9.7 Object Superclass
Classes can form a hierarchy where each of the subclasses has access to all of the public methods of a parent. At the top of the hierarchy is the Object class, which is a part of the built-in java.lang package. All other classes can use the Object class through inheritance.
The Object class contains several useful methods that are used and inherited by its subclasses. Two such methods are equals() and toString().
The equals method compares two objects and is often overridden by a subclass. Using the Object level version of the equals() method, Java returns true only if the two objects are represented by the same object in memory. This is often not very helpful, as == also returns the same result, which is why this method is often overridden by subclasses.
One example where equals() is overridden is the String class. The String class equals() method compares each character of the string to determine if they are the same, not just if they are the same objects.
The toString() method is another commonly used and commonly overridden method from the Object class. When an object name is called with no method, the toString() is returned as a String representation of the object. The generic toString() for an object is to return the object type followed by an @ symbol and then the unsigned hexadecimal representation of the object. This has limited value, which is why it is often overridden.
There are many examples where the toString() method is overridden. For example, in the Person class you’ve created earlier in this chapter, the toString() method is overridden to return the name of the person:
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Which of the following methods would you find in the
Objectclass?
For this exercise, you are given 3 Object class objects and 3 Ball class objects. In theory, all three Objects are the same and all 3 Ball objects are the same, but based on the way they are created and the way that Java evaluates equality, not all three objects will be equal.
Your task is to start by printing out the 6 Object and Ball objects. Remember, the Object class has a toString() method and since a Ball object extends the Object class, we should see similarities between the Object and Ball class.
After printing them out, you should notice which objects are the same. Write two true and two false statements for each group, one using == and one using the equals() method. Will these be the same?