4.4. Basic Java Syntax: Flow Control Structures (if, else, switch)

Basic Java Syntax: Flow Control Structures

Flow control structures are fundamental in any programming language, as they allow the program to make decisions and execute blocks of code according to certain conditions. In Java, the main flow control structures are if, else, and switch. Let's explore each of them in detail.

The if

Statement

The if statement is the most basic flow control structure. It evaluates a Boolean expression and, if the result is true, executes a block of code. The basic syntax is as follows:

if (condition) {
    // Block of code to be executed if the condition is true
}

For example:

int temperature = 30;
if (temperature > 25) {
    System.out.println("It's hot in here!");
}

In this case, the message "It's hot in here!" will be printed if the temperature is greater than 25.

The else

Statement

We often want to execute an alternative block of code if the if condition is false. This is done with the else statement. The syntax is as follows:

if (condition) {
    // Block of code to be executed if the condition is true
} else {
    // Block of code to be executed if the condition is false
}

For example:

int score = 75;
if (score >= 70) {
    System.out.println("You passed!");
} else {
    System.out.println("You didn't pass.");
}

In this example, "You passed!" will be printed if the score is 70 or higher, otherwise "You did not pass." will be printed.

The else if

Statement

To test multiple conditions in sequence, we can use the else if statement. This allows us to have multiple checks, and each one is evaluated in order until one is true or they are all false. The syntax is as follows:

if (first condition) {
    // Block of code to be executed if the first condition is true
} else if (second condition) {
    // Block of code to be executed if the second condition is true
} else {
    // Block of code to be executed if all previous conditions are false
}

For example:

int age = 20;
if (age < 13) {
    System.out.println("You are a child.");
} else if (age < 20) {
    System.out.println("You are a teenager.");
} else {
    System.out.println("You are an adult.");
}

In this example, "You are an adult." will be printed, because the age is 20, which does not satisfy the first two conditions, but falls into the else condition.

The switch

Instruction

The switch statement is an alternative to using multiple if and else if when you want to compare a single variable with a series of values constants. The syntax of switch is as follows:

switch (expression) {
    case value1:
        // Block of code to be executed if the expression is equal to value1
        break;
    case value2:
        // Block of code to be executed if the expression is equal to value2
        break;
    // ...
    default:
        // Block of code to be executed if the expression is not equal to any of the previous values
}

For example:

char grade = 'B';
switch (grid) {
    case 'A':
        System.out.println("Excellent!");
        break;
    case 'B':
        System.out.println("Great!");
        break;
    case 'C':
        System.out.println("Good!");
        break;
    case 'D':
        System.out.println("You passed!");
        break;
    case 'F':
        System.out.println("Better try again.");
        break;
    default:
        System.out.println("Invalid note.");
        break;
}

In this example, "Great!" will be printed, as the variable grade has the value 'B'. The break instruction is used to exit the switch block as soon as a case is satisfied, preventing subsequent cases from being evaluated unnecessarily.

Final Considerations

Control flow structures are essential for creating dynamic programs that can respond to different conditions and input data. Mastering if, else, else if and switch allows the Java programmer to build complex and efficient logic. Practicing these structures with varied exercises is the best way to become proficient in their use and understand how they can be combined to solve real programming problems.