HashMap entries with duplicate keys...

Hi, In spite of overriding equals, why HashMap is allowing duplicate entries...?

import java.util.HashMap;

public class Main
	{
		public static void main(String[] args)
			{
				HashMap<Employee, String> hm = new HashMap<Employee, String>(0);
				hm.put(new Employee(1, "A"), "AA");
				System.out.println(hm);
				
				hm.put(new Employee(1, "A"), "BB");
				System.out.println(hm);
			}
	}

class Employee
{
	private Integer empId;
	private String empName;
	
	public Employee(Integer i, String n)
	{
		this.empId = i;
		this.empName = n;
	}
	public Integer getEmpId()
		{
			return empId;
		}
	
	public void setEmpId(Integer empId)
		{
			this.empId = empId;
		}
	
	public String getEmpName()
		{
			return empName;
		}
	public void setEmpName(String empName)
		{
			this.empName = empName;
		}
	
	@Override
	public String toString()
		{
			return this.empId+":"+this.empName;
		}
	
	@Override
	public boolean equals(Object obj)
		{
			Employee e2 = (Employee) obj;
			System.out.println("equals : "+this.empId.equals(e2.empId));
			return this.empId.equals(e2.empId) && this.empName.equals(e2.empName) ;
		}
	
	
}
// output
/*
{1:A=AA}
{1:A=BB, 1:A=AA}
*/

Thanks...

Hello J World wrote:
Hi, In spite of overriding equals, why HashMap is allowing duplicate entries...?

you should override hashCode method too. because HashMap entry checks the equality of both hashCode and equals . if any one fails then HashMap add the new entry value. @see HashMap#put(K, V)

<edit> also your overridden equals method can be fragile . have a look how to override an equals method</edit>

Hello J World ?

and welcome to JavaRanch

.

Seetharaman Venkatasamy wrote:

Hello J World wrote:
Hi, In spite of overriding equals, why HashMap is allowing duplicate entries...?

you should override hashCode method too. because HashMap entry checks the equality of both hashCode and equals . if any one fails then HashMap add the new entry value. @see HashMap#put(K, V)

<edit> also your overridden equals method can be fragile . have a look how to override an equals method</edit>

yaa..I know i must apply instatnceof check as well...but
What about the default implementation of hashcode...?
As per my understanding it puts all the elements in a single bucket...doesn't it...?

Thanks...

Jigar Gohil wrote:
What about the default implementation of hashcode...?

@see java.lang.Object#hashCode

No it doesn't. It's more likely to put all objects in different buckets. But you really should read the Javadoc comments of Object.equals and Object.hashCode. They explain you exactly which rules you must follow if you override one or both.