Control Structures in Java

Control structures dictate the flow of execution in a program. Java provides various control structures like loops and conditionals to control how code is executed based on conditions or repeatedly for a certain number of times.

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1. Loops: for, while, do-while

Loops allow you to execute a block of code multiple times, either for a fixed number of iterations or while a condition is true.

for Loop

The for loop is typically used when the number of iterations is known beforehand.

Syntax:

for (initialization; condition; increment/decrement) {
    // code to be executed
}

Example:

for (int i = 0; i < 5; i++) {
    System.out.println("i = " + i);
}

Output:

i = 0
i = 1
i = 2
i = 3
i = 4

• Initialization: Executes once at the start.
• Condition: If true, the loop continues; if false, it terminates.
• Increment/Decrement: Updates the loop counter after each iteration.

Enhanced for Loop (for-each)

The enhanced for loop is used for iterating over arrays or collections.

Syntax:

for (type variable : collection) {
    // code to be executed
}

Example:

int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println(num);
}

Output:

1
2
3
4
5

while Loop

The while loop is used when the number of iterations isn't known in advance, but depends on a condition.

Syntax:

while (condition) {
    // code to be executed
}

Example:

int i = 0;
while (i < 5) {
    System.out.println("i = " + i);
    i++;
}

Output:

i = 0
i = 1
i = 2
i = 3
i = 4

• The loop runs as long as the condition is true.

do-while Loop

A do-while loop is similar to the while loop, but the condition is checked after the loop body is executed, ensuring that the body is executed at least once.

Syntax:

do {
    // code to be executed
} while (condition);

Example:

int i = 0;
do {
    System.out.println("i = " + i);
    i++;
} while (i < 5);

Output:

i = 0
i = 1
i = 2
i = 3
i = 4

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2. Conditionals: if-else and switch

Conditionals allow decision-making in a program by executing different blocks of code based on conditions.

if-else Statement

An if statement checks a condition and executes the block of code if the condition is true. You can extend it with an optional else or else if block.

Syntax:

if (condition) {
    // code to be executed if condition is true
} else if (anotherCondition) {
    // code to be executed if anotherCondition is true
} else {
    // code to be executed if all conditions are false
}

Example:

int age = 20;

if (age >= 18) {
    System.out.println("You are an adult.");
} else if (age >= 13) {
    System.out.println("You are a teenager.");
} else {
    System.out.println("You are a child.");
}

Output:

You are an adult.

switch Statement

The switch statement allows you to choose between multiple options based on the value of an expression.

Syntax:

switch (expression) {
    case value1:
        // code to be executed for value1
        break;
    case value2:
        // code to be executed for value2
        break;
    // more cases
    default:
        // code to be executed if no cases match
}

Example:

int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday");
        break;
    default:
        System.out.println("Invalid day");
}

Output:

Wednesday

• The break statement ensures the switch statement exits after the matched case is executed.
• The default case is optional and gets executed if no case matches the expression.

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3. Iteration using for and while

Both the for and while loops allow iteration over sequences like arrays, lists, or ranges of values.

Iterating Over an Array Using for Loop:

int[] numbers = {1, 2, 3, 4, 5};

for (int i = 0; i < numbers.length; i++) {
    System.out.println("Number: " + numbers[i]);
}

Iterating Over a List Using while Loop:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("John");
        names.add("Doe");
        names.add("Alice");

        int i = 0;
        while (i < names.size()) {
            System.out.println("Name: " + names.get(i));
            i++;
        }
    }
}

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Conclusion:

• Loops like for, while, and do-while are essential for repeating code.
• Conditionals like if-else and switch help control decision-making by executing different blocks based on conditions.
• Using these control structures effectively is crucial in writing efficient and clean Java programs.