Custom Class as Key for Hash Map

[Midhun Agnihotram]

Hi All,

We created a custom class to act as a key for HashMap as follows :

import org.apache.commons.lang.builder.HashCodeBuilder;

import com.CommonUtils;

public class CustomKey {

	private String id1 = null;
	private String id2 = null;
	private String id3 = null;
	private String id4 = null;
	private String id5 = null;

	public CustomKey(String id1, String id2, String id3, String id4, String id5){
		super();
		this.id1 = id1;
		this.id2 = id2;
		this.id3 = id3;
		this.id4 = id4;
		this.id5 = id5;
	}

	public String getid1() {
		return id1;
	}

	public void setid1(String id1) {
		this.id1 = id1;
	}

	public String getid2() {
		return id2;
	}

	public void setid2(String id2) {
		this.id2 = id2;
	}

	public String getid3() {
		return id3;
	}

	public void setid3(String id3) {
		this.id3 = id3;
	}

	public String getid4() {
		return id4;
	}

	public void setid4(String id4) {
		this.id4 = id4;
	}
	
	@Override
	public int hashCode(){
		return new HashCodeBuilder(41, 59).append(id1).append(id2).append(id3).append(id4).append(id5).hashCode();
	}
	
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		CustomKey other = (CustomKey) obj;
		if(!CommonUtils.fieldChanged(this.id1, other.id1) && !CommonUtils.fieldChanged(this.id2, other.id2)
			&& !CommonUtils.fieldChanged(this.id3, other.id3) && !CommonUtils.fieldChanged(this.id4, other.id4)
			&& !CommonUtils.fieldChanged(this.id5, other.id5)){
				return true;
			}else{
				return false;
			}
	}

	public String getid5() {
		return id5;
	}

	public void setid5(String id5) {
		this.id5 = id5;
	}
}

The CommonUtils.fieldChanged function is as follows :

public static boolean fieldChanged(String v, String p){
		 if ((v == null && p != null) ||
			 (v != null && p == null) ||
			 (v != null && p != null && !v.equals(p)))
			 return true;
		 else
			 return false;
	 }

When this class is used as a Key in HashMap and retrieve the value, I get a null - although in Debug mode I see the same key present in the HashMap too. I believe that this has got to do something with the equals / hashCode methods. Can anybody point to as what is wrong ?


Thanks,
Midhun

[Jesper de Jong]

To work correctly as a key in hash-based collections such as HashMap, your class must indeed have correctly implemented equals() and hashCode() methods. The documentation of the method hashCode() in class Object explains what the contract for this method is. The most important thing to remember from it is that if two objects are equal (i.e. equals() return true), then the hashCode() methods of both objects must return the same value.

In case of your code, the equals() and hashCode() methods seem to be OK - I don't see any obvious problems.

Maybe there is something else in your code, in the rest of the program somewhere, that causes the problem.

[James Sabre]

Any class being used as a key for a hash map should be immutable (at least as far as the hashcode() and equals() methods are concerned) since any change to a field that is used by either hashcode() or equals() will destroy the lookup of objects in the map. Since the fields in your class can be modified you don't have this property.

[Costi Ciudatu]

The actual bug in your code is that you're calling hashCode() on the HashCodeBuilder (that class doesn't even overwrite the hashCode() method).
The proper usage of HashCodeBuilder is to call toHashCode() after setting it up:

return new HashCodeBuilder(41, 59).append(id1).append(id2).append(id3).append(id4).append(id5).toHashCode();  

[Jesper de Jong]

Good catch Costi, you're right!

But what James said is indeed also important. Objects that you use as keys in a HashMap must not be mutable, because if you modify the fields after you've put the object in the HashMap, so that the result of hashCode() changes, will confuse the HashMap, so that it can't find your objects anymore.