hashCode与equals的区别与联系(经典)
hashCode与equals的区别与联系(经典)
一、equals方法的作用1、默认情况（没有覆盖equals 方法）下equals方法都是调用Object类的equals方法，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（是不是同一个对象）。2 、要是类中覆盖了equals方法，那么就要根据具体的代码来确定equals方法的作用了，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：[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 = age; https://www.wendangku.net/doc/027332236.html, = name; } public int getAge()
{ return age; } public String getName() { return name; } public void setAge(int age) { this.age = age; } public void setName(String name) { https://www.wendangku.net/doc/027332236.html, = name; } } 测试代码如下：[java] view plaincopyimport java.util.HashSet; import
java.util.LinkedList; import java.util.Set; public class EqualsTest { public static void main(String[]
args) { LinkedList<Student> list = new LinkedList<Student>(); Set<Student> set = new HashSet<Student>(); Student
stu1 = new Student(3,"张三"); Student stu2 = new Student(3,"张三"); System.out.println("stu1 == stu2 : "+(stu1 == stu2));
System.out.println("stu1.equals(stu2) :
"+stu1.equals(stu2)); list.add(stu1);
list.add(stu2); System.out.println("list size:"+ list.size()); set.add(stu1);
set.add(stu2); System.out.println("set size:"+ set.size()); } } 运行结果：stu1 == stu2 : falsestu1.equals(stu2) : falselist size:2set size:2结果分析：Student类没有覆盖equals方法，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，所以stu1.equals(stu2) 是false。3、我们覆盖一下equals方法（age和name属性），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：[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 = age; https://www.wendangku.net/doc/027332236.html, = name; } public int getAge() { return age; }
public String getName() { return name; } public void setAge(int age) { this.age =
age; } public void setName(String name)
{ https://www.wendangku.net/doc/027332236.html, = name; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Student other = (Student) obj;
if (age != other.age) return false;
if (name == null) { if (https://www.wendangku.net/doc/027332236.html, != null) return false; } else if (!name.equals(https://www.wendangku.net/doc/027332236.html,)) return false; return true; } } 运行结果：stu1 == stu2 : false
stu1.equals(stu2) : true
list size:2
set size:2
结果分析：因为Student两个对象的age和name属性相等，而且又是通过覆盖equals方法来判断的，所示
stu1.equals(stu2) 为true。注意以上几次测试list和set的
size都是2二、HashCode4、通过以上的代码运行，我们知道equals方法已经生效。接下来我们在覆盖一下hashCode 方法（通过age和name属性来生成hashcode）并不覆盖equals方法，其中Hash码是通过age和name生成的。覆盖hashcode后的Student类：[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 = age; https://www.wendangku.net/doc/027332236.html, = name; } public int getAge() { return age; }
public String getName() { return name; } public void setAge(int age) { this.age =
age; } public void setName(String name)
{ https://www.wendangku.net/doc/027332236.html, = name; } @Override public int hashCode() { final int prime = 31;
int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 :
name.hashCode()); return result; } } 运行结果：stu1 == stu2 : false
stu1.equals(stu2) : false
list size:2
hashCode :775943
hashCode :775943
set size:2结果分析：我们并没有覆盖equals方法只覆盖了hashCode方法，两个对象虽然hashCode一样，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，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：[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 = age; https://www.wendangku.net/doc/027332236.html, = name; } public int getAge() { return age; } public String getName() { return name; }
public void setAge(int age) { this.age =
age; } public void setName(String name)
{ https://www.wendangku.net/doc/027332236.html, = name; } @Override public int hashCode() { final int prime = 31;
int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 :
name.hashCode());
System.out.println("hashCode : "+ result); return result; } @Override public boolean
equals(Object obj) { if (this == obj)
return true; if (obj == null) return false; if (getClass() != obj.getClass())
return false; Student other = (Student) obj;
if (age != other.age) return false;
if (name == null) { if (https://www.wendangku.net/doc/027332236.html, != null) return false; } else if (!name.equals(https://www.wendangku.net/doc/027332236.html,)) return false; return true; } }
