Java Exercises

// Basic Java Exercise
public class Exercise {
    public static void main(String[] args) {
        System.out.println("Practice Java Exercises");
    }
}

// ===============================
// 1. PRINT HELLO WORLD
// ===============================
class HelloWorld {

  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}


// ===============================
// 2. CHECK EVEN OR ODD
// ===============================
class EvenOdd {

  public static void main(String[] args) {

    int num = 10;

    if (num % 2 == 0) {
      System.out.println("Even Number");
    } else {
      System.out.println("Odd Number");
    }
  }
}


// ===============================
// 3. FACTORIAL PROGRAM
// ===============================
class Factorial {

  public static void main(String[] args) {

    int num = 5;
    int fact = 1;

    for (int i = 1; i <= num; i++) {
      fact *= i;
    }

    System.out.println(fact);
  }
}


// ===============================
// 4. REVERSE STRING
// ===============================
class ReverseString {

  public static void main(String[] args) {

    String str = "Java";
    String rev = "";

    for (int i = str.length() - 1; i >= 0; i--) {
      rev += str.charAt(i);
    }

    System.out.println(rev);
  }
}


// ===============================
// 5. ARRAY SUM
// ===============================
class ArraySum {

  public static void main(String[] args) {

    int[] arr = {1, 2, 3, 4};
    int sum = 0;

    for (int num : arr) {
      sum += num;
    }

    System.out.println(sum);
  }
}


// ===============================
// 6. PALINDROME NUMBER
// ===============================
class Palindrome {

  public static void main(String[] args) {

    int num = 121;
    int temp = num;
    int rev = 0;

    while (num > 0) {
      rev = rev * 10 + num % 10;
      num /= 10;
    }

    if (temp == rev) {
      System.out.println("Palindrome");
    } else {
      System.out.println("Not Palindrome");
    }
  }
}


// ===============================
// 7. COLLECTION EXERCISE
// ===============================
import java.util.*;

class ListExercise {

  public static void main(String[] args) {

    List<String> list = new ArrayList<>();

    list.add("Java");
    list.add("Spring Boot");

    System.out.println(list);
  }
}

• 1 Hello World program
• 2 Even or odd number
• 3 Factorial program
• 4 Palindrome check

• 1 Reverse string
• 2 Count characters
• 3 Check palindrome string
• 4 Remove spaces

• 1 Find array sum
• 2 Find maximum value
• 3 Sort array
• 4 Search element

• 1 Class and object creation
• 2 Inheritance examples
• 3 Polymorphism practice
• 4 Interface implementation

• 1 Code daily
• 2 Understand logic deeply
• 3 Debug your programs
• 4 Build mini projects

• 1 Used in coding interview preparation.
• 2 Used in college lab exercises.
• 3 Used in Java bootcamps.
• 4 Used for improving programming logic.
• 5 SaaS products use Java Exercises in services, dashboards, background jobs, and API workflows.
• 6 ERP and banking systems apply Java Exercises with validation, logging, review, and rollback plans.
• 7 E-commerce and healthcare platforms use Java Exercises carefully because reliability and data correctness matter.

• 1 A Java program first evaluates the surrounding context, then applies the Java Exercises rules to the current data.
• 2 The important mental model is input, transformation, result, and failure path.
• 3 In production, the same flow usually sits inside a larger layer such as a controller, service, repository, job, or UI component.

• 1 Choose the simplest implementation first, then measure real workloads.
• 2 Watch for repeated work inside loops, unnecessary allocations, and slow I/O in hot paths.
• 3 Prefer clear data structures and stable APIs before micro-optimizing syntax.

• 1 Treat external input as untrusted until it is validated.
• 2 Avoid hardcoded secrets and never print sensitive values in examples or logs.
• 3 Use established libraries for authentication, encryption, parsing, and database access.

• 1 Copying syntax without understanding the data flow.
• 2 Ignoring edge cases and error states.
• 3 Using the concept where a simpler solution is clearer.
• 4 Skipping the small working example before adding framework code.
• 5 Ignoring null, empty, duplicate, and boundary inputs.
• 6 Mixing business logic, input handling, and output formatting in one place.
• 7 Using broad error handling that hides the real failure.
• 8 Forgetting to test the behavior after refactoring.
• 9 Adding clever code that future maintainers will struggle to read.
• 10 Not checking performance on realistic input sizes.

• 1 Start with the smallest working example.
• 2 Use descriptive names and consistent formatting.
• 3 Test the behavior with normal and edge-case inputs.
• 4 Start with clear requirements and one minimal working example.
• 5 Use meaningful names that explain business intent.
• 6 Keep examples small enough to debug line by line.
• 7 Validate input at every trust boundary.
• 8 Handle errors explicitly and preserve useful context.
• 9 Prefer simple control flow over deeply nested logic.
• 10 Separate domain logic from I/O and framework code.
• 11 Write tests for normal, boundary, and failure cases.
• 12 Review security assumptions before production use.
• 13 Measure performance before optimizing.
• 14 Document non-obvious decisions close to the code or in project notes.
• 15 Use official documentation when behavior is version-specific.
• 16 Keep dependencies current and remove unused code.
• 17 Avoid hardcoded secrets, credentials, and environment-specific paths.
• 18 Log operational events without exposing sensitive data.
• 19 Design examples so learners can safely modify and rerun them.
• 20 Prefer maintainability over short-term cleverness.

• 1 Beginner: rewrite the example with different names and values.
• 2 Intermediate: add validation and handle one expected failure case.
• 3 Advanced: place Java Exercises inside a small service-style design with tests.

• 1 Build a small Java console feature that demonstrates Java Exercises.
• 2 Accept input, process it with the concept, print a clear result, and handle invalid input.
• 3 Add a README note explaining the design choice and two edge cases you tested.

• 1 If the program does not compile, check spelling, imports, braces, and file/class names first.
• 2 If output is unexpected, print intermediate values and verify each branch of the logic.
• 3 If the design feels complex, reduce it to the smallest working example and add pieces back one at a time.

• 1 Practice Java Exercises with a second example from a business domain such as inventory, payroll, banking, or e-commerce.
• 2 Review related Java topics that cover data flow, error handling, testing, and clean design.
• 3 Compare your solution with official documentation and simplify anything you cannot explain clearly.