Equals and Hashcode

同事问了个关于equals和hashcode的问题，我写了个测试说明了一下有啥不同，为什么SUN的文档会说改写equals方法要改写hashcode方法（不改写当然可以，但在使用中会有些不一样的地方，从下面的代码可以看出来），要求equals为true时，hashcode值一样，equals为假时，hashcode并不要求一定要不一样。下面给出测试代码与测试结果来说明：

 

package test;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class EqualsNHashcodeTest {
 public static void main(String[] args) {
  Set<SampleModel> s1 = new HashSet<SampleModel>();
  Set<SampleModel1> s2 = new HashSet<SampleModel1>();
  Set<SampleModel2> s3 = new HashSet<SampleModel2>();
  List<SampleModel> l1 = new ArrayList<SampleModel>();
  List<SampleModel1> l2 = new ArrayList<SampleModel1>();
  List<SampleModel2> l3 = new ArrayList<SampleModel2>();
  List<SampleModel> l4 = new ArrayList<SampleModel>();
  List<SampleModel1> l5 = new ArrayList<SampleModel1>();
  List<SampleModel2> l6 = new ArrayList<SampleModel2>();
  Set<SampleModel> s4 = new HashSet<SampleModel>();
  Set<SampleModel1> s5 = new HashSet<SampleModel1>();
  Set<SampleModel2> s6 = new HashSet<SampleModel2>();

  SampleModel m1 = new SampleModel("anny");
  SampleModel m2 = new SampleModel("lily");
  SampleModel m3 = new SampleModel("lily");

  SampleModel1 n1 = new SampleModel1("anny");
  SampleModel1 n2 = new SampleModel1("lily");
  SampleModel1 n3 = new SampleModel1("lily");

  SampleModel2 p1 = new SampleModel2("anny");
  SampleModel2 p2 = new SampleModel2("lily");
  SampleModel2 p3 = new SampleModel2("lily");
  System.out.println(m2.equals(m3));
  System.out.println(p2.equals(p3));
  System.out.println(p2 + " vs " + p3);
  System.out.println(p2 == p3);

  s1.add(m1);
  s1.add(m2);
  s1.add(m3);
  System.out.println(s1);

  l1.add(m1);
  l1.add(m2);
  l1.add(m3);
  System.out.println(l1);

  s2.add(n1);
  s2.add(n2);
  s2.add(n3);
  System.out.println(s2);

  l2.add(n1);
  l2.add(n2);
  l2.add(n3);
  System.out.println(l2);

  s3.add(p1);
  s3.add(p2);
  s3.add(p3);

  System.out.println(s3);

  l3.add(p1);
  l3.add(p2);
  l3.add(p3);
  System.out.println(l3);

  l4.add(m1);
  l4.add(m2);
  System.out.println(l4.contains(m1));
  System.out.println(l4.contains(m2));
  System.out.println(l4.contains(m3));

  l5.add(n1);
  l5.add(n2);
  System.out.println(l5.contains(n1));
  System.out.println(l5.contains(n2));
  System.out.println(l5.contains(n3));

  l6.add(p1);
  l6.add(p2);
  System.out.println(l6.contains(p1));
  System.out.println(l6.contains(p2));
  System.out.println(l6.contains(p3));
  
  s4.add(m1);
  s4.add(m2);
  System.out.println(s4.contains(m1));
  System.out.println(s4.contains(m2));
  System.out.println(s4.contains(m3));

  s5.add(n1);
  s5.add(n2);
  System.out.println(s5.contains(n1));
  System.out.println(s5.contains(n2));
  System.out.println(s5.contains(n3));

  s6.add(p1);
  s6.add(p2);
  System.out.println(s6.contains(p1));
  System.out.println(s6.contains(p2));
  System.out.println(s6.contains(p3));

 }
}

class SampleModel {

 String name;

 public SampleModel(String name) {
  this.name = name;
 }

 @Override
 public boolean equals(Object obj) {
  if (!(obj instanceof SampleModel))
   return false;
  SampleModel s = (SampleModel) obj;
  return this.name.equals(s.name);
 }

}

class SampleModel1 {
 String name;

 public SampleModel1(String name) {
  this.name = name;
 }

 @Override
 public int hashCode() {
  char[] cs = this.name.toCharArray();
  int temp = 0;
  for (char c : cs) {
   temp += Integer.MAX_VALUE % (int) c;
  }
  return temp;
 }
}

class SampleModel2 {

 public SampleModel2(String name) {
  this.name = name;
 }

 String name;

 @Override
 public boolean equals(Object obj) {
  if (!(obj instanceof SampleModel2))
   return false;
  SampleModel2 s = (SampleModel2) obj;
  return this.name.equals(s.name);
 }

 @Override
 public int hashCode() {
  char[] cs = this.name.toCharArray();
  int temp = 0;
  for (char c : cs) {
   temp += Integer.MAX_VALUE % (int) c;
  }
  return temp;
 }
}

 

编译及运行结果：

E:\PlatEclipseWorkSpace\TestProject>javac test/EqualsNHashcodeTest.java

E:\PlatEclipseWorkSpace\TestProject>java test.EqualsNHashcodeTest
true
true
test.SampleModel2@125 vs test.SampleModel2@125
false
[test.SampleModel@61de33, test.SampleModel@ca0b6, test.SampleModel@14318bb]
[test.SampleModel@61de33, test.SampleModel@14318bb, test.SampleModel@ca0b6]
[test.SampleModel1@120, test.SampleModel1@125, test.SampleModel1@125]
[test.SampleModel1@120, test.SampleModel1@125, test.SampleModel1@125]
[test.SampleModel2@120, test.SampleModel2@125]
[test.SampleModel2@120, test.SampleModel2@125, test.SampleModel2@125]
true
true
true
true
true
false
true
true
true
true
true
false
true
true
false
true
true
true

 

看下代码对下结果应该比较好理解，就不写很详细的注释了。而且注意LIST与SET中的不同。

更详细的建议去看文档和源代码，再对不明确的地方做下测试会更清楚些。