Top Java Developer Interview Questions

What is the difference between '==' and '.equals()' in Java?

What is the purpose of the 'finally' block in Java?

What is the difference between 'interface' and 'abstract class' in Java?

What is the purpose of the 'synchronized' keyword in Java?

What is the difference between 'HashMap' and 'Hashtable' in Java?

What is the purpose of the 'volatile' keyword in Java?

What is the difference between 'ArrayList' and 'LinkedList' in Java?

What is the purpose of the 'transient' keyword in Java?

What is the difference between 'Java' and 'JavaScript'?

What is the purpose of the 'assert' statement in Java?

What is the difference between 'JDK' and 'JRE'?

What is the purpose of the 'ClassLoader' in Java?

What is the difference between 'static' and 'instance' variables in Java?

What is the purpose of the 'finalize' method in Java?

What is the difference between 'checked' and 'unchecked' exceptions in Java?

What is the purpose of the 'wait' and 'notify' methods in Java?

What is the difference between 'synchronized' and 'ReentrantLock' in Java?

What is the purpose of the 'ThreadLocal' class in Java?

What is the difference between 'HashMap' and 'ConcurrentHashMap' in Java?

What is the purpose of the 'Atomic' classes in Java?

Write a Java program to find the maximum element in an array

public class Main {
  public static void main(String[] args) {
    
    int[] array = {
      1, 2, 3, 4, 5}
      ;
      int max = array[0];
      for (int i = 1;
      i < array.length;
      i++) {
        
        if (array[i] > max) {
          
          max = array[i];
        }
      }
      System.out.println(max);
    }
  }

Write a Java program to reverse a string

public class Main {
  public static void main(String[] args) {
    
    String str = 'Hello World';
    String reversed = new StringBuilder(str).reverse().toString();
    System.out.println(reversed);
  }
}

Write a Java program to find the first duplicate in an array

public class Main {
  public static void main(String[] args) {
    
    int[] array = {
      1, 2, 3, 2, 4}
      ;
      for (int i = 0;
      i < array.length;
      i++) {
        
        for (int j = i + 1;
        j < array.length;
        j++) {
          
          if (array[i] == array[j]) {
            
            System.out.println(array[i]);
            return;
          }
        }
      }
    }
  }

Write a Java program to find the maximum sum of a subarray

public class Main {
  public static void main(String[] args) {
    
    int[] array = {
      -2, 1, -3, 4, -1, 2, 1, -5, 4}
      ;
      int maxSum = array[0];
      int currentSum = array[0];
      for (int i = 1;
      i < array.length;
      i++) {
        
        currentSum = Math.max(array[i], currentSum + array[i]);
        maxSum = Math.max(maxSum, currentSum);
      }
      System.out.println(maxSum);
    }
  }

Write a Java program to find the minimum window that contains all elements of another array

public class Main {
  public static void main(String[] args) {
    
    String str1 = 'abc';
    String str2 = 'abcabc';
    int minWindow = Integer.MAX_VALUE;
    for (int i = 0;
    i <= str2.length() - str1.length();
    i++) {
      
      String window = str2.substring(i, i + str1.length());
      if (containsAll(window, str1)) {
        
        minWindow = Math.min(minWindow, window.length());
      }
    }
    System.out.println(minWindow);
  }
  public static boolean containsAll(String window, String str) {
    
    for (char c : str.toCharArray()) {
      
      if (window.indexOf(c) == -1) {
        
        return false;
      }
    }
    return true;
  }
}

Write a Java program to find the maximum length of a subarray with equal number of 0s and 1s

public class Main {
  public static void main(String[] args) {
    
    int[] array = {
      1, 0, 1, 1, 1, 0, 0}
      ;
      int maxLength = 0;
      for (int i = 0;
      i < array.length;
      i++) {
        
        int zeros = 0;
        int ones = 0;
        for (int j = i;
        j < array.length;
        j++) {
          
          if (array[j] == 0) {
            
            zeros++;
          }
          else {
            ones++;
          }
          if (zeros == ones) {
            
            maxLength = Math.max(maxLength, j - i + 1);
          }
        }
      }
      System.out.println(maxLength);
    }
  }

Write a Java program to find the first missing positive integer in an array

public class Main {
  public static void main(String[] args) {
    
    int[] array = {
      3, 4, -1, 1}
      ;
      int missing = 1;
      for (int i = 0;
      i < array.length;
      i++) {
        
        if (array[i] == missing) {
          
          missing++;
          i = -1;
        }
      }
      System.out.println(missing);
    }
  }

Write a Java program to find the maximum product of two numbers in an array

public class Main {
  public static void main(String[] args) {
    
    int[] array = {
      1, 20, 3, -10, 5}
      ;
      int maxProduct = Integer.MIN_VALUE;
      for (int i = 0;
      i < array.length;
      i++) {
        
        for (int j = i + 1;
        j < array.length;
        j++) {
          
          maxProduct = Math.max(maxProduct, array[i] * array[j]);
        }
      }
      System.out.println(maxProduct);
    }
  }