CS 213 Fall 2021 Note #5 – Polymorphism Dr. • Polymorphism
Three important features of object-oriented programming are encapsulation, inheritance, and polymorphism. The inheritance relationship enables a subclass to inherit data and operations from its superclass and define additional data and operations specific to the subclass. A subclass is a specialization of its superclass; every instance of a subclass is also an instance of its superclass, but not vice versa. For example, every circle object is a Shape object, but not every Shape object is a circle object. Therefore, you can always pass an instance of a subclass to a parameter of its superclass type. Consider the code below, what is the output?
public class PolymorphismDemo {
public static void main(String[] args) {
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displayObject(new Circle(1, “red”, false));
displayObject(new Rectangle(1, 1, “black”, true)); }
public static void displayObject(Shape object) { System.out.println(object.toString() + ” color is ”
Console output:
+ object.getColor());
Circle color is red Rectangle color is black
An object of a subclass can be used wherever its superclass object is used. This is commonly known as polymorphism (from a Greek word meaning “many forms”). In simple terms, polymorphism means that a variable of a supertype can refer to a subtype object. The supertype is a “generalization” of the subtypes. Below is another example.
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 hide();
width = newW; height = newH; show();
( int newW, int newH) {
int deltaX, int deltaY ) {
public void move (
hide(); x = x + y = y + show();
deltaX; 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
CS 213 Fall 2021 Note #5 – Polymorphism Dr.
Box b = new Box( 50, 40, 20, 30 );
ColoredBox cb = new ColoredBox( 80, 60, 10, 20, Color.red ); cb = b; //invalid; assignment expected a ColoredBox
b = cb; //valid; assignment expected a Box, got a child is OK
In the above code segment, cb is an instance of ColoredBox class, and b is an instance of the supertype Box class. If you assign the reference of b to cb, you will get a compile error since cb is an instance of the subclass ColoredBox, which is more specific and includes additional data fields that the Box class doesn’t have. On the other hand, although b is an instance of Box class, you can assign the reference of cb to b since the ColoredBox is a subtype of Box. The problem is,
1. Given cb.move(3, 4); which show() method is called when move is executed?
2. Given b.show(); which show() method is called, Box’s show or ColoredBox’s show?
As another example, which toString() method, Shape class’s or Object class’s, is invoked by o based on the following code?
Object o = new Shape(“red”, false); System.out.println(o.toString());
A method can be implemented in several classes along the inheritance chain. The JVM decides which method is invoked at runtime. When a program is running and a method is activated, the Java Runtime System checks the data type of the actual object and uses the method from that type (rather than the method from the type of the reference variable.
A variable must be declared a type. The type that declares a variable is called the variable’s declared type. Here, o’s declared type is Object. A variable of a reference type can hold a null value or a reference to an instance of the declared type. The instance may be created using the constructor of the declared type or its subtype. The actual type of the variable is the actual class for the object referenced by the variable. Here, o’s actual type is Shape, because o references an object created using new Shape(“red”, false). As for which toString() method is invoked by o is determined by o’s actual type. This is known as dynamic binding. The “power” of polymorphism is extensibility. Write code that can handle changes in the future without being modified!
Matching a method signature and binding a method implementation are two separate issues. The declared type of the reference variable decides which method to match at compile time. The compiler finds a matching method according to the parameter type, number of parameters, and order of the parameters at compile time. A method may be implemented in several classes along the inheritance chain. The JVM dynamically binds the implementation of the method at runtime, decided by the actual type of the variable.
Dynamic binding works as follows: Suppose that an object o is an instance of classes C1. There is an inheritancechainC1, C2,…,Cn-1,andCn,whereC1 isasubclassofC2,C2 isasubclassofC3,…,and Cn-1 is a subclass of Cn, as shown below. That is, Cn is the most general class, and C1 is the most specific class. In Java’s inheritance chain, Cn is the Object class. If object o is an instance of C1 and invokes a method p, the JVM searches for the implementation of the method p in the order of C1, C2, . . . , Cn-1, and Cn, until the p method is found. Once an implementation is found, the search stops and the first- found implementation is invoked.
