CS 213 Fall 2021 Note #4 – Inheritance Dr.
Inheritance is an important and powerful feature for reusing software. Suppose you need to define classes to model circles, rectangles, and triangles. These classes have many common features. What is the best way to design these classes so as to avoid redundancy and make the system easy to comprehend and easy to maintain?
Business is dynamic and running in a fast pace. Software help the businesses make important decisions and stay competitive. Often times, software must change to meet new requirements and business needs. What would be the best approach to minimize the impact on the running software and incorporate new functionalities into the software?
• Superclass (parent class) and Subclass (child class) in Java
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You use a class to model objects of the same type. Different classes may have some common properties and behaviors, which can be generalized in a class that can be shared by other classes. You can define a specialized class that extends the generalized class. The specialized classes inherit the properties and methods from the general class.
Consider the geometric objects. Suppose you want to design the classes to model geometric objects such as circles and rectangles. Geometric objects have many common properties and behaviors. For example, they can be drawn in a certain color and be filled or unfilled. In this case, a general Shape class can be defined to model the geometric objects sharing the properties color and filled and their appropriate getter and setter methods. Shape class also contains the toString() method, which returns a string representation of the object, and it is an overriding method implemented in most classes.
Since a circle is a special type of geometric object, it shares common properties and methods with other geometric objects. Therefore, it makes sense to define the Circle class that extends the Shape class. Likewise, Rectangle can also be defined as a special type of geometric objects. The following figure shows the relationship among these classes. A triangular arrow pointing to the generalized class is used to denote the generalization (inheritance) relationship between the two classes involved.
CS 213 Fall 2021 Note #4 – Inheritance Dr.
In Java’s terminology, a class C1 extended from another class C2 is called a subclass, and C2 is called a superclass. A superclass is also referred to as a parent class or a base class, and a subclass as a child class, an extended class, or a derived class.
A subclass inherits accessible data fields and methods from its superclass and may also add new data fields and methods. In the example above, Circle and Rectangle are subclasses of Shape class, and – Shape class is the superclass for Circle and Rectangle. A class defines an abstract data type (ADT.) A type defined by a subclass is called a subtype, and a type defined by its superclass is called a supertype. In other words, Circle is a subtype of Shape, and Shape is a supertype for Circle.
The subclass and its superclass are said to form a is-a relationship. A Circle object is a special type of general Shape. The Circle class inherits all accessible data fields and methods from the Shape class. In addition, it has a new data field, radius and its associated getter and setter methods. The Circle class also contains additional methods for returning the area, perimeter and diameter of the circle object.
The Rectangle class inherits all accessible data fields and methods from the Shape class. In addition, it has the data fields width and height and their associated getter and setter methods. It also contains the methods for returning the area and perimeter of the rectangle. Note that you may have used the terms width and length to describe the sides of a rectangle in geometry. The common terms used in computer science are width and height, where width refers to the horizontal length, and height to the vertical length.
In Java, the keyword extends is used to tell the compiler that the subclass extends the superclass. For example, the Circle class above extends the Shape class with the following syntax and inherits the data fields and all the public methods defined in the Shape class.
public class Circle extends Shape subclass superclass
• The “super” keyword
In Java, every class has one and only one direct superclass (single inheritance.) Child class inherits data and operations from the parent, but NOT constructors. Child class can override parent methods it wants to change. Classes that don’t explicitly inherit from another class inherit from Object class. You can use a child class wherever a parent class is expected. There are some rules a child class must follow. The first line of the constructor must invoke a parent constructor via “super” (superclass). If you override a method and want to get the parent’s version, use the super key word. For example, super(), or super(arguments). If a parent wants to give the children direct access to its data fields, it must declare the data fields as protected.
The statement super() at the first line in the subclass constructor invokes the default constructor of its superclass, and the statementsuper(arguments)invokes the superclass constructor that matches the arguments. The statement super() or super(arguments) must be the first statement of the subclass’s constructor. Invoking a superclass constructor’s name in a subclass causes a syntax error. Using “super” is the only way to explicitly invoke a superclass constructor. For example,
public Circle(double radius, String color, boolean filled) { super(color, filled); //invoke the superclass constructor this.radius = radius;
CS 213 Fall 2021 Note #4 – Inheritance Dr. A constructor may invoke an overloaded constructor or its superclass constructor. If neither is invoked
explicitly, the compiler automatically puts super() as the first statement in the constructor. For example:
In any case, constructing an instance of a class invokes the constructors of all the superclasses along the inheritance chain. When constructing an object of a subclass, the subclass constructor first invokes its superclass constructor before performing its own tasks. If the superclass is derived from another class, the superclass constructor invokes its parent-class constructor before performing its own tasks. This process continues until the last constructor along the inheritance hierarchy is called. This is called constructor chaining. If possible, you should provide a default constructor for every class to make the class easy to extend and to avoid errors.
