CONTROL STRUCTURES IN JAVA

Control Structures

Statements that control the flow of execution in a program. They allow decisions, repetitions, and jumps in the program sequence.

Types of Control Structures

Java has three main types of control structures:

1. Selection – decision-making (if, if-else, switch)

2. Repetition – looping (while, do-while, for)

3. Branching – flow control (break, continue, return)

Selection Statements

Decision-making statements used to control which set of instructions should be executed based on conditions. Includes if, if-else, and switch statements.

if Statement

Executes a block of code only if a given condition is true.
Example:

if (a % 2 == 0) {
   System.out.println("Even number");
}

if-else Statement

Executes one block of code if the condition is true and another block if the condition is false.
Example:

if (a % 2 == 0)
   System.out.println("Even");
else
   System.out.println("Odd");

switch Statement

Used for decision-making when a variable has multiple possible values. It is an easier alternative to multiple if-else statements.
Example:

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

case Keyword

Defines each possible value for a switch variable and the code block that executes when matched.

break in Switch

Terminates the current case in a switch statement, preventing the code from “falling through” to the next case.

default Keyword

A case in the switch statement that executes when none of the other case values match.

Rules for switch Statements

• Case constants must be unique.

• Data type of cases must match the switch variable.

• default is optional but should be last.

Repetition Statements

Used to repeat a block of code multiple times as long as a condition is true. Also called loops.

Loop Control Variable

The variable that determines how many times a loop runs, usually appearing in the loop condition.

while Loop

A pre-test loop that repeats code while the condition is true. The condition is checked before each iteration.
Example:

int i = 1;
while (i <= 5) {
   System.out.println(i);
   i++;
}

do-while Loop

A post-test loop that executes the block once before checking the condition. Ensures the loop body runs at least once.
Example:

int j = 1;
do {
   System.out.println(j);
   j++;
} while (j <= 10);

for Loop

A loop used when the number of repetitions is known. It combines initialization, condition, and update in one line.
Syntax:

for (init; condition; update) {
   statement;
}

Components of a for Loop

• Initialization – sets starting value

• Test/Condition – determines whether loop continues

• Increment/Decrement – updates loop variable after each iteration

Common for Loop Mistake

Adding a semicolon right after the for statement causes the loop to execute incorrectly or skip its block.

Iteration

One complete execution of a loop’s body.

Branching Statements

Statements that transfer control to another part of the program. Includes break, continue, and return.

break Statement

Exits a loop or switch statement immediately. It stops the current control structure’s execution.

Labeled Break

A form of break statement that allows you to exit from an outer loop in nested loops.

continue Statement

Skips the rest of the current loop iteration and moves to the next iteration of the loop.

return Statement

Ends a method’s execution and optionally sends a value back to the method’s caller.

Forms of return Statement

• return value; → returns a value

• return; → returns nothing (used in void methods)

• return expression; → returns an evaluated value

Example of return Usage

public static void hello() {
   System.out.println("Hello " + welcome());
}
static String welcome() {
   return "Welcome to Java Programming";
}

Nested Loop

A loop placed inside another loop. Commonly used in multiplication tables or matrix operations.