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Monday 6 July 2015

Java Important Interview Programs - 1

1. Find out duplicate number between 1 to N numbers.


Description:
You have got a range of numbers between 1 to N, where one of the number is
repeated. You need to write a program to find out the duplicate number.

Code:
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package kundan;
import java.util.ArrayList;
import java.util.List;
public class DuplicateNumber {
    public int findDuplicateNumber(List<Integer> numbers){
         
        int highestNumber = numbers.size() - 1;
        int total = getSum(numbers);
        int duplicate = total - (highestNumber*(highestNumber+1)/2);
        return duplicate;
    }
     
    public int getSum(List<Integer> numbers){
         
        int sum = 0;
        for(int num:numbers){
            sum += num;
        }
        return sum;
    }
     
    public static void main(String a[]){
        List<Integer> numbers = new ArrayList<Integer>();
        for(int i=1;i<30;i++){
            numbers.add(i);
        }
        //add duplicate number into the list
        numbers.add(22);
        DuplicateNumber dn = new DuplicateNumber();
        System.out.println("Duplicate Number: "+dn.findDuplicateNumber(numbers));
    }
}

Output:
Duplicate Number: 22

2. Find out middle index where sum of both ends are equal.


Description:
You are given an array of numbers. Find out the array index or position
where sum of numbers preceeding the index is equals to sum of numbers
succeeding the index.

Code:
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package kundan;
public class FindMiddleIndex {
    public static int findMiddleIndex(int[] numbers) throws Exception {
        int endIndex = numbers.length - 1;
        int startIndex = 0;
        int sumLeft = 0;
        int sumRight = 0;
        while (true) {
            if (sumLeft > sumRight) {
                sumRight += numbers[endIndex--];
            } else {
                sumLeft += numbers[startIndex++];
            }
            if (startIndex > endIndex) {
                if (sumLeft == sumRight) {
                    break;
                } else {
                    throw new Exception(
                            "Please pass proper array to match the requirement");
                }
            }
        }
        return endIndex;
    }
    public static void main(String a[]) {
        int[] num = { 2, 4, 4, 5, 4, 1 };
        try {
            System.out.println("Starting from index 0, adding numbers till index "
                            + findMiddleIndex(num) + " and");
            System.out.println("adding rest of the numbers can be equal");
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
    }
}

Output:
Starting from index 0, adding numbers till index 2 and
adding rest of the numbers can be equal

3. Write a singleton class.


Description:
Singleton class means you can create only one object for the given class. You can create a singleton class by making its constructor as private, so that you can restrict the creation of the object. Provide a static method to get instance of the object, wherein you can handle the object creation inside the class only. In this example we are creating object by using static block.

Code:
package kundan;

public class MySingleton {

 private static MySingleton myObj;
 
 static{
  myObj = new MySingleton();
 }
 
 private MySingleton(){
 
 }
 
 public static MySingleton getInstance(){
  return myObj;
 }
 
 public void testMe(){
  System.out.println("Hey.... it is working!!!");
 }
 
 public static void main(String a[]){
  MySingleton ms = getInstance();
  ms.testMe();
 }
}

4. Write a program to create deadlock between two threads.


Description:
Deadlock describes a situation where two or more threads are blocked forever, waiting for each other. 
Deadlocks can occur in Java when the synchronized keyword causes the executing thread to block while waiting 
to get the lock, associated with the specified object. Since the thread might already hold locks associated with 
other objects, two threads could each be waiting for the other to release a lock. In such case, they will end up waiting forever.

Code:
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package kundan;
public class MyDeadlock {
    String str1 = "Java";
    String str2 = "UNIX";
     
    Thread trd1 = new Thread("My Thread 1"){
        public void run(){
            while(true){
                synchronized(str1){
                    synchronized(str2){
                        System.out.println(str1 + str2);
                    }
                }
            }
        }
    };
     
    Thread trd2 = new Thread("My Thread 2"){
        public void run(){
            while(true){
                synchronized(str2){
                    synchronized(str1){
                        System.out.println(str2 + str1);
                    }
                }
            }
        }
    };
     
    public static void main(String a[]){
        MyDeadlock mdl = new MyDeadlock();
        mdl.trd1.start();
        mdl.trd2.start();
    }
}

5. Write a program to reverse a string using recursive algorithm.


Description:
Write a program to reverse a string using recursive methods.
You should not use any string reverse methods to do this.

Code:
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package kundan;
public class StringRecursiveReversal {
    String reverse = "";
     
    public String reverseString(String str){
         
        if(str.length() == 1){
            return str;
        } else {
            reverse += str.charAt(str.length()-1)
                    +reverseString(str.substring(0,str.length()-1));
            return reverse;
        }
    }
     
    public static void main(String a[]){
        StringRecursiveReversal srr = new StringRecursiveReversal();
        System.out.println("Result: "+srr.reverseString("kundan"));
    }
}

Output:
Result: nadnuk

6. Write a program to reverse a number.


Description:
Write a program to reverse a number using numeric operations. Below example shows how to reverse a number using numeric operations.

Code:
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package kundan;
public class NumberReverse {
    public int reverseNumber(int number){
         
        int reverse = 0;
        while(number != 0){
            reverse = (reverse*10)+(number%10);
            number = number/10;
        }
        return reverse;
    }
     
    public static void main(String a[]){
        NumberReverse nr = new NumberReverse();
        System.out.println("Result: "+nr.reverseNumber(17868));
    }
}

Output:
Result: 86871

7. Write a program to convert decimal number to binary format.


Description:
Write a program to convert decimal number to binary format using numeric operations. Below example shows how to convert decimal number to binary format using numeric operations.

Code:
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package kundan;
public class DecimalToBinary {
    public void printBinaryFormat(int number){
        int binary[] = new int[25];
        int index = 0;
        while(number > 0){
            binary[index++] = number%2;
            number = number/2;
        }
        for(int i = index-1;i >= 0;i--){
            System.out.print(binary[i]);
        }
    }
     
    public static void main(String a[]){
        DecimalToBinary dtb = new DecimalToBinary();
        dtb.printBinaryFormat(25);
    }
}

Output:
11001

8. Write a program to find perfect number or not.


Description:
A perfect number is a positive integer that is equal to the sum
of its proper positive divisors, that is, the sum of its positive
divisors excluding the number itself. Equivalently, a perfect number
is a number that is half the sum of all of its positive divisors.
The first perfect number is 6, because 1, 2 and 3 are its proper
positive divisors, and 1 + 2 + 3 = 6. Equivalently, the number 6
is equal to half the sum of all its positive divisors:
  ( 1 + 2 + 3 + 6 ) / 2 = 6.

Code:
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package kundan;
public class IsPerfectNumber {
    public boolean isPerfectNumber(int number){
         
        int temp = 0;
        for(int i=1;i<=number/2;i++){
            if(number%i == 0){
                temp += i;
            }
        }
        if(temp == number){
            System.out.println("It is a perfect number");
            return true;
        } else {
            System.out.println("It is not a perfect number");
            return false;
        }
    }
     
    public static void main(String a[]){
        IsPerfectNumber ipn = new IsPerfectNumber();
        System.out.println("Is perfect number: "+ipn.isPerfectNumber(28));
    }
}

Output:
28
It is a perfect number
Is perfect number: true

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