.equals(stu2) 是false。



3、我们覆盖一下equals方法&＃xff08;age和name属性&＃xff09;&＃xff0c;让Student类其通过判断对象的内容是否相等来确定对象是否相等。



覆盖后的Student类&＃xff1a;





[java] view plaincopy

//学生类

public class Student {

private int age;

private String name;



public Student() {

}

public Student(int age, String name) {

super();

this.age &＃61; age;

this.name &＃61; name;

}

public int getAge() {

return age;

}

public String getName() {

return name;

}

public void setAge(int age) {

this.age &＃61; age;

}

public void setName(String name) {

this.name &＃61; name;

}

&＃64;Override

public boolean equals(Object obj) {

if (this &＃61;&＃61; obj)

return true;

if (obj &＃61;&＃61; null)

return false;

if (getClass() !&＃61; obj.getClass())

return false;

Student other &＃61; (Student) obj;

if (age !&＃61; other.age)

return false;

if (name &＃61;&＃61; null) {

if (other.name !&＃61; null)

return false;

} else if (!name.equals(other.name))

return false;

return true;

}



}



运行结果&＃xff1a;

stu1 &＃61;&＃61; stu2 : false

stu1.equals(stu2) : true

list size:2

set size:2



结果分析&＃xff1a;因为Student两个对象的age和name属性相等&＃xff0c;而且又是通过覆盖equals方法来判断的&＃xff0c;所示stu1.equals(stu2) 为true。注意以上几次测试list和set的size都是2



二、HashCode



4、通过以上的代码运行&＃xff0c;我们知道equals方法已经生效。接下来我们在覆盖一下hashCode方法&＃xff08;通过age和name属性来生成hashcode&＃xff09;并不覆盖equals方法&＃xff0c;其中Hash码是通过age和name生成的。



覆盖hashcode后的Student类&＃xff1a;





[java] view plaincopy

//学生类

public class Student {

private int age;

private String name;



public Student() {

}

public Student(int age, String name) {

super();

this.age &＃61; age;

this.name &＃61; name;

}

public int getAge() {

return age;

}

public String getName() {

return name;

}

public void setAge(int age) {

this.age &＃61; age;

}

public void setName(String name) {

this.name &＃61; name;

}

&＃64;Override

public int hashCode() {

final int prime &＃61; 31;

int result &＃61; 1;

result &＃61; prime * result &＃43; age;

result &＃61; prime * result &＃43; ((name &＃61;&＃61; null) ? 0 : name.hashCode());

return result;

}

}



运行结果&＃xff1a;

stu1 &＃61;&＃61; stu2 : false

stu1.equals(stu2) : false

list size:2

hashCode :775943

hashCode :775943

set size:2



结果分析&＃xff1a;我们并没有覆盖equals方法只覆盖了hashCode方法&＃xff0c;两个对象虽然hashCode一样&＃xff0c;但在将stu1和stu2放入set集合时由于equals方法比较的两个对象是false&＃xff0c;所以就没有在比较两个对象的hashcode值。



5、我们覆盖一下equals方法和hashCode方法。



Student代码如下&＃xff1a;





[java] view plaincopy

//学生类

public class Student {

private int age;

private String name;

public Student() {

}

public Student(int age, String name) {

super();

this.age &＃61; age;

this.name &＃61; name;

}

public int getAge() {

return age;

}

public String getName() {

return name;

}

public void setAge(int age) {

this.age &＃61; age;

}

public void setName(String name) {

this.name &＃61; name;

}

&＃64;Override

public int hashCode() {

final int prime &＃61; 31;

int result &＃61; 1;

result &＃61; prime * result &＃43; age;

result &＃61; prime * result &＃43; ((name &＃61;&＃61; null) ? 0 : name.hashCode());

System.out.println("hashCode : "&＃43; result);

return result;

}

&＃64;Override

public boolean equals(Object obj) {

if (this &＃61;&＃61; obj)

return true;

if (obj &＃61;&＃61; null)

return false;

if (getClass() !&＃61; obj.getClass())

return false;

Student other &＃61; (Student) obj;

if (age !&＃61; other.age)

return false;

if (name &＃61;&＃61; null) {

if (other.name !&＃61; null)

return false;

} else if (!name.equals(other.name))

return false;

return true;

}



}





运行结果&＃xff1a;



stu1 &＃61;&＃61; stu2 : false



stu1.equals(stu2) :true



list size:2



hashCode :775943



hashCode :775943



set size:1



结果分析&＃xff1a;stu1和stu2通过equals方法比较相等&＃xff0c;而且返回的hashCode值一样&＃xff0c;所以放入set集合中时只放入了一个对象。



6、下面我们让两个对象equals方法比较相等&＃xff0c;但hashCode值不相等试试。



Student类的代码如下&＃xff1a;





[java] view plaincopy

//学生类

public class Student {

private int age;

private String name;

private static int index&＃61;5;

public Student() {

}

public Student(int age, String name) {

super();

this.age &＃61; age;

this.name &＃61; name;

}

public int getAge() {

return age;

}

public String getName() {

return name;

}

public void setAge(int age) {

this.age &＃61; age;

}

public void setName(String name) {

this.name &＃61; name;

}

&＃64;Override

public int hashCode() {

final int prime &＃61; 31;

int result &＃61; 1;

result &＃61; prime * result &＃43; (age&＃43;index&＃43;&＃43;);

result &＃61; prime * result &＃43; ((name &＃61;&＃61; null) ? 0 : name.hashCode());