The keyword super can also be used to reference a method other than the constructor in the superclass. The syntax is: super.method(arguments); note that the following syntax will give you an error: super.super.method();
• Overriding Methods
A subclass inherits methods from a superclass. Sometimes, it is necessary for the subclass to modify the implementation of a method defined in the superclass. This is referred to as method overriding. toString() and equals() methods are good examples of overriding the behaviors of the superclasses. Because, in most cases, there are additional data fields in the subclasses, how you determine 2 objects of the subclasses are equal is different. For example, the toString() method in the Circle class.
public String toString() {
return super.toString() + “\nradius is ” + radius;
The overriding method must have the same signature as the overridden method and same or compatible return type. Compatible means that the overriding method’s return type is a subtype of the overridden method’s return type.
An instance method can be overridden only if it is accessible. Thus, a private method cannot be overridden, because it is not accessible outside its own class. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated.
Like an instance method, a static method can be inherited. However, a static method cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden. The hidden static methods can be invoked using the syntax SuperClassName.staticMethodName.
More examples below.
CS 213 Fall 2021 Note #4 – Inheritance Dr.
public class Box {
private int x, y; //upper-left hand corner of the Box private int width, height;
public Box( int startX, int startY, int w, int h) {
x = startX;
y = startY;
width = w;
height = h;
public void show() {
// Some code that draws the box
public void hide() {
public void resize ( int newW, int newH) {
width = newW;
height = newH;
public void move ( int deltaX, int deltaY ) {
x = x + deltaX;
y = y + deltaY;
public int area() {
return width * height;
parent class
public class ColoredBox extends Box {
private Color color;
public ColoredBox (int startX, int startY, int w,
int h, Color c ) {
super( startX, startY, w, h );
color = c; }
public void show() {
super.show();
public Color getColor() {
return color;
// if you want to call it
child class
public class Bicycle {
private int cadence;
private int gear;
private int speed;
public Bicycle(int startCadence, int startSpeed,
int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
public void setCadence(int newValue) { cadence = newValue;
public void setGear(int newValue) {
gear = newValue;
public void applyBrake(int decrement) { speed -= decrement;
public void speedUp(int increment) {
speed += increment;
parent class
public class MountainBike extends Bicycle { // the MountainBike subclass adds one field
private int seatHeight;
// the MountainBike subclass has one constructor public MountainBike(int startHeight,
int startCadence,
int startSpeed,
int startGear) {
super(startCadence, startSpeed, startGear);
seatHeight = startHeight; }
// the MountainBike subclass adds one method
public void setHeight(int newValue) { seatHeight = newValue;
child class
• Overriding vs. Overloading
Overloading means to define multiple methods with the same name but different signatures. Overriding means to provide a new implementation for a method in the subclass. For example, in the first code segment, the method p(double i) in class A overrides the same method defined in class B. In the second
CS 213 Fall 2021 Note #4 – Inheritance Dr. code segment however, the classAhas two overloaded methods:p(double i)andp(int i). The
method p(double i) is inherited from B.
public class TestOverloading {
public static void main(String args[]) {
A a = new A(); a.p(10); a.p(10.0);
public void p(double i) {
System.out.println(i * 2); }
class A extends B { public void p(int i) {
System.out.println(i); //overload the method in B }
public class TestOverriding {
public static void main(String args[]) {
A a = new A(); a.p(10); a.p(10.0);
public void p(double i) {
System.out.println(i * 2); }
class A extends B {
public void p(double i) {
System.out.println(i); //override the method in B }
When you run the TestOverriding class, both a.p(10) and a.p(10.0) invoke the p(double i) method defined in class A to display 10.0. When you run the TestOverloading class, a.p(10) invokes the p(int i) method defined in class A to display 10 and a.p(10.0) invokes the p(double i) method defined in class B to display 20.0.
Note that, overridden methods are in different classes related by inheritance; overloaded methods can be either in the same class, or in different classes related by inheritance. Overridden methods have the same signature; overloaded methods have the same name but different parameter lists. To avoid mistakes, you
CS 213 Fall 2021 Note #4 – Inheritance Dr.
can use a special Java syntax, called override annotation, to place @Override before the overriding method in the subclass. This annotation denotes that the annotated method is required to override a method in its superclass. If a method with this annotation does not override its superclass’s method, the compiler will report an error. For example, if toString() is mistyped as tostring, a compile error is reported. If isn’t used, the compiler won’t report an error. Using the @Override annotation avoids mistakes.
• The Object Class and its toString() Method
Every class in Java is descended from the java.lang.Object class. If no inheritance is specified when a class is defined, the superclass of the class is Object by default. See the Java library class hierarchy here: https://docs.oracle.com/en/java/javase/14/docs/api/overview-tree.html.
Classes such as String, StringBuilder, Loan, and Shape are implicitly subclasses of the Object class. It is important to be familiar with the methods provided by the Object class so that you can use them in your classes. For example, the toString() method. Invoking toString() on an object returns a string that describes the object. By default, it returns a string consisting of a class name of which the object is an instance, an at sign and the object’s memory address in hexadecimal. For example, the output for the following code looks something like
representation of the object.
This message is not very helpful or
informative. Usually you should override the toString() method
so that it returns a descriptive string
Loan loan = new Loan();
System.out.println(loan.toString());
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