Apache Commons equals/hashCode builder

If you do not want to depend on a 3rd party library (maybe you are running an a device with limited resources) and you even do not want to type your own methods, you can also let the IDE do the job, e.g. in eclipse use

Source -> Generate hashCode() and equals()...

You will get 'native' code which you can configure as you like and which you have to support on changes.

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Example (eclipse Juno):

import java.util.Arrays;
import java.util.List;

public class FooBar {

    public String string;
    public List<String> stringList;
    public String[] stringArray;

    /* (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((string == null) ? 0 : string.hashCode());
        result = prime * result + Arrays.hashCode(stringArray);
        result = prime * result
                + ((stringList == null) ? 0 : stringList.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 (getClass() != obj.getClass())
            return false;
        FooBar other = (FooBar) obj;
        if (string == null) {
            if (other.string != null)
                return false;
        } else if (!string.equals(other.string))
            return false;
        if (!Arrays.equals(stringArray, other.stringArray))
            return false;
        if (stringList == null) {
            if (other.stringList != null)
                return false;
        } else if (!stringList.equals(other.stringList))
            return false;
        return true;
    }

}

Folks, wake up! Since Java 7 there are helper methods for equals and hashCode in the standard library. Their usage is fully equivalent to usage of Guava methods.

The commons/lang builders are great and I have been using them for years without noticeable performance overhead (with and without hibernate). But as Alain writes, the Guava way is even nicer:

Here's a sample Bean:

public class Bean{

    private String name;
    private int length;
    private List<Bean> children;

}

Here's equals() and hashCode() implemented with Commons/Lang:

@Override
public int hashCode(){
    return new HashCodeBuilder()
        .append(name)
        .append(length)
        .append(children)
        .toHashCode();
}

@Override
public boolean equals(final Object obj){
    if(obj instanceof Bean){
        final Bean other = (Bean) obj;
        return new EqualsBuilder()
            .append(name, other.name)
            .append(length, other.length)
            .append(children, other.children)
            .isEquals();
    } else{
        return false;
    }
}

and here with Java 7 or higher (inspired by Guava):

@Override
public int hashCode(){
    return Objects.hash(name, length, children);
}

@Override
public boolean equals(final Object obj){
    if(obj instanceof Bean){
        final Bean other = (Bean) obj;
        return Objects.equals(name, other.name)
            && length == other.length // special handling for primitives
            && Objects.equals(children, other.children);
    } else{
        return false;
    }
}

Note: this code originally referenced Guava, but as comments have pointed out, this functionality has since been introduced in the JDK, so Guava is no longer required.

As you can see the Guava / JDK version is shorter and avoids superfluous helper objects. In case of equals, it even allows for short-circuiting the evaluation if an earlier Object.equals() call returns false (to be fair: commons / lang has an ObjectUtils.equals(obj1, obj2) method with identical semantics which could be used instead of EqualsBuilder to allow short-circuiting as above).

So: yes, the commons lang builders are very preferable over manually constructed equals() and hashCode() methods (or those awful monsters Eclipse will generate for you), but the Java 7+ / Guava versions are even better.

And a note about Hibernate:

be careful about using lazy collections in your equals(), hashCode() and toString() implementations. That will fail miserably if you don't have an open Session.

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Note (about equals()):

a) in both versions of equals() above, you might want to use one or both of these shortcuts also:

@Override
public boolean equals(final Object obj){
    if(obj == this) return true;  // test for reference equality
    if(obj == null) return false; // test for null
    // continue as above

b) depending on your interpretation of the equals() contract, you might also change the line(s)

    if(obj instanceof Bean){

to

    // make sure you run a null check before this
    if(obj.getClass() == getClass()){ 

If you use the second version, you probably also want to call super(equals()) inside your equals() method. Opinions differ here, the topic is discussed in this question:

right way to incorporate superclass into a Guava Objects.hashcode() implementation?

(although it's about hashCode(), the same applies to equals())

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Note (inspired by Comment from kayahr)

Objects.hashCode(..) (just as the underlying Arrays.hashCode(...)) might perform badly if you have many primitive fields. In such cases, EqualsBuilder may actually be the better solution.