System.out.println("result :"&＃43;result);

return result;

}

&＃64;Override

public boolean equals(Object obj) {

if (this &＃61;&＃61; obj)

return true;

if (obj &＃61;&＃61; null)

return false;

if (getClass() !&＃61; obj.getClass())

return false;

Student other &＃61; (Student) obj;

if (age !&＃61; other.age)

return false;

if (name &＃61;&＃61; null) {

if (other.name !&＃61; null)

return false;

} else if (!name.equals(other.name))

return false;

return true;

}



}

运行结果&＃xff1a;

stu1 &＃61;&＃61; stu2 : false

stu1.equals(stu2) : true

list size:2

hashCode :776098

hashCode :776129

set size:2



结果分析&＃xff1a;虽然stu1和stu2通过equals方法比较相等&＃xff0c;但两个对象的hashcode的值并不相等&＃xff0c;所以在将stu1和stu2放入set集合中时认为是两个不同的对象。



7、修改stu1的某个属性值



Student代码如下&＃xff1a;





[java] view plaincopy

//学生类

public class Student {

private int age;

private String name;

public Student() {

}

public Student(int age, String name) {

super();

this.age &＃61; age;

this.name &＃61; name;

}

public int getAge() {

return age;

}

public String getName() {

return name;

}

public void setAge(int age) {

this.age &＃61; age;

}

public void setName(String name) {

this.name &＃61; name;

}

&＃64;Override

public int hashCode() {

final int prime &＃61; 31;

int result &＃61; 1;

result &＃61; prime * result &＃43; age;

result &＃61; prime * result &＃43; ((name &＃61;&＃61; null) ? 0 : name.hashCode());

System.out.println("hashCode : "&＃43; result);

return result;

}

&＃64;Override

public boolean equals(Object obj) {

if (this &＃61;&＃61; obj)

return true;

if (obj &＃61;&＃61; null)

return false;

if (getClass() !&＃61; obj.getClass())

return false;

Student other &＃61; (Student) obj;

if (age !&＃61; other.age)

return false;

if (name &＃61;&＃61; null) {

if (other.name !&＃61; null)

return false;

} else if (!name.equals(other.name))

return false;

return true;

}



}



测试代码如下&＃xff1a;



[java] view plaincopy

import java.util.HashSet;

import java.util.LinkedList;

import java.util.Set;





public class EqualsTest {

public static void main(String[] args) {

LinkedList list &＃61; new LinkedList();

Set set &＃61; new HashSet();

Student stu1 &＃61; new Student(3,"张三");

Student stu2 &＃61; new Student(3,"张三");

System.out.println("stu1 &＃61;&＃61; stu2 : "&＃43;(stu1 &＃61;&＃61; stu2));

System.out.println("stu1.equals(stu2) : "&＃43;stu1.equals(stu2));

list.add(stu1);

list.add(stu2);

System.out.println("list size:"&＃43; list.size());



set.add(stu1);

set.add(stu2);

System.out.println("set size:"&＃43; set.size());

stu1.setAge(34);

System.out.println("remove stu1 : "&＃43;set.remove(stu1));

System.out.println("set size:"&＃43; set.size());

}



}



运行结果&＃xff1a;

stu1 &＃61;&＃61; stu2 : false

stu1.equals(stu2) : true

list size:2

hashCode : 775943

hashCode : 775943

set size:1

hashCode : 776904

remove stu1 : false

set size:1



结果分析&＃xff1a;



当我们将某个对象存到set中时&＃xff0c;如果该对象的属性参与了hashcode的计算&＃xff0c;那么以后就不能修改该对象参与hashcode计算的那些属性了&＃xff0c;否则会引起意向不到的错误的。正如测试中&＃xff0c;不能够移除stu1对象。

总结&＃xff1a;



1、equals方法用于比较对象的内容是否相等&＃xff08;覆盖以后&＃xff09;



2、hashcode方法只有在集合中用到



3、当覆盖了equals方法时&＃xff0c;比较对象是否相等将通过覆盖后的equals方法进行比较&＃xff08;判断对象的内容是否相等&＃xff09;。



4、将对象放入到集合中时&＃xff0c;首先判断要放入对象的hashcode值与集合中的任意一个元素的hashcode值是否相等&＃xff0c;如果不相等直接将该对象放入集合中。如果hashcode值相等&＃xff0c;然后再通过equals方法判断要放入对象与集合中的任意一个对象是否相等&＃xff0c;如果equals判断不相等&＃xff0c;直接将该元素放入到集合中&＃xff0c;否则不放入。



5、将元素放入集合的流程图&＃xff1a;





6、HashSet中add方法源代码&＃xff1a;





[java] view plaincopy

public boolean add(E e) {

return map.put(e, PRESENT)&＃61;&＃61;null;

}

map.put源代码:

[java] view plaincopy

 public V put(K key, V value) { 

 if (key &＃61;&＃61; null) 

 return putForNullKey(value); 

 int hash &＃61; hash(key.hashCode()); 

 int i &＃61; indexFor(hash, table.length); 

 for (Entry e &＃61; table[i]; e !&＃61; null; e &＃61; e.next) { 

 Object k; 

 if (e.hash &＃61;&＃61; hash && ((k &＃61; e.key) &＃61;&＃61; key || key.equals(k))) { 

 V oldValue &＃61; e.value; 

 e.value &＃61; value; 

 e.recordAccess(this); 

 return oldValue; 

 } 

 } 



 modCount&＃43;&＃43;; 

 addEntry(hash, key, value, i); 

 return null; 

 }