Jakarta Commons Cookbook

1.6. Automating hashCode( ) and equals( )

Problem

You need a way to automatically implement equals() and hashCode( ) methods.

Solution

Commons Lang EqualsBuilder and HashCodeBuilder provide methods to automate both the equals( ) and hashCode( ). Example 1-2 briefly demonstrates these two builders using the PoliticalCandidate bean from the previous two recipes.

Example 1-2. Automating hashCode( ) and equals( )

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

public class PoliticalCandidate {

    // Member variables - omitted for brevity

    // Constructors - omitted for brevity

    // get/set methods - omitted for brevity

    // A hashCode which creates a hash from the two unique identifiers
    public int hashCode( ) {
        return new HashCodeBuilder(17, 37)
                       .append(firstName)
                       .append(lastName).toHashCode( );
    }

    // An equals which compares two unique identifiers
    public boolean equals(Object o) {
        boolean equals = false;
        if ( o != null && 
            PoliticalCandidate.class.isAssignableFrom(o) ) {
            PoliticalCandidate pc = (PoliticalCandidate) o;
            equals = (new EqualsBuilder( )
                       .append(firstName, ps.firstName)
                       .append(lastName, ps.lastName)).isEquals( );
        }
        return equals;
    }
}

Discussion

HashCodeBuilder has a constructor that takes two integer primitives. These primitives are used as an offset when creating a hash code; both numbers should be odd, nonzero, and prime. The HashCodeBuilder in Example 1-2 is configured to use the firstName and the lastName of ...