InstanceOfJava

Enum in java Example

Posted by: Instanceofjava Posted date: April 19, 2016 / comment : 0 Core java Interview Questions

Java Enum:

In this tutorial I will explain what is enum, how to use enum in different areas of a Java program and an example program on it.

An Enum type is a special data type which is added in Java 1.5 version. It is an abstract class in Java API which implements Cloneable and Serializable interfaces. It is used to define collection of constants. When you need a predefined list of values which do not represent some kind of numeric or textual data, at this moment, you have to use an enum.
Common examples include days of week (values of SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, and SATURDAY), directions, and months in a year. Etc.
You can declare simple Java enum type with a syntax that is similar to the Java class declaration syntax. Let’s look at several short Java enum examples to get you started.

Enum declaration Example 1:

public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}

Enum declaration Example 2:

public enum Month{
JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER,
OCTOBER, NOVEMBER, DECEMBER
}

Enums are constants; they are by default static and final. That’s why the names of an enum type's fields are in uppercase letters.
Make a note that the enum keyword which is used in place of class or interface. The Java enum keyword signals to the Java compiler that this type definition is an enum.
You can refer to the constants in the enum above like this:

Day day = Day.MONDAY;

Here the ‘day’ variable is of the type ‘Day’ which is the Java enum type defined in the example above. The ‘day’ variable can take one of the ‘Day’ enum constants as value (SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY). In this case ‘day’ is set to MONDAY.
If you use enum instead of integers or String codes, you increase compile-time checking and avoid errors from passing in invalid constants, and you document which values are legal to use.
How to use a Java enum type in various areas in a Java Program:
We have seen how to declare simple Java enum types, let’s take a look at how to use them in various areas. We have to use a Java enum type in a variety of situations, including in a Java 5 for loop, in a switch statement, in an if/else statement, and more. For simple, Enum comparison, there are 3 ways to do it. They are, Switch-Case Statement, == Operator, .equals() method. Like that there are other places where you have to use Enum.

Let's take a look at how to use our simple enum types with each of these Java constructs.

Enum in IF statement
Enum in Switch statement
Enum Iteration in for-each loop
Enum Fields
Enum Methods

1. Enum in IF statements:

We know Java Enums are constants. Sometimes, we have a requirement to compare a variable of Enum constant against the possible other constants in the Enum type. At this moment, we have to use IF statement as follows.
Day day = ----- //assign some Day constants to it.

If(day ==Day.MONDAY){
….//your code
} else if (day == Day.TUESDAY){
….//your code
} else if (day == Day.WEDNESDAY){
….//your code
} else if (day == Day. THURSDAY){
….//your code
} else if (day == Day.FRIDAY){
….//your code
} else if (day == Day.SATURDAY){
….//your code
} else if (day == Day.SUNDAY){
….//your code
}

Here, you can use “.equals()” instead of “==”. But, my preference is to use “==” because, it will never throws NullPointerException, safer at compile-time, runtime and faster.
The code snippet compares the ‘day’ variable against each of the possible other Enum constants in the ‘Day’ Enum.

2. Enums in Switch Statements:

Just assume that your Enum have lot of constants and you need to check a variable against the other values in Enum. For a small Enum IF Statement will be OK. If you use same IF statement for lot of Enum constants then our program will be increased and it is not a standard way to write a Java program. At this Situation, use Switch Statement instead of IF statement.

You can use enums in switch statements as follows:
Day day = ... //assign some Day constant to it

switch (day) {
case SUNDAY : //your code; break;
case MONDAY //your code; break;
case TUESDAY : //your code; break;
case WEDNESDAY : //your code; break;
case THURSDAY: //your code; break;
case FRIDAY : //your code; break;
case SATURDAY : //your code; break;
}

Here, give your code in the comments, “//your code”. The code should be a simple Java operation or a method call..etc

3.Enum Iteration in for-each loop
4.Enum Fields
5.Enum Methods

Please click here Enum in java part 2

Unreachable Blocks in java

Posted by: Instanceofjava Posted date: April 19, 2016 / comment : 1 Core java Interview Questions , exception handling interview questions

Unreachable Catch Blocks:

The block of statements to which the control would never reach under any case can be called as unreachable blocks.
Unreachable blocks are not supported by java.
Thus catch block mentioned with the reference of "Exception" class should and must be always last catch block. Because Exception is super class of all exceptions.

