OVERRIDING/OVERLOADING
Srinivas
EXAM OBJECTIVES
 Given
a code example, determine if a method is
correctly overriding or overloading another
method, and identify legal return values
(including covariant returns), for the method.
 Given
a scenario, develop code that declares
and/or invokes overridden or overloaded
methods and code that declares and/or invokes
superclass, overridden, or overloaded
constructors.
OVERRIDING
 What
is overriding?
 The
child class provides alternative
implementation for parent class method.
 The
key benefit of overriding is the ability to
define behavior that's specific to a particular
subclass type.
 Overridden
 Overriding
method: In the superclass.
method: In the subclass.
METHOD OVERRIDING
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Example:
public class Car
{
public void maxSpeed()
{
System.out.println("Max speed is 60
mph");
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}
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class Ferrari extends Car
{
public void maxSpeed()
{
System.out.println("Max speed is 120
mph");
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}
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public void msc(){}
 }
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public class TestCar
{
public static void main(String args[])
{
Ferrari f=new Ferrari();
f.maxSpeed();
}
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public static void main(String args[])
{
Car f=new Ferrari();
f.maxSpeed();
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}
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}
 In
the preceding code, the test class uses a Car
reference to invoke a method on a Ferrari object.
 This is possible because Ferrari IS-A Car.
 The compiler will allow only methods in class Car
to be invoked when using a reference to a Car.
 Car
 JVM
f=new Ferrari();
f.msc();
sees the real object at the other end of the
reference and invokes the proper method.
RULES FOR METHOD
OVERRIDING

EXACT argument list matching.
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public class Car
{
public void maxSpeed(int speed)
{
System.out.println("Max speed is ” +speed+ “
mph");
}
}
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class Ferrari extends Car
{
public void maxSpeed(float speed)
{
System.out.println("Max speed is ” +speed+ “
mph");
}
public void msc(){}
}
RULES FOR METHOD
OVERRIDING
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The return type must be the same as, or a subtype of, the return type
declared in the original overridden method in the superclass.
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class Alpha
{
Alpha doStuff(char c)
{
return new Alpha();
}
}
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class Beta extends Alpha
{
Beta doStuff(char c)
{
return new Beta();// legal override in Java 1.5
}
}
RULES FOR METHOD
OVERRIDING

The access level can't be more restrictive than the overridden method's.
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public class Car
{
public void maxSpeed()
{
System.out.println("Max speed is 60 mph");
}
}
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class Ferrari extends Car
{
private void maxSpeed()
{
System.out.println("Max speed is 120 mph");
}
public void msc(){}
}
RULES FOR METHOD
OVERRIDING

The access level CAN be less restrictive than that of the overridden
method.
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public class Car
{
protected void maxSpeed()
{
System.out.println("Max speed is 60 mph");
}
}
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class Ferrari extends Car
{
public void maxSpeed()
{
System.out.println("Max speed is 120 mph");
}
public void msc(){}
}
RULES FOR METHOD
OVERRIDING
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Possible only through inheritance.
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public class Car
{
protected void maxSpeed()
{
System.out.println("Max speed is 60 mph");
}
}
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class Ferrari
{
public void maxSpeed()
{
System.out.println("Max speed is 120 mph");
}
public void msc(){}
}
RULES FOR METHOD
OVERRIDING
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o
o
o
o
The overriding method CAN throw any
unchecked (runtime) exception, regardless of
whether the overridden method declares the
exception.
An unchecked exception, also called runtime
exception, is detected only at runtime.
Examples of unchecked exceptions:
ArithmeticException
ArrayIndexOutOfBoundsException
NullPointerException
NumberFormatException
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public class NoRunExc
{
public void divideBy()
{
int result= 100/1;
System.out.println(result);
}
public static void main(String[] args)
{
NoRunExc n=new NoRunExc();
n.divideBy();
NoRunExc r=new RunExc();
r.divideBy();
}
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}
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class RunExc extends NoRunExc
{
public void divideBy() throws ArithmeticException
{
int result= 100/0;
System.out.println(result);
}
}
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RULES FOR METHOD
OVERRIDING
The overriding method must NOT throw
checked exceptions that are new or broader
than those declared by the overridden
method.
 A checked exception is an exception that
is checked at compile time. The compiler will
complain if a checked exception is not
handled appropriately.
 Examples of checked exceptions:
 IOException
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◦ FileNotFoundException
◦ EOFException
RIGHT
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import java.io.*;
public class Exc
{
public void openFile() throws IOException
{
try
{
FileWriter fw=new FileWriter("Filename.txt");
}
catch (IOException e)
{
e.printStackTrace();
}
}
}
class Exc2 extends Exc
{
public void openFile() throws FileNotFoundException
{
try
{
FileReader fw=new FileReader("Filename.txt");
}
catch (FileNotFoundException e)
{
e.printStackTrace();
}
}
}
WRONG
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import java.io.*;
public class Exc
{
public void openFile() throws FileNotFoundException
{
try
{
FileWriter fw=new FileWriter("Filename.txt");
}
catch (IOException e)
{
e.printStackTrace();
}
}
}
class Exc2 extends Exc
{
public void openFile() throws IOException
{
try
{
FileReader fw=new FileReader("Filename.txt");
}
catch (FileNotFoundException e)
{
e.printStackTrace();
}
}
}
RULES FOR METHOD
OVERRIDING

