The Contract of equals
If you need logical equality (instead of object identity),
override equals(),
and obey the contract when overriding it
(think of value classes).
Given non-null references x, y, and z, the contract includes:
- Reflexive:
x.equals(x). - Symmetric:
x.equals(y) == y.equals(x). - Transitive:
x.equals(y) && y.equals(z) == x.equals(z). - Consistent: Multiple calls of
x.equals(y)should yield the same result. - Non-Nullity:
x.equals(null) == false.
And you should always also override hashCode() when overriding equals().
Nevertheless, it is not always possible to obey both the contract and object-oriented principles while extending a class.
public class Point {
private final int x;
private final int y;
@Override
public boolean equals(Object o) {
if (!(o instanceof Point)) {
return false;
}
Point p = (Point) o;
return x == p.x && y == p.y;
}
...
}
public class ColorPoint extend Point {
private final Color color;
// Violate symmetry.
@Override
public boolean equals(Object o) {
if (!(o instanceof ColorPoint)) {
return false;
}
return super.equals(o) && color == ((ColorPoint) o).color;
}
// Violate Liskov substitution principle.
@Override
public boolean equals(Object o) {
if (o == null || getClass() != o.getClass()) {
return false;
}
Point p = (Point) o;
return x == p.x && y == p.y;
}
...
}(So think twice when you want to make a “trivial” extension; use composition instead.)
