Introduction
Loops in Java allow executing a block of code multiple times based on a condition. There are three main types of loops:
do-while loop: Similar to while, but executes at least once.
for loop: Used when the number of iterations is known.
while loop: Used when the number of iterations is unknown and depends on a condition.
For Loop
Example 1: Print numbers from 1 to 5
We will use a for loop to print numbers from 1 to 5.
public class ForLoopExample {
public static void main(String[] args) {
for (int i = 1; i <= 5; i++) {
System.out.println(i);
}
}
}
Explanation: The loop starts with i = 1 and runs until i <= 5, incrementing i in each iteration.
While Loop
Example 2: Print even numbers from 2 to 10
We will use a while loop to print even numbers from 2 to 10.
public class WhileLoopExample {
public static void main(String[] args) {
int i = 2;
while (i <= 10) {
System.out.println(i);
i += 2;
}
}
}
Explanation: The loop starts with i = 2 and runs while i <= 10, printing i and increasing it by 2 each time.
Do-While Loop
Example 3: Print “Hello” three times
We will use a do-while loop to print “Hello” three times.
public class DoWhileExample {
public static void main(String[] args) {
int count = 0;
do {
System.out.println("Hello");
count++;
} while (count < 3);
}
}
Explanation: The loop executes the block once before checking the condition count < 3.
Real-World Examples
Example 4: Count the number of digits in a number
Let’s count the number of digits in a hardcoded number.
public class CountDigits {
public static void main(String[] args) {
int number = 12345;
int count = 0;
while (number > 0) {
number = number / 10;
count++;
}
System.out.println("Number of digits: " + count);
}
}
Explanation: The loop repeatedly divides number by 10 and increments count until number becomes 0.
Example 5: Reverse a number
Let’s reverse a hardcoded number.
public class ReverseNumber {
public static void main(String[] args) {
int number = 1234;
int reverse = 0;
while (number > 0) {
int digit = number % 10;
reverse = reverse * 10 + digit;
number = number / 10;
}
System.out.println("Reversed number: " + reverse);
}
}
Explanation: The loop extracts the last digit using %, adds it to reverse, and removes the last digit using /.
Interview Questions
1. What is the difference between while and do-while loops?
Answer: A while loop checks the condition first, whereas a do-while loop executes at least once before checking the condition.
2. What is the output of the following code?
public class LoopTest {
public static void main(String[] args) {
int i = 5;
while (i > 0) {
System.out.println(i);
i--;
}
}
}
Output:
5
4
3
2
1
3. Will this loop run infinitely?
public class InfiniteLoop {
public static void main(String[] args) {
for (int i = 1; i > 0; i++) {
System.out.println(i);
}
}
}
Answer: Yes, because i keeps increasing and never becomes negative.
4. What happens if we use break in a loop?
Answer: The loop terminates immediately when break is encountered.
5. What happens if we use continue in a loop?
Answer: The loop skips the current iteration and proceeds to the next iteration.
6. What will be the output of the following code?
public class LoopExample {
public static void main(String[] args) {
for (int i = 1; i <= 5; i++) {
if (i == 3) {
continue;
}
System.out.println(i);
}
}
}
Output:
1
2
4
5
Explanation: When i == 3, continue skips that iteration.
7. What is the best way to loop through an array in Java?
Answer: Use an enhanced for loop.
public class ArrayLoop {
public static void main(String[] args) {
int[] numbers = {10, 20, 30, 40};
for (int num : numbers) {
System.out.println(num);
}
}
}
8. Can a for loop run without a condition?
Answer: Yes, an infinite loop can be written as for(;;) {}.
9. What is the difference between break and return?
Answer: break exits a loop, while return exits the entire method.
10. How can we optimize nested loops?
Answer: Reduce nesting, use break, and consider using data structures like maps or sets.
Leave a Reply