Computer Knowledge

Object-Oriented Programming

2,686 Questions

Object-oriented programming questions test core computer science concepts like classes, inheritance, polymorphism, and encapsulation. The focus includes Java program structures, method overloading, and memory allocation for objects. This topic is essential for technical sections in various recruitment tests.

Java class definitionsMethod overriding rulesPolymorphism conceptsGeneric type parametersMemory allocation in objects

Object-Oriented Programming Questions

Multiple choice technology programming languages

Given: 1. class Example { 2. public static void main(String[] args) { 3. Short s = 15; 4. Boolean b; 5. // insert code here 6. } 7. } Which, inserted independently at line 5, will compile? (Choose all that apply.)

  1. b = (Number instanceof s);

  2. b = (s instanceof Short);

  3. b = s.instanceof(Short);

  4. b = (s instanceof Number);

  5. b = s.instanceof(Object);

  6. b = (s instanceof String);
Reveal answer Fill a bubble to check yourself
B,D Correct answer
Explanation

Short is a wrapper class and inherits from Number. Options B and D are correct: s instanceof Short checks if s is a Short instance (true), and s instanceof Number checks if s is a Number instance (also true, since Short extends Number). Option A has reversed instanceof syntax. Options C, E use dot notation instead of instanceof keyword.

Multiple choice technology programming languages

Given: 1. class Zippy { 2. String[] x; 3. int[] a [] = {{1,2}, {l}}; 4. Object c = new long [4] ; 5. Object[] d = x; 6. } What is the result?

  1. Compilation succeeds.

  2. Compilation fails due only to an error on line 3

  3. Compilation fails due only to an error on line 4

  4. Compilation fails due only to an error on line 5

  5. Compilation fails due to errors on lines 3 and 5

  6. Compilation fails due to errors on lines 3, 4, and 5
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice technology programming languages

Given: 1. class Eco { 2. public static void main(String[] args) { 3. Eco e1 = new Eco(); 4. Eco e2 = new Eco(); 5. Eco e3 = new Eco(); 6. e3.e = e2; 7. e1.e = e3; 8. e2 = null; 9. e3 = null; 10. e2.e = el; 11. e1 = null; 12. } 13. Eco e; 14. } At what point is only a single object eligible for GC?

  1. After line 8 runs

  2. After line 9 runs.

  3. After line 10 runs.

  4. After line 11 runs.

  5. An exception is thrown at runtime.

  6. Never in this program
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

Line 10 attempts e2.e = el, but e2 was set to null on line 8. Accessing e2.e throws NullPointerException. Note: el is a typo (should be e1), but the null dereference happens first.

Multiple choice technology programming languages

Given: class Clidders { public final void flipper() { System.out.print(" Flip a Clidder"); } } public class Clidlets extends Clidders { public void flipper() { System.out.print(" Flip a Clidlet"); super.flipper(); } public static void main(String [] args) { new Clidlets().flipper(); } } What is the result?

  1. Flip a Clidlet

  2. Flip a Clidder

  3. Flip a Clidder Flip a Clidlet

  4. Flip a Clidlet Flip a Clidder

  5. Compilation fails
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

A method marked as final cannot be overridden in subclasses. The parent class Clidders declares flipper() as final, but the child class Clidlets attempts to override it with its own flipper() method. This violates Java's rules for final methods, causing a compilation error.

Multiple choice technology programming languages

Given:class Clidder { private final void flipper() { System.out.print(" Flip a Clidder"); }}public class Clidlet extends Clidder { public final void flipper() { System.out.print(" Flip a Clidlet"); } public static void main(String [] args) { new Clidlet().flipper(); }}What is the result?

  1. Flip a Clidlet

  2. Flip a Clidder

  3. Flip a Clidlet Flip a Clidder

  4. Flip a Clidder Flip a Clidlet

  5. Compilation fails.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Private methods are not visible to subclasses and therefore cannot be overridden. The Clidder class has a private final flipper(), but since it's private, Clidlet can define its own public flipper() method without any conflict. This is not overriding but defining a new method. The code compiles and outputs 'Flip a Clidlet'.

Multiple choice technology programming languages

Given: public class MyOuter { public static class MyInner { public static void foot) { } } } Which, if placed in a class other than MyOuter or MyInner, instantiates an instance of the nested class?

  1. MyOuter.MyInner m = new MyOuter.Mylnner();

  2. MyOuter.MyInner mi = new MyInner();

  3. MyOuter m = new Myouter(); MyOuter.MyInner mi = m.new Myouter.MyInner();

  4. MyInner mi = new MyOuter.MyInner();
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

MyInner is a static nested class. To instantiate it from outside MyOuter, we use the syntax MyOuter.MyInner m = new MyOuter.MyInner();. Option A has a typo ('Mylnner' with an 'l' instead of 'I' in the constructor call), but conceptually it represents this correct instantiation syntax.

Multiple choice technology programming languages

Given: public class Foo { Foo() {System.out.print("foo");} class Bar{ Bar() {System.out.print("bar");} public void go() {System.out.print("hi");} } public static void main(String[] args) { Foo f = new Foo (); f.makeBar(); } void makeBar() { (new Bar() {}).go(); } } What is the result?

