Best implementation for hashCode method for a collection

Question

How do we decide on the best implementation of hashCode() method for a collection (assuming that equals method has been overridden correctly) ?

Answer 1

For a simple class it is often easiest to implement hashCode() based on the class fields which are checked by the equals() implementation.

public class Zam {
    private String foo;
    private String bar;
    private String somethingElse;

    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }

        if (obj == null) {
            return false;
        }

        if (getClass() != obj.getClass()) {
            return false;
        }

        Zam otherObj = (Zam)obj;

        if ((getFoo() == null && otherObj.getFoo() == null) || (getFoo() != null && getFoo().equals(otherObj.getFoo()))) {
            if ((getBar() == null && otherObj. getBar() == null) || (getBar() != null && getBar().equals(otherObj. getBar()))) {
                return true;
            }
        }

        return false;
    }

    public int hashCode() {
        return (getFoo() + getBar()).hashCode();
    }

    public String getFoo() {
        return foo;
    }

    public String getBar() {
        return bar;
    }
}

The most important thing is to keep hashCode() and equals() consistent: if equals() returns true for two objects, then hashCode() should return the same value. If equals() returns false, then hashCode() should return different values.