Program:

package com.instanceofjava;
public class ExcpetionDemo {
public static void main(String agrs[])
{
try
{
//statements
}
catch(Exception e)
{
System.out.println(e);
}
catch(ArithmeticException e)//unreachable block.. not supported by java. leads to error
System.out.println(e);
}
}

\Java Example program on unreachable catch block

package com.instanceofjava;
public class ExcpetionDemo {
public static void main(String agrs[])
{
try {
System.out.println("Excpetion handling interview questions Java unreachable block");
} catch (IllegalThreadStateException e) {
e.printStackTrace();
}catch (IllegalArgumentException e) {
e.printStackTrace();
}catch (Exception e) {
e.printStackTrace();
}
}
}

Output:

Excpetion handling interview questions Java unreachable block

package com.instanceofjava;
public class ExcpetionDemo {
public static void main(String agrs[])
{
try {
System.out.println("Excpetion handling interview questions Java unreachable block");
}
catch (Exception e) {
e.printStackTrace();
}catch (IllegalThreadStateException e) {//Error: Unreachable catch block for
//IllegalThreadStateException. It is already handled by the catch block for Exception
e.printStackTrace();
}catch (IllegalArgumentException e) {
e.printStackTrace();
}
}
}

Iterator and Custom Iterator in java with example programs

Posted by: Instanceofjava Posted date: April 19, 2016 / comment : 5 java Collections interview Questions

Java Iterator

Iterator is an interface which is made for Collection objects like List, Set. It comes inside java.util package and it was introduced in java 1.2 as public interface Iterator. To generate successive elements from a Collection, we can use java iterator. It contains three methods: those are,

boolean hasNext(): this method returns true if this Iterator has more element to iterate.
remove(): method remove the last element return by the iterator this method only calls once per call to next().
Object next() : Returns the next element in the iteration.

Every collection classes provides an iterator() method that returns an iterator to the beginning of the collection. By using this iterator object, we can access each element in the collection, one element at a time. In general, to use an iterator to traverse through the contents of a collection follow these steps:

Obtain Iterator object by calling Collections’s iterator() method
Make a loop to decide the next element availability by calling hasNext() method.
Get each element from the collection by using next() method.

Java Program which explains how to use iterator in java

Here is an example demonstrating Iterator. It uses an ArrayList object, You can apply to any type of collection.

import java.util.ArrayList;
import java.util.Iterator;
public class IteratorExample {
public static void main(String args[]) {
// create an array list. But, you can use any collection
ArrayList<String> arraylist = new ArrayList<String>();
// add elements to the array list
arraylist .add("Raja");
arraylist .add("Rani");
arraylist .add("Shankar");
arraylist .add("Uday");
arraylist .add("Philips");
arraylist .add("Sachin");
// use iterator to display contents of arraylist
System.out.println("Original contents of arraylist : ");
Iterator<String> itr = arraylist .iterator();
while (itr.hasNext()) {
Object obj = itr.next();
System.out.print(obj + "\n");
}
}
}

Output:

Original contents of arraylist :
Raja
Rani
Shankar
Uday
Philips
Sachin

Iterator is the best choice if you have to iterate the objects from a Collection. But, Iterator having some limitations.

Those are,

Iterator is not index based
If you need to update the collection object being iterated, normally you are not allowed to because of the way the iterator stores its position. And it will throw ConcurrentModificationException.
It does not allow to traverse to bi directions.

