hashCode与equals的区别与联系 / 四六文摘

引用。。。。http://blog.csdn.net/afgasdg/article/details/6889383#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：12345678910111213141516171819202122232425//学生类public class Student {private int age;private String name;public Student() {}public Student(int age, String name) {super();this.age = age;this.name = name;}public int getAge() {return age;}public String getName() {return name;}public void setAge(int age) {this.age = age;}public void setName(String name) {this.name = name;}}测试代码如下：1234567891011121314151617181920212223import 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：1234567891011121314151617181920212223242526272829303132333435363738394041424344//学生类public class Student {private int age;private String name;public Student() {}public Student(int age, String name) {super();this.age = age;this.name = name;}public int getAge() {return age;}public String getName() {return name;}public void setAge(int age) {this.age = age;}public void setName(String name) {this.name = name;}@Overridepublic 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 (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}}运行结果：stu1 == stu2 : falsestu1.equals(stu2) : truelist size:2set 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：123456789101112131415161718192021222324252627282930313233//学生类public class Student {private int age;private String name;public Student() {}public Student(int age, String name) {super();this.age = age;this.name = name;}public int getAge() {return age;}public String getName() {return name;}public void setAge(int age) {this.age = age;}public void setName(String name) {this.name = name;}@Overridepublic 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 : falsestu1.equals(stu2) : falselist size:2hashCode :775943hashCode :775943set 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：12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152//学生类public class Student {private int age;private String name;public Student() {}public Student(int age, String name) {super();this.age = age;this.name = name;}public int getAge() {return age;}public String getName() {return name;}public void setAge(int age) {this.age = age;}public void setName(String name) {this.name = name;}@Overridepublic 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;}@Overridepublic 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 (other.name != null)return false;} else if (!name.equals(other.name))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：1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253//学生类public class Student {private int age;private String name;private static int index=5;public Student() {}public Student(int age, String name) {super();this.age = age;this.name = name;}public int getAge() {return age;}public String getName() {return name;}public void setAge(int age) {this.age = age;}public void setName(String name) {this.name = name;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + (age+index++);result = prime * result + ((name == null) ? 0 : name.hashCode());System.out.println("result :"+result);return result;}@Overridepublic 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 (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}}运行结果：stu1 == stu2 : falsestu1.equals(stu2) : truelist size:2hashCode :776098hashCode :776129set 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：12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152//学生类public class Student {private int age;private String name;public Student() {}public Student(int age, String name) {super();this.age = age;this.name = name;}public int getAge() {return age;}public String getName() {return name;}public void setAge(int age) {this.age = age;}public void setName(String name) {this.name = name;}@Overridepublic 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;}@Overridepublic 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 (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}}测试代码如下：1234567891011121314151617181920212223242526import 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 : falsestu1.equals(stu2) : truelist size:2hashCode : 775943hashCode : 775943set size:1hashCode : 776904remove stu1 : falseset 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：123public boolean add(E e) {return map.put(e, PRESENT)==null;}map.put源代码:12345678910111213141516171819public 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;}

hashCode与equals的区别与联系

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