cn-1 c2 c1
CS 213 Fall 2021 Note #5 – Polymorphism Dr. • LSP (Liskov Substitution Principle)
1. A child class (subclass) can be used wherever a parent class (superclass) is expected; child class must be completely substitutable for their parent class
2. For every overriding method in a child class: require no more, promise no less; make sure a child class just extend without replacing the functionality of the superclass
3. Use the notion of “is-a” and LSP to determine if your inheritance is good – don’t overdo inheritance!
• Casting Objects and the instanceof Operator
One object reference can be typecast into another object reference. This is called casting object. For example, the statement Object o = new Student(), known as implicit casting, is valid because an instance of Student is an instance of Object, which is a superclass of all Java classes. Suppose you want to assign the object reference o to a variable of the Student type using the following statement.
Student s = o;
In this case, a compile error occurs. Why does the statement Object o = new Student() work, but Student s = o doesn’t? The reason is that a Student object is always an instance of Object, but anObjectis not necessarily an instance ofStudent. Even though you can see thatois really a Student object, the compiler is not clever enough to know it. To tell the compiler o is a Student object, use explicit casting. The syntax is similar to the one used for casting among primitive data types. Enclose the target object type in parentheses and place it before the object to be cast, as follows.
Student s = (Student) o;
To help understand casting, you may also consider the analogy of fruit, apple, and orange, with the Fruit class as the superclass for Apple and Orange. An apple is a fruit, so you can always safely assign an instance of Apple to a variable for Fruit. However, a fruit is not necessarily an apple, so you have to use explicit casting to assign an instance of Fruit to a variable of Apple.
It is always possible to cast an instance of a subclass to a variable of a superclass (known as upcasting) because an instance of a subclass is always an instance of its superclass. When casting an instance of a superclass to a variable of its subclass (known as downcasting), explicit casting must be used to confirm your intention to the compiler with the (SubclassName) cast notation. For the casting to be successful, you must make sure the object to be cast is an instance of the subclass. If the superclass object is not an instance of the subclass, a runtime ClassCastException occurs. For example, if an object is not an instance of Student, it cannot be cast into a variable of Student. It is a good practice, therefore, to ensure the object is an instance of another object before attempting a casting. This can be accomplished by using the instanceof operator.
public void someMethod(Object obj) { if (obj instanceof Circle) {
Circle circle = (Circle) obj;
System.out.println(“circle radius: ” + circle.getRadius()); }
You may be wondering why casting is necessary. The variable obj is declared Object. The declared type decides which method to match at compile time. Using obj.getRadius() will cause a compile error, because the Object class does not have the getRadius() method. The compiler cannot find a match for obj.getRadius(). Therefore, it is necessary to cast obj into the Circle type to tell the compiler that obj is
CS 213 Fall 2021 Note #5 – Polymorphism Dr.
also an instance of Circle. Why not declare obj as a Circle type in the first place? To enable generic programming, it is a good practice to declare a variable with a supertype that can accept an object of any subtype. As another example, the overriding equals() method.
public boolean (Object obj) { if (obj instanceof Student) {
Student s = (Student) obj; return s.name.equals(name) &&
return false; }
s.startDate.equals(startDate);
The object member access operator dot (.) precedes the casting operator. Use parentheses to ensure that casting is done before the dot operator.
• The Protected Data and Methods
A protected member of a class can be directly accessed from a subclass. So far you have used the private and public keywords to specify whether data fields and methods can be accessed from outside of the class. Private members can be accessed only from inside of the class, and public members can be accessed from any other classes. Often it is desirable to allow subclasses to access data fields or methods defined in the superclass, but not to allow non-subclasses in different packages to access these data fields and methods. To accomplish this, you can use the protected keyword. This way you can access protected data fields or methods in a superclass from its subclasses.
A subclass may override a protected method defined in its superclass and change its visibility to public. However, a subclass cannot weaken the accessibility of a method defined in the superclass. For example, if a method is defined as public in the superclass, it must be defined as public in the subclass.
The “final” keyword. You may occasionally want to prevent classes from being extended. In such cases, use the final modifier to indicate a class is final and cannot be a parent class. For example,
public final class Student { } //cannot be extended
public class International extends Student { } //will get a compile error
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