To overcome these on other side, we have For-each loop. It was introduced in Java 1.5
Advantages of For each loop :

Index Based
Internally the for-each loop creates an Iterator to iterate through the collection.
If you want to replace items in your collection objects, use for-each loop

Consider the following code snippet, to understand the difference between how to use Iterator and for-each loop.

Iterator<String> itr = aList.iterator();
while (itr.hasNext()) {
Object obj = itr.next();
System.out.print(obj + "\n");
}
Is same as,
For (String str : aList){
System.out.println(str);
}

Same code has been rewritten using enhanced for-loops which looks more readable. But enhanced for-loops can’t be used automatically in Collection objects that we implement. Then what do we do? This is where Iterable interface comes in to picture. Only, if our Collection implemented Iterable interface, we can iterate using enhanced for-loops. The advantage of implementing Iterable interface So, we should implement Java Iterable interface in order to make our code more elegant.
Here I am going to write a best Practice to develop a custom iterator. Following is the Syntax.

public class MyCollection<E> implements Iterable<E>{
public Iterator<E> iterator() {
return new MyIterator<E>();
}
public class MyIterator <T> implements Iterator<T> {
public boolean hasNext() {
//implement...
}
public T next() {
//implement...;
}
public void remove() {
//implement... if supported.
}
public static void main(String[] args) {
MyCollection<String> stringCollection = new MyCollection<String>();
for(String string : stringCollection){
}
}
}
}

package com.instanceofjava.customIterator;
public class Book {
String name;
String author;
long isbn;
float price;
public Book(String name, String author, long isbn, float price) {
this.name = name;
this.author = author;
this.isbn = isbn;
this.price = price;
}
public String toString() {
return name + "\t" + author + "\t" + isbn + "\t" + ": Rs" + price;
}
}

package com.instanceofjava.customIterator;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import com.pamu.test.Book;
import java.lang.Iterable;
public class BookShop implements Iterable<Book> {
List<Book> avail_books;
public BookShop() {
avail_books = new ArrayList<Book>();
}
public void addBook(Book book) {
avail_books.add(book);
}
public Iterator<Book> iterator() {
return (Iterator<Book>) new BookShopIterator();
}
class BookShopIterator implements Iterator<Book> {
int currentIndex = 0;
@Override
public boolean hasNext() {
if (currentIndex >= avail_books.size()) {
return false;
} else {
return true;
}
}
@Override
public Book next() {
return avail_books.get(currentIndex++);
}
@Override
public void remove() {
avail_books.remove(--currentIndex);
}
}
//main method
public static void main(String[] args) {
Book book1 = new Book("Java", "JamesGosling", 123456, 1000.0f);
Book book2 = new Book("Spring", "RodJohnson", 789123, 1500.0f);
Book book3 = new Book("Struts", "Apache", 456789, 800.0f);
BookShop avail_books = new BookShop();
avail_books.addBook(book1);
avail_books.addBook(book2);
avail_books.addBook(book3);
System.out.println("Displaying Books:");
for(Book total_books : avail_books){
System.out.println(total_books);
}
}
}

Here, we created a BookShop class which contains books. This class able to use for-each loop only if it implements Iterable interface.Here, we have to provide the implementation for iterator method. we define my BookShopIterator as inner class.
Inner classes will provide more security, So that your class is able to access with in the Outer class and this will achieve data encapsulation.
The best reusable option is to implement the interface Iterable and override the method iterator().The main advantage of Iterable is, when you implement Iterable then those object gets support for for:each loop syntax.
Execute above program and check output. Practice makes perfect.

Basic operators in java

Posted by: Instanceofjava Posted date: March 22, 2016 / comment : 0 Core java Interview Questions

Types of Operators in Java:

Arithmetic Operators
Bit-wise Operators
Logical Operators
Relational Operators
Assignment Operators
Misc Operators

Java operator precedence

Arithmetic Operators

Arithmetic operators are used in mathematical expressions.
Following are the arithmetic operators in java.

