B. General Questions about Java

Question 1

What are wrapper classes?

Answer 1

A wrapper class converts java primitives into objects. So a primitive wrapper class is a wrapper class that encapsulates, hides, or wraps data types from the eight primitive data types so that these can be used to create instantiated objects with methods in another class or in other classes. The primitive wrapper classes are found in the Java API.

Question 2

What is casting? and the types of casting with details?

Answer 2

Type casting is when you assign a value of one primitive data type to another type.

Widening Casting (automatically) - converting a smaller type to a larger type size
byte -> short -> char -> int -> long -> float -> double

Narrowing Casting (manually) - converting a larger type to a smaller size type
double -> float -> long -> int -> char -> short -> byte

Question 3

Can you use a primitive type when using Arraylist? Why or why not?

Answer 3

No. Because the list can only store objects and primitive types are not objects. Unless we cast them using the wrapper class.

Question 4

What is the difference between valueOf() and parseFloat()?

Answer 4

Float.parseFloat(String) converts the string to primitive float.
valueOf(String) - returns the Float object initialized with the value provided.

Question 5

Does Java support multiple inheritance?

Answer 5

No, Java does not support multiple inheritance. Each class can extend only one class but is able to implement more than one interfaces.

Question 6

Do you use “implement” or “extend” from one class to another?

Answer 6

Extend

Question 7

If the parentclass has a protected variable, can the subclass access it? Why or why not?

Answer 7

Yes. Because subclasses can acess public and protected variables, but not private.

Question 8

Why and when would you use inheritance?

Answer 8

It is useful for code reusability. To reuse attributes and methods of an existing class when creating a new class.

Question 9

What is inheritance?

Answer 9

Inheritance can be defined as the process where one class (called the subclass or child class) acquires the properties (methods and fields) of another class (called the superclass or parent class).

Inheritance provides reusability of code and can be used to add additional features to an existing class, without modifying it.

Question 10

What does the word Polymorphism mean?

Answer 10

Polymorphism means “many forms”.

Question 11

What is Polymorphism?

Answer 11

Polymorphism is the ability of programming languages to present the same inferface for different underlying data types. A plymorphic type is a type whose operation can also be applied to values of some other type.

In other words, when using inheritance, we inherit attributes and methods from the superclass/parent class. Polymorphism uses those attribues and/or methods to perform different tasks. This allows the developer to perform a single action in different ways.

Question 12

What are the types of Polymorphism?

Answer 12

There are two types of Polymorphism in Java:
• Compile-time polymorphism (Static binding) – Method overloading
• Runtime polymorphism (Dynamic binding) – Method overriding
We can perform polymorphism by Method Overloading and Method Overriding.

Question 13

What is the keyword used to allow one class to inherit the properties of another class?

Answer 13

extends

Question 14

Are constructors members of a class?

Answer 14

No. Constructors are not members, so they are not inherited by subclasses. But the subclass automatically acquires the default constructor of the superclass.

Question 15

What keyword do you use to call a parameterized constructor of the superclass?

Answer 15

super(values);

Question 16

What word do you use when a class inherits the properties of an interface?

Answer 16

Implements

Question 17

Can interfaces be extended by a class?

Answer 17

No.

Question 18

public interface Animal {
}

public class Mammal implements Animal {
}

public class Dog extends Mammal {
}

In this example, what keyword can you use to check wheter Mammal is actually an Animal, and dog is actually an Animal?

Answer 18

instancef

   public static void main(String args[]) {
      Mammal m = new Mammal();
      Dog d = new Dog();

  System.out.println(m instanceof Animal);
  System.out.println(d instanceof Mammal);
  System.out.println(d instanceof Animal);    }

Output:
true
true
true

Question 19

HAS-A relationship

These relationships are mainly based on the _____.

Answer 19

Usage. This determines whether a certain class HAS-A certain thing

Question 20

Name all the members that a subclass inherits from its superclass?

Answer 20

Fields, methods and nested classes.

Extra info: Constructors are not members, but can be invoked from the subclass.

Question 21

In Java, an Is-A relationship depends on _____

Answer 21

Inheritance.

Question 22

In Java, a Has-A relationship is also known as ______

Answer 22

Composition

Question 23

Can a class extend more than one class? Why or why not?

Answer 23

No.

However, a class can implement one or more interfaces.

Question 24

How would you define polymorphisim in simple words?

Answer 24

We can define polymorphism as the ability of a message to be displayed in more than one form.

Question 25

What is method overloading?

Answer 25

Method overloading is the compile-time polymorphism where two or more methods share the same name, with different parameters or signature and different return type.

Question 26

What is method overriding?

Answer 26

Method overriding is the runtime polymorphism having same method with same parameters or signature, but associated in different classes.

Question 27

Why do we need Method Overloading ?

Answer 27

If we need to do same kind of the operation with different ways i.e. for different inputs. It is hard to find many different meaningful names for single action.

Question 28

Method overloading can be done by changing:

Answer 28

The number of parameters in two methods.
The data types of the parameters of methods.
The Order of the parameters of methods.

Question 29

2. Which of the below are reserved keyword in Java?

A. array
B. goto
C. null
D. int

Answer 29

The goto and int are reserved keywords in java. array and null are not keywords in java.

Question 30

What’s the difference between a “class variable” and an “instance variable” in Java?

Answer 30

Class variables are declared with the keyword static.

Instance variables are declared without the static keyword.

Question 31

Are class variables stored inside the class memory or inside the object memory?

Answer 31

class memory

Question 32

Are instance variables stored inside the class memory or inside the object memory?

Answer 32

object memory.

Extra points: Each object will have its own copy of instance variables.

Question 33

Can we extend “final” classes in Java?

Answer 33

No

Question 34

True or false.

The “extend” keyword is used to extend a class in java.

Answer 34

False.

You can use the “extend” keyword to extend two interfaces.

Question 35

What will be the output of below program?

public class Test {
	public static void main(String[] args) {
		Super s = new Subclass();
		s.foo();
	}
}

class Super {
	void foo() {
		System.out.println("Super");
	}
}

class Subclass extends Super {
	static void foo() {
		System.out.println("Subclass");
	}
}

A. Compile time error
B. Super
C. Subclass
D. Runtime error

Answer 35

Correct Answer: A

Subclass foo() method can’t be static, it will give compile time error. This static method cannot hide the instance method from Super.

Remember… Instance variables are declared without the static keyword.

Question 36

Do instance variables have default values?

Answer 36

Yes.

For numbers, the default value is 0.
For boolean it is false, and
for Objects it is null.

Question 37

What is the default value of a boolean variable?

Answer 37

False

Question 38

What is the default value for numbers?

Answer 38

0

Question 39

What is the default value of Object variable?

Answer 39

null.

The “object” variable has default value of null if defined as an instance/static variable.

Question 40

Values can be assigned during the ______ or within the ______.

Answer 40

declaration

constructor

Question 41

What is an applet?

Answer 41

A java applet is program that can be included in a HTML page and be executed in a java enabled client browser. Applets are used for creating dynamic and interactive web applications.

Question 42

What is the size of double variable?

Answer 42

64 bit

Question 43

What is the default value of byte variable?

Answer 43

0

Question 44

True or False

A class always has a default constructor.

Answer 44

false

Question 45

What is an applet?

Answer 45

An applet is a Java program that runs in a Web browser.

Question 46

What is the default value of String variable?

Answer 46

null

Question 47

What of the following is the default value of a local variable?

A - null

B - 0

C - Depends upon the type of variable

D - Not assigned

Answer 47

Answer : D
Explaination
Local variables are not assigned any value by default.

Question 48

What is the default value of char variable?

Answer 48

‘\u0000’

char variable has default value of ‘\u0000’ if defined as an instance/static variable.

Question 49

True or false

Can a constructor be made private?

Answer 49

True

Yes, class with private constructor can not instantiated using new keyword.

Question 50

What is an Interface?

A - An interface is a collection of abstract methods.

B - Interface is an abstract class.

C - Interface is an concrete class.

D - None of the above.

Answer 50

Answer : C
Explaination
An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Question 51

Can constructor be made final?

Answer 51

No, this is not possible.

Question 52

What is a concrete class?

Answer 52

A concrete class is a class that has an implementation for all of its methods.

Question 53

An abstract class is a ____ class that cannot be used to create objects.

To access it, it must be ____ from another class.

Answer 53

restricted

inherited

Question 54

Both abstract class and interface are used for ______.

Answer 54

abstraction

Question 55

True or False

A final method in a superclass can be overwritreen.

Answer 55

False.

A final method in a superclass cannot be overridden in a subclass.

This guarantees that the final method implementation will be used by all direct and indirect subclasses in the hierarchy.

Question 56

Yes or no

Are abstract classes declared using the abstract modifier?

Answer 56

Yes

Question 57

Yes or no

Can concrete classes be declared using abstract modifier?

Answer 57

no

Question 58

True of False

An abstract class may or may not contain abstract methods.

Answer 58

True

Question 59

True or False

A concrete class cannot contain an abstract method.

Answer 59

True

Question 60

True or False
The subclass of abstract class in java must implement all the abstract methods unless the subclass is also an abstract class.

Answer 60

True