Unit 12: Overriding
After reading this unit, students should
- be aware that every class inherits from
Object
- be familiar with the
equals
andtoString
methods - understand what constitutes a method signature
- understand method overriding
- appreciate the power of method overriding
- understand what Java annotations are for, and know when to use
@Override
- be exposed to the
String
class and its associated methods, especially the+
operator
Object
and String
In Java, every class that does not extend another class inherits from the class Object
implicitly. Object
is, therefore, the "ancestor" of all classes in Java and is at the root of the class hierarchy.
The Object
class does not encapsulate anything in particular. It is a very general class that provides useful methods common to all objects. The two useful ones that we are going to spend time with are:
equals(Object obj)
, which checks if two objects are equal to each other, andtoString()
, which returns a string representation of the object as aString
object.
The toString
Method
The toString
method is very special, as this is invoked implicitly by Java, by default, to convert a reference object to a String
object during string concatenation using the operator +
.
We showed you that in Python, 4 + "Hello"
would result in a type mismatch error. In Java, however, 4 + "Hello"
will result in the string "4Hello"
. In this example, the primitive value 4 is converted to a string before concatenation.
A more interesting scenario is what happens if we try to concatenate, say, a Circle
object with a string. Let's say we have:
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You will see that s
now contains the string "Circle c is Circle@1ce92674 " (the seemingly gibberish text after @` is the reference to the object and so your result will be different).
What happened here is that the +
operator sees that one of the operands is a string but the other is not, so it converts the one that is not a string to a string by calling its toString()
method automatically for us. This is equivalent to1
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Recall that in our Circle
class (up to version 0.5) we do not have any toString
method. The toString
method that we invoked here is the toString
method inherited from its parent Object
.
jshell
and toString
Recall that jshell
is a REPL tool. After evaluating an expression, jshell
prints the resulting value out. If the resulting value is a reference type, jshell
will invoke toString
to convert the reference type to a string first, before printing the string.
Customizing toString
for Circle
The Object::toString
method (that is our notation for the method toString
from the class Object
) is not very user friendly. Ideally, when we print a Circle
object, say, for debugging, we want to see its center and its radius. To do so, we can define our own toString
method in Circle
. Let's upgrade our Circle
class to do this:
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The body of the method toString
simply constructs a string representation for this circle object and returns it. With this toString
implemented, the output will look something like this:
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Note that when the center this.c
is converted to a string, the toString
method of Point
is invoked. We leave the implementation of Point::toString
as an exercise.
Method Overriding
What we just did is called method overriding in OOP. Inheritance is not only good for extending the behavior of an existing class but through method overriding, we can alter the behavior of an existing class as well.
Let's define the method signature of a method as the method name and the number, type, and order of its parameters, and the method descriptor as the method signature plus the return type.
When a subclass defines an instance method with the same method descriptor as an instance method in the parent class, we say that the instance method in the subclass overrides the instance method in the parent class2. In the example above, Circle::toString
has overridden Object::toString
.
The @Override
Annotation
Line 37 in the example above contains the symbol @Override
. This symbol is an example of annotation in Java. An annotation is not part of the program and does not affect the bytecode generated. Instead, it is a hint to the compiler. Remember that the compiler is our friend who will do its best to help detect errors early, during compilation. We must do our part to help the compiler help us. Here, @Override
is a hint to the compiler that the following method, toString
, is intended to override the method in the parent class. In case, there is a typo and overriding is not possible, the compiler will let us know.
It is therefore recommended and expected that all overriding methods in your code are annotated with @Override
.
Using super
To Access Overridden Methods
After a subclass overrides a method in the superclass, the methods that have been overridden can still be called, with the super
keyword.
For instance, the following Circle::toString
calls Object::toString
to prefix the string representation of the circle with Circle@1ce92674
.
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