+ : Addition
- : Subtraction
* : Multiplication
/ : Division
% : Modulus

Bit wise Operators:

Bit wise operators operates on individual bits of operands.
Following are the Bit-wise operators in java.
Bitwise operators in java

~ : Bit wise unary not
& : Bit wise And
| : Bit wise OR
^ : Bit wise Exclusive OR
>> : Shift Right
>>> : Shift right zero fill
<< :Shift left

Logical Operators:

&& : Logical AND
|| : Logical OR
! : Logical NOT

Relational Operators in java

== : Equal to
!= : Not Equal to
< : Less Than
> : Greater Than
>= : Greater Than or Equal to
<= : Less Than or Equal to

Assignment Operators:

= : Simple Assignment
+= : Add and assign
-= : Subtract and assign
*= : Multiply and assign
%= : Modulus and assign
/= : Divide and assign
&= : Bit wise AND assignment
|= : Bit wise assignment OR
^= : Bit wise Exclusive OR assignment
>>= : Shift Right assignment
>>>= : Shift right zero fill assignment
<<= : Shift left assignment

Misc Operators:

Increment and Decrement operators:

++ : increment
-- : decrement

Top 20 Basic Java Interview Programs on Increment Decrement operators

Conditional Operator: (?:)

Also known as ternary operator
Example: int a= 1>2?1:2;

Ternary operator in java

Instanceof Operator

Instance of operator is used to test whether that object is belong to that class type or not.
If that object belongs to that class it returns true .otherwise it returns false.
Instance of operator is also known as comparison operator.because it compares with the instance of type.

Instanceof operator in java

Top 30 frequently asked java interview programs for freshers

Posted by: Instanceofjava Posted date: March 22, 2016 / comment : 3

Java Example program convert Decimal to Binary

Posted by: Instanceofjava Posted date: March 21, 2016 / comment : 0 Core java Interview Questions

On of the way to convert decimal number to its binary representation using the Integer.toBinaryString() method. This method takes an integer as an argument and returns its binary representation as a string. Here is an example:

public static String decimalToBinary(int decimal) {

return Integer.toBinaryString(decimal);

}

We can call this method and pass a decimal number as an argument, and it will return the binary representation of that number. For example:

int decimal = 10;

String binary = decimalToBinary(decimal);

System.out.println(binary); // Output: 1010

In the above method will return a string representation of the binary number, if we want to have it as an integer you can use Integer.parseInt(binary, 2) to convert it to an int.

Another way to convert a decimal number to its binary representation is by using bit shifting and bitwise operations. You can repeatedly divide the decimal number by 2 and keep track of the remainder. The remainder represents the least significant bit of the binary representation. You can then reverse the order of the remainders to get the binary representation of the decimal number.

In summary, In Java you can use the Integer.toBinaryString() method to convert a decimal number to its binary representation as a string, and you can use Integer.parseInt(binary, 2) to convert it to an int. Also, you can use bit shifting and bitwise operations to convert a decimal number to its binary representation.

Another way to convert a decimal number to its binary representation is by using bit shifting and bitwise operations.

This method involves repeatedly dividing the decimal number by 2 and keeping track of the remainders. The remainders represent the least significant bits of the binary representation.

public static String decimalToBinaryUsingBitOperation(int decimal) {
    StringBuilder binary = new StringBuilder();
    while (decimal > 0) {
        binary.append(decimal % 2);
        decimal = decimal >> 1;
    }
    return binary.reverse().toString();
}

the function uses a while loop to divide the decimal number by 2 and keep track of the remainders. The remainders are appended to a StringBuilder object. The while loop continues until the decimal number is 0. At the end of the loop, the binary representation of the decimal number is obtained by reversing the order of the remainders using the reverse() method of the StringBuilder. The binary representation is then returned as a string.