  1. Compilation fails.

  2. An error occurs at runtime.

  3. foobarhi

  4. barhi

  5. hi

  6. foohi
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

new Foo() prints 'foo'. makeBar() instantiates an anonymous subclass of inner class Bar using new Bar() {}, which executes Bar's constructor and prints 'bar'. It then calls go() which prints 'hi', resulting in 'foobarhi'.

Multiple choice technology programming languages

package overriding; class Super1 { void testSuper(int empId) { System.out.println("I am in Super " + empId); } } public class TestOverrideSub1 extends Super1 { private void testSuper(int empId) { System.out.println("I am in Sub " + empId); } public static void main(String[] args) { Super1 s1 = new TestOverrideSub1(); s1.testSuper(144670); } }

  1. I am in Sub 144670

  2. Compiler Error

  3. I am in Super 144670

  4. Exception
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Overriding cannot reduce visibility. The superclass has default (package-private) access, while the subclass tries private. This violates Java's overriding rules, causing a compiler error.

Multiple choice technology programming languages

package overriding; class Super3 { void testDivide(int i, int j) throws Exception { double p = i / j; System.out.println("p in Super =" + p); } } public class TestOverrideSub3 extends Super3 { void testDivide(int i, int j) throws ArithmeticException { double p = i / j; System.out.println("p in Sub =" + p); } public static void main(String[] args) { Super3 s3 = new TestOverrideSub3(); try { s3.testDivide(10, 5); } catch (Exception e) { System.out.println("Exception " + e.getMessage()); } } }

  1. p in Super =2.0

  2. p in Sub =2.0

  3. Compiler error

  4. Exception
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The subclass narrows the thrown exception from Exception to ArithmeticException, which is allowed because ArithmeticException is a subclass of Exception. Due to polymorphism, the subclass version runs, printing p in Sub.

Multiple choice technology programming languages

package overriding; class Super2 { void testSuper(int empId) { System.out.println("I am in Super " + empId); } } public class TestOverrideSub2 extends Super2 { public void testSuper(int empId) { System.out.println("I am in Sub " + empId); } public static void main(String[] args) { Super2 s2 = new TestOverrideSub2(); s2.testSuper(144670); } }

  1. I am in Sub 144670

  2. I am in Super 144670

  3. Compiler error

  4. Exception
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The subclass widens access from default to public, which is valid in overriding. When calling through a superclass reference pointing to a subclass object, Java uses the actual object's type (Sub), so the subclass version executes.

Multiple choice technology programming languages

package overriding; class Super4 { final void testSuper() { System.out.println("I am Super"); } } public class TestOverrideSub4 extends Super4{ void testSuper() { System.out.println("I am Sub"); } public static void main(String[] args) { Super4 s4 = new TestOverrideSub4(); s4.testSuper(); } }

  1. Compiler Error

  2. I am Sub

  3. I am Super

  4. Exception
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A method marked final cannot be overridden. The superclass declares testSuper() as final, but the subclass attempts to override it. This is a compiler error.

Multiple choice technology programming languages

package overriding; class Super5 { int testAdd(int x,int y) { System.out.println("Super Add "+(x+y)); return(x+y); } } public class TestOverrideSub5 extends Super5{ long testAdd(int x,int y) { System.out.println("Sub Add "+(x+y)); return(x+y); } public static void main(String[] args) { Super5 s5 = new TestOverrideSub5(); s5.testAdd(5,5); } }

  1. Compiler Error

  2. Exception

  3. Sub Add 10

  4. Super Add 10
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Overriding requires identical method signatures including return type. Changing return type from int to long with the same parameters is not a valid override in Java. This causes a compiler error.

Multiple choice technology programming languages

package simplejava; public class TestMain1 { TestMain1() { System.out.println("I am in Constructor"); } TestMain1(String message) { System.out.println("Message " + message); } public void main(String[] args) { TestMain1 nm = new TestMain1(); TestMain1 nm1 = new TestMain1("Hello"); } }

  1. Exception

  2. Compiler Error

  3. I am in Constructor

  4. Message Hello
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The main method is missing the 'static' keyword. Java requires the main method to be public static void to serve as an entry point - the JVM must be able to call it without instantiating an object. A non-static main method cannot be invoked by the JVM, causing an exception at runtime.

Multiple choice technology programming languages

_______ is a construct that defines objects of the same type.

  1. A class

  2. An object

  3. A method

  4. A data field
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A class is a blueprint or template that defines the structure and behavior of objects of the same type. An object is a specific instance (option B is wrong). A method is a function defined in a class (option C is wrong). A data field is a single variable within a class (option D is wrong).

Multiple choice technology programming languages

An object is an instance of a __________.

  1. program

  2. class

  3. method

  4. data
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

An object is created as an instance of a class. The class defines the type, and when you instantiate it with 'new', you create an object that belongs to that class type. A program contains many objects, methods are behaviors defined in classes, and data is what objects store.