Control Flow Statements

In Java, control flow means the order in which your program runs instructions. By default, it runs from top to bottom, one line after another. But sometimes, you want your program to make choices, repeat actions, or skip certain things. This is where control flow statements come in—they help your program decide what to do and when.

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Types of Control Flow Statements in Java

Decision-Making Statements

These statements help your program choose what to do based on conditions.

• if Statement: Runs the code only if a condition is true.

if (temperature > 30) {
    System.out.println("It's a hot day!");
}

• if-else Statement: Runs one block if true, another if false.

if (score >= 60) {
    System.out.println("You passed!");
} else {
    System.out.println("You failed.");
}

• if-else-if Ladder: Checks multiple conditions, one by one.

if (marks >= 90) {
    grade = 'A';
} else if (marks >= 80) {
    grade = 'B';
} else {
    grade = 'C';
}

• Nested if: An if statement inside another if, for checking more details.

if (accountActive) {
    if (balance >= amount) {
        withdraw(amount);
    }
}

• switch Statement: A simple way to pick one option from many.

switch (dayOfWeek) {
    case 1: System.out.println("Monday"); break;
    case 2: System.out.println("Tuesday"); break;
    default: System.out.println("Unknown");
}

⚠️ break is mandatory to prevent fall-through.

• Modern switch Statement (Java 14+)

String dayName = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    default -> "Unknown";
};

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Looping Statements

Loops let your program repeat actions, saving you from writing the same code again and again.

• for Loop: Repeats a fixed number of times.

for (int i = 1; i <= 5; i++) {
    System.out.print(i + " "); // Prints numbers 1 to 5
}

• while Loop: Repeats as long as a condition is true. The condition is checked before running the loop.

int count = 1;
while (count <= 3) {
    System.out.print(count + " "); // Prints 1 2 3
    count++;
}

• do-while Loop: Similar to while, but the loop runs at least once because the condition is checked after running code.

int num = 5;
do {
    System.out.print(num + " "); // Prints 5 at least once
    num--;
} while (num > 0);

• Enhanced for Loop: Easy way to loop through elements in a list or array.

String[] colors = {"Red", "Green", "Blue"};
for (String color : colors) {
    System.out.println(color);
}

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Jump Statements

Jump statements help you skip or exit loops or methods to control the flow better.

• break: Stops a loop or switch immediately.

for (int i = 1; i <= 10; i++) {
    if (i == 6) break; // Stop loop at 6
    System.out.print(i + " ");
}

• continue: Skips the current loop iteration and moves to the next one.

for (int i = 1; i <= 5; i++) {
    if (i == 3) continue; // Skip number 3
    System.out.print(i + " ");
}

• return: Ends a method and optionally sends back a value.

public int square(int n) {
    return n * n; // Returns the square of n
}

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Nested Control Flow

You can put loops inside loops or decisions inside loops to handle more complex situations.

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        System.out.println(i + "-" + j);
    }
}

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Best Practices

• Use switch when comparing one variable against many constant values.
• Use enhanced for loop to avoid mistakes when looping through arrays or lists.
• Avoid writing too many nested blocks inside each other, because it makes code hard to read.
• Always use braces {} after if or loop statements to be clear, even if it’s just one line.
• Choose meaningful variable names to make your code easier to understand.

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Practical Examples

Check if a number is prime:

boolean isPrime(int n) {
    if (n <= 1) return false;
    for (int i = 2; i <= Math.sqrt(n); i++) {
        if (n % i == 0) return false;
    }
    return true;
}

Print Fibonacci series:

int n = 10, a = 0, b = 1;
System.out.print(a + " " + b + " ");
for (int i = 2; i < n; i++) {
    int next = a + b;
    System.out.print(next + " ");
    a = b;
    b = next;
}

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Common Mistakes to Avoid

• Trying to use an array index that doesn't exist.
• Forgetting break in switch and causing wrong code to run.
• Creating infinite loops by not changing the loop variable.
• Writing complicated nested blocks that are hard to read and debug.

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Tips for Debugging

• Use System.out.println to print variables and check the flow.
• Try using breakpoints and step-by-step debugging in an IDE.
• Explain your code out loud (rubber duck debugging) to find mistakes.
• Keep code simple and clear.

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Performance Tips

• Use System.arraycopy() to copy arrays quickly.
• Move calculations that don’t change outside loops.
• Use break and continue carefully to speed up loops.
• Store array lengths in a variable when looping instead of calling .length each time.

for (int i = 0, len = arr.length; i < len; i++) {
    // loop body
}