Tutorial Java: Core java Interview Questions

Famous java interview question: difference between collections and collection in java
Major difference between Collection and Collections is Collection is an interface and Collections is a class.
Both are belongs to java.util package
Collection is base interface for list set and queue.
Collections is a class and it is called utility class.
Collections utility class contains some predefined methods so that we can use while working with Collection type of classes(treeset, arraylist, linkedlist etc.)
Collection is base interface for List , Set and Queue.

Collection vs Collections

public interface Collection<E>
extends Iterable<E>

public class Collections
extends Object

difference between collections and collection in java

Collections utility class contains static utility methods so that we can use those methods by using class name without creating object of Collections class object
Lest see some methods of Collections class.

addAll: public static <T> boolean addAll(Collection<? super T> c,T... elements)
reverseOrder: public static <T> Comparator<T> reverseOrder()
shuffle: public static void shuffle(List<?> list)
sort:public static <T extends Comparable<? super T>> void sort(List<T> list)

How to Relate Collection and Collections

ArrayList is a Collection type of class means it is implementing Collection interface internally
Now lets see a java example program to sort ArrayList of elements using Collections.sort() method.

public class ArrayList<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, Serializable

1.Basic Java example program to sort arraylist of integers using Collections.sort() method

package com.javasortarraylistofobjects;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
public class SortArrayListExample{
public static void main(String[] args) {
//create an ArrayList object
ArrayList<Integer> arrayList = new ArrayList<Integer>();
//Add elements to Arraylist
arrayList.add(10);
arrayList.add(7);
arrayList.add(11);
arrayList.add(4);
arrayList.add(9);
arrayList.add(6);
arrayList.add(2);
arrayList.add(8);
arrayList.add(5);
arrayList.add(1);
System.out.println("Before sorting ArrayList ...");
Iterator itr=arrayList.iterator();
while (itr.hasNext()) {
System.out.println(itr.next());
}
/*
To sort an ArrayList object, use Collection.sort method. This is a
static method. It sorts an ArrayList object's elements into ascending order.
*/
Collections.sort(arrayList);
System.out.println("After sorting ArrayList ...");
Iterator itr1=arrayList.iterator();
while (itr1.hasNext()) {
System.out.println(itr1.next());
}
}
}

Output:

Before sorting ArrayList ...
10
7
11
4
9
6
2
8
5
1
After sorting ArrayList ...
1
2
4
5
6
7
8
9
10
11

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Basic Java Example programs Core java Interview Questions

Java Program to find missing numbers in an array

Kris Kimcil 07.54 Komentar

To find missing numbers in an array first we need to make sure that array is sorted.
After sorting we need to check that array each element with next element then we can find the difference.
if Array is not sorted :To sort array use Arrays.sort(array);
If difference is 1 then no need to do any thing because numbers are in order.
If difference is not equal to 1 then we need to print all those numbers or pick those numbers and place it in one array.
this would be the logic to find missing numbers in an array
Here there may be a chance of array not starts with 1 then we need to check first itself whether array starts with 1 or not if not we need to print 1 to starting element of array.
for example int a[]={4,5,6,8}; then we need to print 1 2 3 7.

Lets see a java example program to find missing numbers in an array.

package arraysInterviewQuestions;
public class PrintMissingNumbers {
private static void findMissingNumber(int[] number){
// take max length as last number in array
int k[] = new int[number[number.length-1]];
int m=0;
if(number[0]!=1){
for (int x = 1; x < number[0]; x++) {
k[m] = x;
m++;
}
}
for (int i = 0; i < number.length -1; i++) {
int j = i+1;
int difference = number[j] - number[i] ;
if(difference != 1 ){
for (int x = 1; x < difference; x++) {
k[m] = number[i] + x;
m++;
}
}
}
System.out.println("missing numbers in array ::");
for (int l = 0; l < m ; l++) {
System.out.println( k[l]+" ");
}
}
public static void main(String[] args) {
int a[]= {2,4,6,9,10,20};
//if Array is not sorted :To sort array use Arrays.sort(a);
findMissingNumber(a);
}
}

program to find missing number in an array in java

Change the array in main method and practice try to find missing elements in an array that contains 1 to 100 numbers.

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Jumat, 17 Juni 2016

Core java Interview Questions

Dynamic polymorphism in java with example program

Kris Kimcil 11.53 Komentar

Polymorphism means defining multiple methods with same name.

Two types of polymorphism
1.Static polymorphism
2.Dynamic polymorphism.

Static polymorphism:

Defining multiple methods with same name and with different type of arguments is known as static polymorphism.
We can achieve this static polymorphism using method overloading.

