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Defining Equals

It's up to each class to decide what makes one object of a class "equal to" another object.

You'll usually follow some common practices when defining an equals method for your class.

  • Remember: equals should return false unless the passed reference meets your criteria for equality to the current object.

Basic Sanity Checks

Check if the passed reference refers to this object.

If the passed reference is a reference to the current object, then of course we are equal.

  • This is called the reflexive property of equals: an object is always equal to itself.
public boolean equals(Object obj) {
  if ( obj == this ) { return true; }
  return false;
}

Check for null

No existing object can be equal to null.

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

Check the passed object's class

If the current object is a Giraffe, there's little chance it is "equal to" a referenced Automobile or Employee or String or even Animal object - to be equal to this object the other object needs to specifically be another Giraffe.

  • Use the getClass method (defined in Object and inherited by all classes) to determine the passed object's class.
public boolean equals(Object obj) {
  if ( obj == this ) { return true; }
  if ( obj == null ) { return false; }
  if ( obj.getClass() != this.getClass() ) { return false; }
  return false;
}

Compare Object State

Once you know you are comparing another object of the same class, the primary purpose of equals is to compare the objects' state.

object state

The identifying properties of an object, and their values.

In simple terms, an object's state consists of the values of the instance fields that define the object.

  • Remember that the parameter passed to equals is of type Object, so you'll need to cast the reference to the current class in order to compare fields.
public boolean equals(Object obj) {
  if ( obj == this ) { return true; }
  if ( obj == null ) { return false; }
  if ( obj.getClass() != this.getClass() ) { return false; }
  MyClass other = (MyClass) obj;
  // Now we can compare fields of "this" and "other" to determine equality.
  return false;
}

Drill

EqualsAndHashcode/com.example.equalsandhashcode.drills.TestNamedObject * Look at the NamedObject class and the TestNamedObject class. What will be printed when you run TestNamedObject? * In NamedObject, create an equals method that (as shown above) first determines if the passed parameter is a reference to another NamedObject, and if so compares its name field to the current object's name.

(Solution: NamedObject.java)

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