Bulk immutability, equals/hashcode in Java

Question

I have a series of classes that represent entities. As I'm trying to follow functional principles, they're methodless and their properties aren't meant to be changed once set.

Implementing equals would be needed, in a straightforward way - two instances are considered equal if so are their properties, with no extra logic.

I'm inexperienced in implementing hashCode, so I'm not sure if it could be programatically implemented.

Coding such classes seems unnecessarily verbose to me - looks like has to be a lightweight alternative to

public class SomeEntity {
    public final int prop0;
    public final long prop1;
    // ...
    public final String propN;

    @Override
    public boolean equals(Object arg){
        // check check that arg instanceof Entity,
        // and that each of their properties equal, one by one.
    }
}

. Are there any methods, annotations, collections or even compilers that can do this for me?

Answer 1

Many IDEs, for example NetBeans, can automatically generate hashCode based on equals, or vice versa.

Java 7 has Objects.deepEquals and Objects.hash utilities for generating equals and hash codes for an arbitrary bunch of fields. As stated in its doc, it's based on Arrays.hashCode, which is available in Java 6. You just need to wrap the fields in a new Object[]{field1, field2, field3...}.

Given that your object is immutable, it may make sense to precompute the hash code and store it in one more member. Or, compute lazily, as it's done in java.lang.String:

private int hash;
public int hashCode() {
    int h = hash;
    if (h == 0) {
       // ... compute hash code into h here ...
       hash = h;
    }
    return h;
}

The purpose of using a local variable h is thread safety. Curiously, it's done wrong in java.util.UUID.

Answer 2

it probably took longer to write the question than it will the actual code.

Java's Guava and its equal and hashcode methods could be useful, it has lots of other goodies too.

Answer 3

See Project Lombok, especially @EqualsAndHashCode. Easy to use, and rids your classes from unnecessary boilerplate code.

Answer 4

Yes. Apache Commons lang.builder