Static polymorphism in java with example program

package com.instanceofjava;
class A{
public void show(int a){
System.out.println("saidesh");
}
public void show(int a,int b){
System.out.println("ajay");
}
public void show(float a){
System.out.println("vinod");
}
public static void main(String args[]){
A a=new A();
a.show(10);
a.show(1,2);
a.show(1.2);
}
}

Output:

saidesh
ajay
vinod

Dynamic polymorphism:

Defining multiple methods with same name and with same signature in super class and sub class.
We can achieve dynamic polymorphism by using method overriding concept in java.
Define a method in super class and override same method with same signature in sub class
Whenever we call the method on t the object based on the object corresponding class method will be executed dynamically.

Java example program on dynamic polymorphism: (Real time example)

package MethodOverridingExamplePrograms;
public class Vehicle{
void Start(){
System.out.println("Vehicle started")
}
}

package MethodOverridingExamplePrograms;
public class Car extends Vehicle{
void Start(){
System.out.println("Car started")
}
}

package MethodOverridingExamplePrograms;
public class Bus extends Vehicle{
void Start(){
System.out.println("Bus started")
}
}

package MethodOverridingExamplePrograms;
public class Bike extends Vehicle{
void Start(){
System.out.println("Bike started")
}
}

package MethodOverridingExamplePrograms;
public class Test{
public static void startVehicle(Vehicle vh){
vh.start();
}
}

Dynamic polymorphism in java with example program

In the above example based on the object creation at run time corresponding class method will be executed.

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Jumat, 10 Juni 2016

Core java Interview Questions java interview programs

toString() method in java with example program

Kris Kimcil 21.53 Komentar

toString() method belongs to object class.
By default it will be called once whenever we try to print object.
toString() method by default will print classname@hex representation of hashcode.
We can override this toString() method from object class.

public String toString(){
return null;
}

Advantage of toString() method in java.

When ever we try to print object by default toString() method will be called and print classname@HEX_hashcode.
We can represent object in String format using toString() method.
if we want to represent string representation of object then we need to override toString() method in our class and return values of the object as a String.

Java example program to print object without using toString() method.

Lets see an example program on printing object of a class.

package tostringexamples;
public class ToStringDemo {
int a,b;
ToStringDemo(int x, int y){
a=x;
b=y;
}
public static void main(String[] args) {
ToStringDemo obj= new ToStringDemo(1,2);
ToStringDemo obj2= new ToStringDemo(3,4);
System.out.println(obj);
System.out.println(obj2);
}
}

Output:

tostringexamples.ToStringDemo@2a139a55
tostringexamples.ToStringDemo@15db9742

In the above program when we print object of ToStringDemo class it prints ToStringDemo@2a139a55.
It means when ever we print object of the class toString() method will be called and by default toString() method print classname@HEX_hashcode.
To test this now we will override the toString() method and prints object of the class and if it calls our method then we can understand that when ever we print object by default toString() method will be called.

Java example program to print object by overriding toString() method.

override tostring method in java example

In above example when we print object it executed overridden toString() method.
By overriding toString() method we can represent object in string form.

toString() method used to convert data to String type.

By using toString method we can convert any wrapper object to string object

Java example program to convert integer to string using toString() method.

package tostringexamples;
public class ToStringDemo {
public static void main(String[] args) {
Integer a=12;
Integer b=37;
String str1=a.toString();
String str2 =b.toString();
System.out.println(str1);
System.out.println(str2);
}
}

Output:

Java example program to convert Float to String using toString() method.

package tostringexamples;
public class ToStringDemo {
public static void main(String[] args) {
Float a=12.0f;
Float b=33.4f;
String str1=a.toString();
String str2 =b.toString();
System.out.println(str1);
System.out.println(str2);
}
}

Output:

12.0
33.4

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Kamis, 28 April 2016

Core java Interview Questions

Control statements in java with examples

Kris Kimcil 22.56 Komentar

Programs in java will be executed in sequential manner means line by line execution of statements.
By using control statements we can control the flow of execution.
We have three types of control statements in java.

Decision making Statements.
Iteration statements.
Branching statements.

1.Decision making statements in java:

In some cases we will have a situation like need to execution set of statements based on a condition
In this scenario we need to use decision making statements in java.

if
if-else
if-else-if
switch

Simple if

Based on some condition if we want to execute some set of statements then we need to use if condition in java.
We need to keep all the required statements inside if block .
If the condition is true then control executes the whole block otherwise it skips the if block of statements.

if( condition ){
// code
}

Java example program on if control statement:

package controlstatementsinjava;
import java.util.Scanner;
public class ControlStatementsInjava {
/**
* @website: www.instanceofjava.com
*/
public static void main(String[] args) {
Scanner in= new Scanner(System.in);
System.out.println("Please enter a number");
int n=in.nextInt();
if(n%2==0){
System.out.println(n+" is even number");
}
if(n%2!=0){
System.out.println(n+" is odd number");
}
}
}

Output:

Please enter a number
5
5 is odd number

If else condition:

If the condition is true then control executes the whole block otherwise it skips the if block of statements
if condition is false if we need to execute another set of statement we can write another if and keep all the statements in that block.
instead of checking multiple times we can simply use else block .
More about in else statement in java

if( condition ){
// code
}else{
//code
}

if else control statement with example programs:

package controlstatementsinjava;
import java.util.Scanner;
public class ControlStatementsInjava {
/**
* @website: www.instanceofjava.com
*/
public static void main(String[] args) {
Scanner in= new Scanner(System.in);
System.out.println("Please enter a number");
int n=in.nextInt();
if(n%2==0){
System.out.println(n+" is even number");
}else{
System.out.println(n+" is odd number");
}
}
}

Output:

Please enter a number
5
5 is odd number

if else ladder :

In some cases we may get requirement with multiple conditions in such cases we need to use if else ladder.(if -else-if-else if -else...)

if( condition ){
// code
}else if{
//code
}else {
//code
}

if else if control statement with example programs:

package controlstatementsinjava;
import java.util.Scanner;
public class ControlStatementsInjava {
/**
* @website: www.instanceofjava.com
*/
public static void main(String[] args) {
Scanner in= new Scanner(System.in);
System.out.println("Please enter your age");
int age=in.nextInt();
if(age<=18){
System.out.println("Your age is between 1-18");
}else if ((age>=18) && (age<=35)){
System.out.println(" Your age is between 18-35");
}
else if ((age>=35) && (age<=50)){
System.out.println(" Your age is between 35-50");
}else{
System.out.println(" Your age is above 50");
}
}
}

Output:

Please enter your age
20
Your age is between 18-35

Switch statement in java:

Instead if writing number of if conditions we can use single switch statement

switch(value)
case n1: statements;
break;
case n2 : statements;
break;
default: statements;
}

Java example program on switch statement in java:

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Rabu, 27 April 2016

Core java Interview Questions

Bitwise operators in java with example

Kris Kimcil 23.27 Komentar

Bitwise operators in java performs operation on bits
Bitwise operators performs bit by bit operations
Bit-wise operators operates on individual bits of operands.

Bit wise Operators in Java:

~ : Bit wise unary not
& : Bit wise And
| : Bit wise OR
^ : Bit wise Exclusive OR

1.Bit wise Unary Not ~:

Bit wise Unary not Inverts the bits.
Lets see a java program on bit wise unary not

Java Example program on bit wise unary not operator ~:

package operatorsinjava;
public class BitwiseUnaryNotDemo {
/**
* @www.instanceofjava.com
*/
public static void main(String[] args) {
int x=2;
System.out.println(~x);
int y= 3;
System.out.println(~y);
int z= 4;
System.out.println(~z);
}
}

Output:

-3
-4
-5

2. Bitwise AND operator:

Bit wise And returns 1 if both operands position values are 1 otherwise 0.
Bitwise operation on two numbers
1 & 2
001
010
-----
000 =0
-----
2 & 3
010
011
-----
010 =2
-----

Java Example program on bitwise AND operator &:

package operatorsinjava;
public class BitwiseAndDemo {
/**
* @www.instanceofjava.com
*/
public static void main(String[] args) {
int x=1;
int y=2;
System.out.println(x&y);
int i=2;
int j=3;
System.out.println(i&j);
System.out.println(4&5);
}
}

Output:

3.Bitwise OR operator in Java | :

Bit wise OR returns 1 if any one operands position values are 1 or both 1 s. otherwise 0.
1 |2
001
010
-----
011 =3
-----
2 | 3
010
011
-----
011 =3
-----

Java Example program on bitwise OR operator |:

package operatorsinjava;
public class BitwiseORDemo {
/**
* @www.instanceofjava.com
*/
public static void main(String[] args) {
int x=1;
int y=2;
System.out.println(x|y);
int i=2;
int j=3;
System.out.println(i|j);
System.out.println(4|5);
}
}

Output:

4.Bit wise Exclusive OR operator in java ^:

Bit wise XOR operator returns 1 if any of the operands bits position 1
Java XOR operator returns 0 if both operands position is 1.
Lest see java Xor on 1 ^ 2
1 ^ 2
001
010
-----
011 =3
-----
Lest see java xor operation on 2 ^3
2 ^ 3
010
011
-----
001 =1
-----

Java Example program on bitwise OR operator |:

package operatorsinjava;
public class BitwiseXORDemo {
/**
* @www.instanceofjava.com
*/
public static void main(String[] args) {
int x=1;
int y=2;
System.out.println(x^y);
int i=2;
int j=3;
System.out.println(i^j);
System.out.println(4^5);
}
}

Output:

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Core java Interview Questions

Ternary operator in java

Kris Kimcil 01.59 Komentar

Ternary operator also known as conditional operator in java.
Ternary operator used to evaluate boolean expression and it consists of three operands.

Ternary operator in java syntax:

variable x = (expression) ? value if true : value if false

Value of x will be decided based on the condition. if condition is true then it assigns first value to the variable
If Expression is false then it assigns second value to the variable.

Java simple example program on ternary operator / conditional operator in java