This method will also work for negative decimal numbers, but the result will be the two's complement representation of the negative number, it will have a leading 1 in the most significant bit.

It's worth noting that, the above method will return the binary representation as a string, if you want to have it as an integer you can use Integer.parseInt(binary, 2) to convert it to an int.

In summary, there are different ways to convert a decimal number to its binary representation in Java. One way is to use the Integer.toBinaryString() method, another way is by using bit shifting and bitwise operations, this method uses a while loop to repeatedly divide the decimal number by 2 and keep track of the remainders, which represent the binary representation of the decimal number. The result can be returned as a string or as an integer by using the Integer.parseInt(binary, 2)

We can convert binary to decimal in three ways
1.Using Integer.toBinaryString(int num);
2.Using Stack
3.Using Custom logic

1.Write a Java Program to convert decimal to binary in java using Integer.toBinaryString(int num)

import java.util.Scanner;
public class ConvertDecimalToBinary{
/**
*www.instanceofjava.com
*/
public static void main(String[] args) {
System.out.println("\nBinary representation of 1: ");
System.out.println(Integer.toBinaryString(1));
System.out.println("\nBinary representation of 4: ");
System.out.println(Integer.toBinaryString(4));
System.out.println("Binary representation of 10: ");
System.out.println(Integer.toBinaryString(10));
System.out.println("\nBinary representation of 12: ");
System.out.println(Integer.toBinaryString(12));
System.out.println("\nBinary representation of 120: ");
System.out.println(Integer.toBinaryString(120));
System.out.println("\nBinary representation of 500: ");
System.out.println(Integer.toBinaryString(500));
}
}

Output:

Binary representation of 1:
1
Binary representation of 4:
100
Binary representation of 10:
1010
Binary representation of 12:
1100
Binary representation of 120:
1111000
Binary representation of 500:
111110100

2.Write a Java Program to convert decimal to binary in java

import java.util.Scanner;
import java.util.Stack;
public class ConvertDecimalToBinary{
/**
*www.instanceofjava.com
*/
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
// Create Stack object
Stack<Integer> stack = new Stack<Integer>();
//Take User input from keyboard
System.out.println("Enter decimal number: ");
int num = in.nextInt();
while (num != 0)
{
int d = num % 2;
stack.push(d);
num /= 2;
}
System.out.print("\nBinary representation is:");
while (!(stack.isEmpty() ))
{
System.out.print(stack.pop());
}
System.out.println();
}
}
}

Output:

Enter decimal number:
12
Binary representation is:1100

3.Write a Java Program to convert decimal to binary in java

import java.util.Scanner;
public class ConvertDecimalToBinary{
/**
*www.instanceofjava.com
*/
public static void convertDeciamlToBinary(int num){
int binary[] = new int[40];
int index = 0;
while(num > 0){
binary[index++] = num%2;
num = num/2;
}
for(int i = index-1;i >= 0;i--){
System.out.print(binary[i]);
}
}
public static void main(String[] args) {
System.out.println("Binary representation of 1: ");
convertDeciamlToBinary(1);
System.out.println("\nBinary representation of 4: ");
convertDeciamlToBinary(4);
System.out.println("\nBinary representation of 10: ");
convertDeciamlToBinary(10);
System.out.println("\nBinary representation of 12: ");
convertDeciamlToBinary(12);
}
}

Output:

Binary representation of 1:
1
Binary representation of 4:
100
Binary representation of 10:
1010
Binary representation of 12:
1100

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Java Example Program to convert binary to decimal

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Enum in java Example

1. Enum in IF statements:

2. Enums in Switch Statements:

Unreachable Blocks in java

Iterator and Custom Iterator in java with example programs

Java Iterator

Output:

Iterator is the best choice if you have to iterate the objects from a Collection. But, Iterator having some limitations.

Basic operators in java

Top 30 frequently asked java interview programs for freshers

Java Example program convert Decimal to Binary