How to implement hashCode and equals method

Question

How should I implement hashCode() and equals() for the following class in Java?

class Emp 
{
  int empid ; // unique across all the departments 
  String name;
  String dept_name ;
  String code ; // unique for the department 
}

Answer 1

in Eclipse right mouse click-> source -> generate hashCode() and equals() gives this:

/* (non-Javadoc)
 * @see java.lang.Object#hashCode()
 */
@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + (code == null ? 0 : code.hashCode());
    return result;
}
/* (non-Javadoc)
 * @see java.lang.Object#equals(java.lang.Object)
 */
@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (!(obj instanceof Emp))
        return false;
    Emp other = (Emp) obj;
    return code == null ? other.code == null : code.equals(other.code);
}

I've selected code as a unique field

Answer 2

try this code, use org.apache.commons.lang3.builder

public int hashCode() {
    return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers
        append(empid).
        append(name).
        append(dept_name ).
        append(code ).
        toHashCode();
}

public boolean equals(Object obj) {

    if (obj == this)
        return true;
    if (!(obj instanceof Person))
        return false;

    Emp rhs = (Emp) obj;
    return new EqualsBuilder().
        // if deriving: appendSuper(super.equals(obj)).
        append(name, rhs.name).
        isEquals();
}

Answer 3

Guava has helper methods for creating them. You tell it which fields to take in consideration and it will handle nulls for you and do the prime number calculation for hashcode.

IDEs can also generate them based on the fields you choose.

The advantage of delegating it to a tool like that is you get a standard solution and will worry less about bugs and maintenance of varied implementations spread all over your project.

Here's an example of using Guava and generated by an IntelliJ plugin: https://plugins.jetbrains.com/plugin/7244?pr=

Answer 4

If code is unique (i.e. your business key), it's best to only use the code for equals and hashCode - it's good practice to seperate business key (code) from object id (id).

Here's a nice read: Hibernate Documentation: Equals and HashCode (valid not only for Hibernate itself)

Answer 5

what ever values you use in equals to determine if two objects are the same, are the the values that you need to use to create a hash code.

public boolean equals(Object o) {

    boolean result = false;

    if(o instanceof CategoryEnum) {

        CategoryEnum ce = (CategoryEnum) o;
        result = ce.toString().equals(name);

    }       
    return result;

}


public int hashCode()
{
  int hash = 6;
  hash += 32 * name.hashCode();
  return hash;
}   

Answer 6

equals()and hashcode(),They have a lot of different places. equals(),if we don't Override it from Object,it represent that whether two variables are pointing to the same object heap?

public  Class Student(){
  private int id;
  private  name;
  public Student(int id,String name){
  this.name=name;
  this.id=id; 
}

public void main(String[] args){
  Student A=new Student(20,'Lily');
  Student B=new Student(20,'Lily');
  boolean flag=A.equals(B)//flag=flase;
/*
 *Although they attribute the same, but they are two different objects, they point to     different memory
 */


@Override
public boolean equals(Object obj) {


  if (obj == null) {
    return false;
  }
  if (this == obj) {
    return true;
  }

  if (this.getClass() != obj.getClass()) {
    return false;
  }
  Student s=(Student)obj;
  return new Integer(this.id).equals(new Integer(s.id))&&this.name.equals(s.name);
  }

/**
  *Sometimes even though we Override  the equals, but we still can not determine whether   the *two objects the same,
  *In the collection object, such as HashSet, this time we have to Override the hashoCode ()
  */

public int hashCode(){
  return id + name.hashCode() ;
}