java - 哈希码()与等于()

Question

我有下面这两个类..

class Emp //implements Comparable
{
      String name,job;
      public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getJob() {
        return job;
    }
    public void setJob(String job) {
        this.job = job;
    }
    public int getSalary() {
        return salary;
    }
    public void setSalary(int salary) {
        this.salary = salary;
    }
    int salary;
      public Emp(String n,String j,int sal)
      {
         name=n;
         job=j;
         salary=sal;
       }
      public void display()
      {
        System.out.println(name+"\t"+job+"\t"+salary);
       }



  public boolean equals(Object o)
      {

         Emp p=(Emp)o;
          return this.name.equals(p.name)&&this.job.equals(p.job) &&this.salary==p.salary;
       }
  /* public int hashCode()
       {
          return name.hashCode()+job.hashCode()+salary;
       }
     */

      /* public int compareTo(Object o)
       {
          Emp e=(Emp)o;
          return this.name.compareTo(e.name);
           //return this.job.compareTo(e.job);
        //   return this.salary-e.salary;

        }*/
} 

还有一个是..

import java.util.*;
class EmpHsDemo
{
      public static void main(String arg[])
      {
          HashSet set=new HashSet();
          set.add(new Emp("Ram","Trainer",34000));
          set.add(new Emp("Ram","Trainer",34000));
          set.add(new Emp("Ravi","Administrator",44000));
          set.add(new Emp("Sachin","Programmer",24000));
          set.add(new Emp("Priyanka","Manager",54000));
          set.add(new Emp("Anupam","Programmer",34000));
          set.add(new Emp("Sachin","Team Leader",54000));
          System.out.println("There are "+set.size()+" elements in the set.");
          System.out.println("Content of set are : ");
          Iterator itr=set.iterator();
          while(itr.hasNext())
          {
            Emp e=(Emp)itr.next();
            System.out.print(e.hashCode()+"\t");   
            e.display();
          }   


          System.out.println("**********************************");
        Emp e1=new Emp("Ravi","Administrator",44000);
        System.out.println("Removing following Emp from the set...");
        System.out.print(e1.hashCode()+"\t");
        e1.display();
        set.remove(e1);
        System.out.println("No. of elements after removal "+set.size());
       /* Emp e2=new Emp("Anupam","Programmer",34000);
        System.out.println("Searching following Emp in the set...");
        System.out.print(e2.hashCode()+"\t");
        e2.display();
        System.out.println("Results of searching is : "+set.contains(e2));*/
      }
}

现在我正在做一项研究是

1. If I comment hashcode() method and not comment equals () method that is it will use it allows the duplicate as Ram is shown twice along with memory address I get the following result..

There are 7 elements in the set.
Content of set are : 
374283533   Priyanka    Manager 54000
1660364311  Ram Trainer 34000
1340465859  Ravi    Administrator   44000
2106235183  Sachin  Programmer  24000
2031692173  Ram Trainer 34000
603737068   Anupam  Programmer  34000
148669801   Sachin  Team Leader 54000
**********************************
Removing following Emp from the set...
1807500377  Ravi    Administrator   44000
No. of elements after removal 7

2 . If I uncomment hashcode() method and equals () method , I get this result

There are 6 elements in the set.
Content of set are : 
1546622676  Sachin  Team Leader 54000
-302767206  Anupam  Programmer  34000
149315535   Ravi    Administrator   44000
199998062   Sachin  Programmer  24000
1407883922  Priyanka    Manager 54000
597555555   Ram Trainer 34000
**********************************
Removing following Emp from the set...
149315535   Ravi    Administrator   44000
No. of elements after removal 5

3 . If I comment equals() method only and not hashcode() then I get the following result

There are 7 elements in the set.
Content of set are : 
1546622676  Sachin  Team Leader 54000
-302767206  Anupam  Programmer  34000
149315535   Ravi    Administrator   44000
199998062   Sachin  Programmer  24000
1407883922  Priyanka    Manager 54000
597555555   Ram Trainer 34000
597555555   Ram Trainer 34000
**********************************
Removing following Emp from the set...
149315535   Ravi    Administrator   44000
No. of elements after removal 7

请告知这三种方法背后的原因是什么..!

Answer 1

不值得阅读您发布的所有信息。

Joshua Bloch 说 hashCode 和 equals 应该一起被覆盖。 Here's你应该怎么做，没有异常(exception)。