BHSawesome2 AP CSA Java 2025+

One commonly overridden Object method is toString(), which is often used to print out the attributes of an object. It is a good idea to write your own toString() method in every class. In a subclass, toString() can call the superclass toString() method using super.toString() and then add on its own attributes.

In the following code, the Person class overrides the toString method from Object method and the Student class then overrides it again. In each class the new toString method adds the new attributes from that class.

One of the important methods inherited from Object is the equals(Object obj) method. This method is used to test if the current object and the passed object called obj are equal. But what does that mean?

As you can see if you run the code below, the equals method inherited from Object only returns true if the two objects references refer to the same object. In other words it is does the same test as ==.

The equals method defined in Object and thus inherited by all classes only considers two object references equivalent if they refer to exactly the same object. But we saw in Unit 2 that the String class provides an equals method that considers two String objects equivalent if they have the same characters in the same order, even if they are actually different objects. How does that work?

It is because the String class has overridden the equals method it inherited from Object to provide a definition of equality that makes more sense.

As we saw in section 9.3 a class can override inherited methods by providing a method with the same method signature (method name, parameter types, and return type). String has done that with equals so when we compare String objects with equals that new method will be called instead of the inherited one.

However, overriding equals is a bit more involved than overriding toString. While the toString method is only required to produce a reasonable human-readable String representation of an object, equals has to meet a more complex set of requirements in order to be useful.

You will not be expected to write your own equals method on the AP exam but it’s worth looking at what those requirements are and how to satisify them. There are five requirements described in the Javadocs for equals in Object that a properly implemented equals must satisfy:

The other way to look at these requirements is as guarantees that are made to you as a user of equals. If you look at it that way, these requirements are quite nice. Imagine how much harder it would be to use the String equals method if you couldn’t rely on the fact that s1.equals(s2) is necessarily the same as s2.equals(s1)!

So even though the Java compiler can’t force you to implement equals correctly, if you ever do want to override it, it’s important that you do. Let’s look at what’s involved.

We’ll write a class Word which represents a word in a particular language. We want two Word objects to be considered equals if and only if they are spelled the same and come from the same language. The latter requirement is because sometimes different languages have words that are spelled the same but with different meanings such as “pie” which in English is a tasty baked treat and in Spanish is what we call a “foot” in English.

The basic recipe for writing your own equals method, is:

Note that the requirements on equals make it almost impossible to correctly override it in a subclass of a class that has already overridden the Object version. To see why, imagine if we made a subclass of Word, ClassifiedWord and added another attribute, partOfSpeech.

If we override equals in the ClassifiedWord to only consider two ClassifiedWord objects equals if their spelling, language, and part of speech match, that will break the symmetry since regularWord.equals(classifiedWord) will invoke the equals from Word which will only compare the spelling and language of the word but classifiedWord.equals(regularWord) will return false assuming the equals in ClassifiedWord checks that other is an instanceof ClassifiedWord. In general you should only provide an overridden equals method in one class in a class hierarchy.

The following code contains the beginning of a class for representing a bank account containing the account holder’s name and the money balance in the account.

Work in pairs to write the following code and test each part before moving on to the next step: