1. Define arrays? How to create and instantiate a one ...

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Pemmaiah.U.T, HOD, Dept. of BCA, Cauvery College, Gonikoppal JAVA Exam Guide

1. Define arrays? How to create and instantiate a one dimensional array reference variable?

Give Example?

An array is a collection of similar data types. Array is a container object that hold

values of homogenous type. It is also known as static data structure because size

of an array must be specified at the time of its declaration.

To use an array in a program, you must declare a variable to reference the array,

and you must specify the type of array the variable can reference. Here is the

syntax for declaring a one dimensional array variable −

Syntax

dataType[] identifier;

or

dataType identifier [];

Initialization of one dimensional Array new operator is used to initialize an array. Example : int[ ] arr = new int[10]; //10 is the size of array. or int[ ] arr = {10,20,30,40,50};

One More Example :

Following statement declares an array variable, myList, creates an array of 10

elements of double type and assigns its reference to myList −

double[] myList = new double[10];

Following picture represents array myList. Here, myList holds ten double values and the

indices are from 0 to 9.



2. Explain the use of for each style of loop with suitable example?

foreach loop is used to access elements of array. Using foreach loop you can access

complete array sequentially without using index of array.

Example :

class Test

{

public static void main(String[] args)

{

int[] arr = {10, 20, 30, 40};

for(int x : arr)

{

System.out.println(x);

}

}

}

Output :

10

20

30



40

3. Explain the use of length member in arrays?

The length member is used in array to find the length of the arrays.

For Example:

Class TestArray{

{

public static void main(String args[])

{

Int array[]={1,2,3,4,5};

System.out.println(array.length);

}

}

4. Define String in Java? How to initiate a Strings in java?

String is a Sequence of character in Java, widely used as objects.

Creating a String object

String can be created in number of ways, here are a few ways of creating string object.

1) Using a String literal

String literal is a simple string enclosed in double quotes " ". A string literal is treated as a

String object.

String str1 = "Hello";

Here str1 is an object of string and have assigned a value “Hello”.

2) Using new Keyword

String str3 = new String("Java");

Here str3 is an object of string and have assigned a value “Java”.



5. List any five methods associated with Strings?

charAt()

charAt() function returns the character located at the specified index. String str = "studytonight"; System.out.println(str.charAt(2)); Output : u

length()

length() function returns the number of characters in a String.

String str = "Count me";

System.out.println(str.length());

Output : 8

replace()

replace() method replaces occurances of character with a specified new

character.

String str = "Change me";

System.out.println(str.replace('m','M'));

Output : Change Me

toLowerCase()

toLowerCase() method returns string with all uppercase characters

converted to lowercase.

String str = "ABCDEF";

System.out.println(str.toLowerCase());

Output : abcdef

toUpperCase()

This method returns string with all lowercase character changed to

uppercase.

String str = "abcdef";

System.out.println(str.toUpperCase());

Output : ABCDEF



6. What are command line arguments?

The java command-line argument is an argument i.e. passed at the time of running the java program. The arguments passed from the console can be received in the java program and it can be used as an input. So, it provides a convenient way to check the behavior of the program for the different values. You can pass N (1,2,3 and so on) numbers of arguments from the command prompt.

Simple example of command-line argument in java

In this example, we are receiving only one argument and printing it. To run this java

program, you must pass at least one argument from the command prompt.

class CommandLineExample

{

public static void main(String args[]){

System.out.println("Your first argument is: "+args[0]);

}

}

compile by > javac CommandLineExample.java

run by > java CommandLineExample CrossMove

Output: Your first argument is: CrossMove

7. Define Class and Objects?

Class can be defined as a template/ blueprint that describe the behaviors /states of a

particular entity.

A class defines new data type. Once defined this new type can be used to create object

of that type.

Object is an instance of class. You may also call it as physical existence of a logical template

class.

A class is declared using class keyword. A class contain both data and code that

operate on that data. The data or variables defined within a class are called instance variables

and the code that operates on this data is known as methods.



Syntax:

public class class_name { Data Members; Methods; }

8. What is the purpose of new operator?

The new operator dynamically allocates memory for an object.

Student is a class and student's name, roll number, age will be its property. Lets see this

in Java syntax

class Student. { String name; int rollno; int age; }

When a reference is made to a particular student with its property then it becomes an

object, physical existence of Student class.

Student std=new Student();

After the above statement std is instance/object of Student class. Here the new keyword

creates an actual physical copy of the object and assign it to the std variable. It will have

physical existence and get memory in heap area. The new operator dynamically allocates

memory for an object

9. Define Constructors? What is the purpose of Constructors?

A constructor is a special method that is used to initialize an object.

Java constructor is invoked at the time of object creation

There are basically two rules defined for the constructor.

Constructor name must be same as its class name

Constructor must have no explicit return type



10. Explain the types of constructors in java?

There are two types of constructors:

Default constructor (no-arg constructor)

Parameterized constructor

Java Default Constructor

A constructor that have no parameter is known as default constructor.

In this example, we are creating the no-arg constructor in the Bike class. It will be

invoked at the time of object creation.

class Bike1{ Bike1(){System.out.println("Bike is created");} public static void main(String args[]){ Bike1 b=new Bike1(); } } Output: Bike is created Java parameterized constructor

A constructor that have parameters is known as parameterized constructor.

Parameterized constructor is used to provide different values to the distinct objects.

In this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor. class Student4{ int id; String name; Student4(int i,String n){ id = i; name = n; } void display(){System.out.println(id+" "+name);}



public static void main(String args[]){ Student4 s1 = new Student4(111,"Pradhan"); Student4 s2 = new Student4(222,"Suman"); s1.display(); s2.display(); } } OutPut: 111 Pradhan

222 Suman

11. What is garbage collections?

Automatic Technique used in Java Programming which is used to de-allocate

memory is called as “Garbage Collection“.

Garbage Collection is Done Automatically by JVM.

As soon as compiler detects that – Object is no longer needed inside program,

Garbage Collection Algorithm gets executed automatically to free up memory

from the heap so that free memory may be used by other objects.

12. What is the use of finalize method () in java?

finalize() is called by the garbage collector on an object when garbage collection

determines that there are no more references to the object.

finalize() method is called by garbage collection thread before collecting object.

It’s the last chance for any object to perform cleanup utility.

Syntax of finalize() method

protected void finalize()

{

//finalize-code

}

13. What is the use of this keyword?

In java, this is a reference variable that refers to the current object.

this() keyword can be used to refer current class instance variable.

this() can be used to invoke current class constructor.

this() keyword can be used to invoke current class method (implicitly)

this() can be passed as an argument in the method call.



this() can be passed as argument in the constructor call.

this() keyword can also be used to return the current class instance.

14. List different access modifiers in Java?

There are 4 types of java access modifiers:

private

default

protected

public

private access modifier

The private access modifier is accessible only within class.

Simple example of private access modifier

In this example, we have created two classes A and Simple. A class contains private

data member and private method. We are accessing these private members from

outside the class, so there is compile time error.

class A{

private int data=40;

private void msg(){System.out.println("Hello java");}

}

public class Simple{


A obj=new A();

System.out.println(obj.data);//Compile Time Error

obj.msg();//Compile Time Error

}

}

default access modifier

If you don't use any modifier, it is treated as default by default. The default modifier is

accessible only within package.

Example of default access modifier

In this example, we have created two packages pack and mypack. We are accessing the A

class from outside its package, since A class is not public, so it cannot be accessed from

outside the package.



//save by A.java

package pack;

class A{

void msg(){System.out.println("Hello");}

}

//save by B.java

package mypack;

import pack.*;

class B{


A obj = new A();//Compile Time Error

obj.msg();//Compile Time Error } }

In the above example, the scope of class A and its method msg() is default so it cannot be

accessed from outside the package.

protected access modifier

The protected access modifier is accessible within package and outside the package but

through inheritance only.

The protected access modifier can be applied on the data member, method and

constructor. It can't be applied on the class.

Example of protected access modifier

In this example, we have created the two packages pack and mypack. The A class of pack

package is public, so can be accessed from outside the package. But msg method of this

package is declared as protected, so it can be accessed from outside the class only through

inheritance.

//save by A.java

package pack;

public class A{

protected void msg(){System.out.println("Hello");}

}

//save by B.java

package mypack;

import pack.*;

class B extends A{




B obj = new B();

obj.msg();

}

}

Output:Hello

public access modifier

The public access modifier is accessible everywhere. It has the widest scope among all

other modifiers.

Example of public access modifier

//save by A.java

package pack;

public class A{

public void msg(){System.out.println("Hello");}

}

//save by B.java

package mypack;

import pack.*;

class B{


A obj = new A();

obj.msg();

}

}

Output:Hello

15. Explain how to pass objects to methods with suitable example?

We can pass Object of any class as parameter to a method in java.

We can access the instance variables of the object passed inside the called

method.

It is good practice to initialize instance variables of an object before passing object

as parameter to method otherwise it will take default initial values.

Example :

void area(Rectangle r1) {



int areaOfRectangle = r1.length * r1.width;

System.out.println("Area of Rectangle : "

+ areaOfRectangle);

}

class RectangleDemo {

public static void main(String args[]) {

Rectangle r1 = new Rectangle(10, 20);

r1.area(r1);// here we passing entire object as parameters through methods

}

16. What is method overloading? Explain with an Example?

If a class have multiple methods by same name but different parameters, it is

known as Method Overloading.

Compile determine which method to execute automatically.

Example:

class Rectangle {

double length;

double breadth;

void area(int length, int width) {

int areaOfRectangle = length * width;

System.out.println("Area of Rectangle : " + areaOfRectangle);

}

void area(double length, double width) {

double areaOfRectangle = length * width;

System.out.println("Area of Rectangle : " + areaOfRectangle);

}

}

class RectangleDemo {

public static void main(String args[]) {

Rectangle r1 = new Rectangle();

r1.area(10, 20);

r1.area(10.50, 20.50);

}



}

Explanation:

We have defined 2 methods with same name and different type of parameters.

When both integer parameters are supplied to the method then it will execute

method with all integer parameters and if we supply both floating point numbers

as parameter then method with floating numbers will be executed.

17. What is the purpose of Static keyword?

Static is a keyword that acts as a non-access modifiers in java that is used to manage

memory.

18. What is a Static variable? Example?

If you declare any variable as static, it is known static variable.

The static variable can be used to refer the common property of all objects (that

is not unique for each object) e.g. company name of employees, college name of

students etc.

The static variable gets memory only once in class area at the time of class loading.

Advantage of static variable

It makes your program memory efficient (i.e it saves memory).

As we have mentioned above, static variable will get the memory only once, if any object changes the value of the static variable, it will retain its value. class Counter2{ static int count=0;//will get memory only once and retain its value Counter2(){ count++; System.out.println(count); } public static void main(String args[]){ Counter2 c1=new Counter2(); Counter2 c2=new Counter2(); Counter2 c3=new Counter2(); } } Output:1



2 3

19. What is a Static methods? Example?

If you apply static keyword with any method, it is known as static method.

A static method belongs to the class rather than object of a class.

A static method can be invoked without the need for creating an instance of a

class.

static method can access static data member and can change the value of it.

Restrictions for static method

There are two main restrictions for the static method. They are:

The static method cannot use non static data member or call non-static method

directly.

this and super cannot be used in static context.

Example:

class Calculate{

static int cube(int x){

return x*x*x;

}


int result=Calculate.cube(5);

System.out.println(result);

}

}

Output: 125

20. Why java main method is static?

Because object is not required to call static method if it were non-static method, jvm

create object first then call main() method that will lead the problem of extra memory

allocation.

21. What is a Static blocks? Example?

Is used to initialize the static data member.

It is executed before main method at the time of class loading.



Example:

class A2{

static{System.out.println("static block is invoked");}


System.out.println("Hello main");

}

}

Output:

static block is invoked

Hello main

22. Explain nesting of classes with example?

When we declares a class within the body of another class or interface then class is called

as nested class.

There are two types of nested classes:

Static inner classes and non-static nested classes.

Non-static nested classes are called inner classes.

Three Types of the Inner Classes are listed below –

Member inner classes

Local inner classes

Anonymous inner classes

Example of Nested Class:

public class OuterClass {

class NestedClass {

}

}

In the above example we can say that InnerClass is class which is enclosed in the

OuterClass.

23. Explain about inner classes in java?

Non-static nested classes are called inner classes. Three Types of the Inner Classes are

listed below –

Member inner classes

Local inner classes

Anonymous inner classes



Member Inner Classes:

A Member Inner Class is a class whose definition is directly enclosed by another

class or interface declaration

Instance of a Member Inner Class can only be created if we have a reference to an

instance of outer class.

A local Inner Class :

Local Inner Class is not a member class of any other class.

Its declaration is not directly within the declaration of the outer class.

Local Inner Class can be declared inside particular block of code and its scope is

within the block.

Anonymous Inner Classes :

A class which does not have name is called as anonymous inner class. Anonymous

class can be created using two methods :

By using the interface

By using the abstract class

Anonymous Inner Classes are used for writing an interface implementation.

24. Explain how to pass variable length arguments with suitable arguments?

The varrags allows the method to accept zero or muliple arguments. Before

varargs either we use overloaded method or take an array as the method

parameter but it was not considered good because it leads to the maintenance

problem.

If we don't know how many argument we will have to pass in the method, varargs

is the better approach.

Syntax of varargs:

The varargs uses ellipsis i.e. three dots after the data type. Syntax is as follows:

return_type method_name(data_type... variableName){}



class VarargsExample1{

static void display(String... values){

System.out.println("display method invoked ");

}


display();//zero argument

display("my","name","is","varargs");//four arguments

}

}

Output:

display method invoked

display method invoked

25. Define inheritance?

Inheritance can be defined as the process where one class acquires the properties

(methods and fields) of another.

The class which inherits the properties of other is known as subclass (derived class,

child class) and the class whose properties are inherited is known as superclass

(base class, parent class).

extends is the keyword used to inherit the properties of a class.

Syntax

class Super {

.....

.....

}

class Sub extends Super {

.....

.....

}

Example:

Class Super{

Int a=10;

}

Class Sub extends Super{

Public static void main(String args[])

{

Int b=20;

System.out.println(a);



System.out.println(b);

}

}

Output: 10

20

26. Why use inheritance in java?

For Method Overriding (so runtime polymorphism can be achieved).

For Code Reusability.

27. What is the purpose of Keyword extends?

extends is the keyword used to inherit the properties of a class.

28. What is the purpose of super keyword?

The super keyword in java is a reference variable that is used to refer immediate parent

class object.

Usage of java super Keyword

super is used to refer immediate parent class instance variable.

super() is used to invoke immediate parent class constructor.

super is used to invoke immediate parent class method.

class Vehicle{

int speed=50;

}

class Bike4 extends Vehicle{

int speed=100;

void display(){

System.out.println(super.speed);//will print speed of Vehicle now

}


Bike4 b=new Bike4();

b.display();

}

}

29. Explain how a super is used to invoke parent class constructor.

The super keyword can also be used to invoke the parent class constructor as given below:



class Vehicle{

Vehicle(){System.out.println("Vehicle is created");}

}


Bike5(){

super();//will invoke parent class constructor

System.out.println("Bike is created");

}


Bike5 b=new Bike5();

} }

As we know well that default constructor is provided by compiler automatically but it also

adds super() for the first statement.If you are creating your own constructor and you don't

have either this() or super() as the first statement, compiler will provide super() as the

first statement of the constructor.