运行结果：stu1 == stu2 : falsestu1.equals(stu2) :truelist size:2hashCode :775943hashCode :775943set size:1结果分析：stu1和stu2通过equals方法比较相等，而且返回的hashCode值一样，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，但hashCode值不相等试试。Student类的代码如下：[java] view plaincopy//学生类public class Student { private int age; private String name; <span
style="color:#ff0000;">private static int
index=5;</span> public Student() { } public Student(int age, String name) { super(); this.age = age; https://www.wendangku.net/doc/027332236.html, = name; } public int getAge() { return age; }
public String getName() { return name; }
public void setAge(int age) { this.age =
age; } public void setName(String name)
{ https://www.wendangku.net/doc/027332236.html, = name; } @Override public int hashCode() { final int prime = 31;
int result = 1; result = prime * result + <span style="color:#ff0000;">(age+index++)</span>; result = prime * result + ((name == null) ? 0 :
name.hashCode()); <span
style="color:#ff0000;">System.out.println("result :"+resu lt);</span> return result; }
@Override public boolean equals(Object obj)
{ if (this == obj) return true;
if (obj == null) return false; if (getClass() != obj.getClass()) return false; Student other = (Student) obj; if (age !=
other.age) return false; if (name == null) { if (https://www.wendangku.net/doc/027332236.html, != null)
return false; } else if (!name.equals(https://www.wendangku.net/doc/027332236.html,)) return false; return true; } } 运行结果：stu1 == stu2 : false
stu1.equals(stu2) : true
list size:2
hashCode :776098
hashCode :776129
set size:2结果分析：虽然stu1和stu2通过equals方法比较相等，但两个对象的hashcode的值并不相等，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：[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 = age;
https://www.wendangku.net/doc/027332236.html, = name; } public int getAge()
{ return age; } public String getName() { return name; } public void setAge(int age) { this.age = age; } public void setName(String name) { https://www.wendangku.net/doc/027332236.html, = name; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + age;
result = prime * result + ((name == null) ? 0 :
name.hashCode());
System.out.println("hashCode : "+ result); return result; } @Override public boolean
equals(Object obj) { if (this == obj)
return true; if (obj == null) return false; if (getClass() != obj.getClass())
return false; Student other = (Student) obj;
if (age != other.age) return false;
if (name == null) { if (https://www.wendangku.net/doc/027332236.html, != null) return false; } else if (!name.equals(https://www.wendangku.net/doc/027332236.html,)) return false; return true; } } 测试代码如下：[java] view plaincopyimport java.util.HashSet; import java.util.LinkedList; import java.util.Set;
public class EqualsTest { public static void
main(String[] args) { LinkedList<Student> list = new LinkedList<Student>();
Set<Student> set = new HashSet<Student>(); Student stu1 = new Student(3,"张三"); Student stu2 = new Student(3,"张三");
System.out.println("stu1 == stu2 : "+(stu1 == stu2)); System.out.println("stu1.equals(stu2) :
"+stu1.equals(stu2)); list.add(stu1);
list.add(stu2); System.out.println("list size:"+ list.size()); set.add(stu1);
set.add(stu2); System.out.println("set size:"+
set.size()); stu1.setAge(34);
System.out.println("remove stu1 : "+set.remove(stu1)); System.out.println("set size:"+ set.size()); } } 运行结果：stu1 == 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：当我们将某个对象存到set中时，如果该对象的属性参与了hashcode的计算，那么以后就不能修改该对象参与hashcode计算的那些属性了，否则会引起意向不到的错误的。正如测试中，不能够移除stu1对象。
总结：1、equals方法用于比较对象的内容是否相等（覆盖以后）2、hashcode方法只有在集合中用到3、当覆盖了equals方法时，比较对象是否相等将通过覆盖后的equals 方法进行比较（判断对象的内容是否相等）。4、将对象放入到集合中时，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，如果不相等直接将该对象放入集合中。如果hashcode值相等，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，如果equals判断不相等，直接将该元素放入到集合中，否则不放入。5、将元素放入集合的流程图：6、HashSet中add方法源代码：[java] view plaincopypublic boolean add(E e) { return map.put(e, PRESENT)==null; } map.put源代码:[java] view plaincopy<pre name="code" class="java"> public V put(K key, V value) { if (key == null) return putForNullKey(value); int hash = hash(key.hashCode()); int i = indexFor(hash, table.length); for
(Entry<K,V> e = table[i]; e != null; e = e.next)
{ Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value;
e.value = value; e.recordAccess(this); return oldValue; } }
modCount++; addEntry(hash, key, value, i); return null; }</pre> <pre></pre>
<pre></pre> 转自
https://www.wendangku.net/doc/027332236.html,/afgasdg/article/details/6889383。