SuperCallToObjectMethod
`super.equals(obj)` and `super.hashCode()` are often bugs when they call the methods defined in `java.lang.Object`

Alternate names: SuperEqualsIsObjectEquals

The problem

Implementations of equals and hashCode should usually not delegate to Object.equals and Object.hashCode.

Those two methods implement equality based on object identity. That implementation is sometimes what the author intended. (This check attempts to identify those cases and not report a warning for them. But in some cases, it still produces a warning when it shouldn’t.)

But when super.equals and super.hashCode call the methods defined in Object, the developer often did not intend to use object identity. Often, developers write something like:

private final int id;

@Override
public boolean equals(Object obj) {
  if (obj instanceof Foo) {
    return super.equals(obj) && id == ((Foo) obj).id;
  }
  return false;
}

@Override
public int hashCode() {
  return super.hashCode() ^ id;
}

This appears to be an attempt to define equality in terms of the id field in this class and any fields in the superclass. However, when the superclass that defines equals or hashCode is Object, the code instead defines equality in terms of a mix of object identity and field values. The result is equivalent to defining it in terms of identity alone—which is equivalent to not overriding equals and hashCode at all!

Typically, the code should be rewritten to remove the super calls entirely:

private final int id;

@Override
public boolean equals(Object obj) {
  if (obj instanceof Foo) {
    return id == ((Foo) obj).id;
  }
  return false;
}

@Override
public int hashCode() {
  return id;
}

Note that the suggested edits for this check instead preserve behavior, which likely means preserving bugs! However, in cases in which object identity is intended, we recommend applying the suggested edit to make that behavior explicit in the code:

// This class's definition of equality is unusual and perhaps not ideal.
// But it is at least explicit.

private final Integer id;

@Override
public boolean equals(Object obj) {
  if (obj instanceof Foo) {
    if (id == null) {
      return this == obj;
    }
    return id.equals(((Foo) obj).id);
  }
  return false;
}

@Override
public int hashCode() {
  return id != null ? id : System.identityHashCode(this);
}

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("SuperCallToObjectMethod") to the enclosing element.