30. Define method overriding?

If subclass (child class) has the same method as declared in the parent class, it is known

as method overriding in java.

Or

Method with same name, same parameters and with same return type is known as

method overriding.

Rules for Java Method Overriding

method must have same name as in the parent class

method must have same parameter as in the parent class.

class Vehicle{

void run(){System.out.println("Vehicle is running");}

}


void run(){System.out.println("Bike is running safely");}


Bike2 obj = new Bike2();

obj.run();

}



In this example, we have defined the run method in the subclass as defined in the parent

class but it has some specific implementation. The name and parameter of the method is

same and there is IS-A relationship between the classes, so there is method overriding.

31. Difference between method overriding and method overloading?

1) Method overloading is used to increase the readability of the program.

Method overriding is used to provide the specific implementation of the method

that is already provided by its super class.

2) Method overloading is performed within class.

Method overriding occurs in two classes that have IS-A (inheritance) relationship.

3) In case of method overloading, parameter must be different.

In case of method overriding, parameter must be same.

4) Method overloading is the example of compile time polymorphism.

Method overriding is the example of run time polymorphism.

5) In java, Return type can be same or different in method overloading.

Return type must be same in method overriding.

32. Define abstract class? Example?

A class that is declared as abstract is known as abstract class. It needs to be extended and its method implemented. It cannot be instantiated. To use an abstract class, you have to inherit it from another class, provide

implementations to the abstract methods in it. abstract class Bike{

abstract void run();

}

class Honda4 extends Bike{

void run()

{

System.out.println("running safely..");// body implementation

}


Bike obj = new Honda4();

obj.run();

}

Output: running safely..

In the above example Bike is an abstract class. This class can’t create any objects.



The methods declared inside the abstract class is called abstract methods. But those methods can’t define their body over there. The implementation should be done in the derived class. i.e., Honda4 is the derived class above. There we can implements run() method.

33. What is the purpose of final variables?

If you make any variable as final, you cannot change the value of final variable(It will be

constant).

Example of final variable

There is a final variable speedlimit, we are going to change the value of this variable, but

It can't be changed because final variable once assigned a value can never be changed.

class Bike9{

final int speedlimit=90;//final variable

void run(){

speedlimit=400;

}


Bike9 obj=new Bike9();

obj.run();

}

}//end of class

Output: compile time error

34. What is final Class in Java?

If you make any class as final, you cannot extend it(inheritance is not possible).

final class Bike{}

class Honda1 extends Bike{

void run(){System.out.println("running safely with 100kmph");}


Honda1 honda= new Honda();

honda.run();

}

}

Output: Compile time error.

In above example Bike class can’t be derived because it is declared as final class .



35. What are final methods in Java?

If you make any method as final, you cannot override it. class Bike{ final void run(){System.out.println("running");} } class Honda extends Bike{ void run(){System.out.println("running safely with 100kmph");} public static void main(String args[]){ Honda honda= new Honda(); honda.run(); } } Output: Compile Time Error

36. What is an Object Class? The Object class is the parent class of all the classes in java by default. In other words, it is the topmost class of java.

37. What is recursion in Java?

Recursion in java is a process in which a method calls itself continuously. A method in java that calls itself is called recursive method. Syntax: returntype methodname(){ //code to be executed methodname();//calling same method } // Program to find factorial public class RecursionExample3 { static int factorial(int n){ if (n == 1) return 1; else return(n * factorial(n-1)); } public static void main(String[] args) { System.out.println("Factorial of 5 is: "+factorial(5)); } } Output:Factorial of 5 is: 120

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