An overriding method doesn't have to
declare any exceptions that it will never
throw, regardless of what the overridden
method declares.
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import java.io.*;
public class Exc
{
public void openFile() throws IOException
{
try
{
FileReader fw=new FileReader("Filename.txt");
}
catch (FileNotFoundException e)
{
e.printStackTrace();
}
}
}
class Exc2 extends Exc
{
public void openFile()
{
System.out.println(“I’ll open the file later");
}
}
RULES FOR METHOD
OVERRIDING
You
cannot override a method marked
final.
You cannot override a method marked
static.
What happens here?
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public class Car
{
public final void maxSpeed()
{
System.out.println("Max speed is 60
mph");
}
}
class Ferrari extends Car
{
public void maxSpeed()
{
System.out.println("Max speed is 120
mph");
}
public void msc(){}
}
Invoking a Superclass Version of an
Overridden Method
 Use
the code in the overridden method.
 Also add extra features in overriding
method.
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public class Car
{
protected void maxSpeed()
{
System.out.println("Max speed is 60 mph");
}
}
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class Ferrari extends Car
{
public void maxSpeed()
{
super.maxSpeed();
System.out.println("I can race");
}
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}
Introduction to overloading
 Same
name, different arguments.
 Code deals with different argument types
rather than forcing the caller to do
conversions prior to invoking your method.
 It CAN have a different return type.
 Argument list MUST be different.
 Access modifier CAN be different.
 New exceptions can be declared.
 A method can be overloaded in the same
class or in a subclass.
In same class
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public class Sort
{
public void sort2(int[] a )
{
//Program to sort integers
}
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public void sort2(String[] a )
{
//Program to sort Strings
}
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public class TestSort
{
public void arr()
{
int[] a={3,8,6,1,2};
String[] s={"Sachin","Sourav","Dravid"};
Sort s=new Sort();
s.sort2(a);
s.sort2(s);
}
public static void main(String[] args)
{
TestSort t=new TestSort();
t.arr();
}
}
In different classes
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public class Sort
{
public void sort2(int[] a )
{
//Program to sort integers
}
}
class FloatSort extends Sort
{
public void sort2(double[] a )
{
//Program to sort floats
}
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public class TestSort
{
public void arr()
{
int[] a={3,8,6,1,2};
double[] f={3.5,6.8,1.4,67.9};
Sort s=new Sort();
s.sort2(a);
FloatSort s2=new FloatSort();
s2.sort2(f);
}
public static void main(String[] args)
{
TestSort t=new TestSort();
t.arr();
}
}
Will this work?
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public class TestSort
{
public void arr()
{
int[] a={3,8,6,1,2};
double[] f={3.5,6.8,1.4,67.9};
Sort s=new Sort();
s.sort2(a);
Sort s2=new FloatSort();
s2.sort2(f);
}
public static void main(String[] args)
{
TestSort t=new TestSort();
t.arr();
}
}
METHODS THAT ARE BOTH
OVERLOADED AND OVERRIDDEN
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public class Animal
{
public void eat()
{
System.out.println("Generic Animal Eating Generically");
}
}
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class Horse extends Animal
{
public void eat()
{
System.out.println("Horse eating hay ");
}
public void eat(String s)
{
System.out.println("Horse eating " + s);
}
}
public abstract interface Frobnicate
{ public void twiddle(String s); }
Given:
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Which is a correct class? (Choose all that apply.)
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A. public abstract class Frob implements Frobnicate {
public abstract void twiddle(String s) { }
}
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B. public abstract class Frob implements Frobnicate { }
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C. public class Frob extends Frobnicate {
public void twiddle(Integer i) { }
}
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D. public class Frob implements Frobnicate {
public void twiddle(Integer i) { }
}
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E. public class Frob implements Frobnicate {
public void twiddle(String i) { }
public void twiddle(Integer s) { }
}
ANSWER
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Answer:
B is correct, an abstract class need not
implement any or all of an interface’s methods.
E is correct, the class implements the interface
method and additionally overloads the twiddle()
method.
A is incorrect because abstract methods have
no body.
C is incorrect because classes implement
interfaces they don’t extend them.
D is incorrect because overloading a method is
not implementing it.
Given:
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class Clidder {
private final void flipper() { System.out.println("Clidder"); }
}
public class Clidlet extends Clidder {
public final void flipper() { System.out.println("Clidlet"); }
public static void main(String [] args) {
new Clidlet().flipper();
}}
What is the result?
A. Clidlet
B. Clidder
C. Clidder
Clidlet
D. Clidlet
Clidder
E. Compilation fails
ANSWER
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A is correct. Although a final method
cannot be overridden, in this case, the
method is private, and therefore hidden.
The effect is that a new, accessible,
method flipper is created. The method
invoked is simply that of the child class,
and no error occurs.
Given:
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1. class Plant {
2. String getName() { return "plant"; }
3. Plant getType() { return this; }
4. }
5. class Flower extends Plant {
6. // insert code here
7. }
8. class Tulip extends Flower { }
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Which statement(s), inserted at line 6, will
compile? (Choose all that apply.)
A. Flower getType() { return this; }
 B. String getType() { return "this"; }
 C. Plant getType() { return this; }
 D. Tulip getType() { return new Tulip(); }
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ANSWER
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A, C, and D are correct. A and D are
examples of co-variant returns, i.e.,
Flower and Tulip are both subtypes of
Plant.
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B is incorrect, String is not a subtype
of Plant.
Given
1. class Programmer {
 2. Programmer debug() { return this; }
 3. }
 4. class SCJP extends Programmer {
 5. // insert code here
 6. }
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Which, inserted at line 5, will compile?
(Choose all that apply.)
A. Programmer debug() { return this; }
 B. SCJP debug() { return this; }
 C. Object debug() { return this; }
 D. int debug() { return 1; }
 E. int debug(int x) { return 1; }
 F. Object debug(int x) { return this; }
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ANSWER
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A, B,E,F are correct.
References:
Head First Java, 2nd Edition,by Kathy Sierra
and Bert Bates.
 SCJP Sun Certified Programmer for Java 5
Study Guide (Exam 310-055),by Kathy
Sierra and Bert Bates.
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