Branching Logic

After this lesson, you will be able to:

• Write if, if-else, and if-else-if chains to control program flow
• Construct boolean expressions using comparison and logical operators
• Choose between if-else-if chains and switch statements based on the problem
• Avoid common conditional bugs (dangling else, missing braces, = vs ==)

Controlling the Flow

Up to now, your programs have been purely sequential — every line runs, top to bottom. Conditionals let you make decisions: execute this code only if a condition is true.

In Python, you wrote:

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("Below B")

In Java, the structure is similar, but the syntax changes:

if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else {
    System.out.println("Below B");
}

Key differences: parentheses () around the condition are required. Curly braces {} replace Python’s indentation. elif becomes else if.

Boolean Expressions

Conditions evaluate to true or false. Java provides:

Comparison operators:

Logical operators:

From CSCD 110: Python uses and, or, not. Java uses &&, ||, !. Same logic, different symbols.

The if-else-if Chain

For mutually exclusive ranges, use an if-else-if chain. Order matters — Java checks each condition top-to-bottom and takes the first match:

public static char getLetterGrade(int score) {
    if (score >= 90) {
        return 'A';
    } else if (score >= 80) {
        return 'B';
    } else if (score >= 70) {
        return 'C';
    } else if (score >= 60) {
        return 'D';
    } else {
        return 'F';
    }
}

Because we use else if, we don’t need to write score >= 80 && score < 90. If the code reaches the second branch, we already know score < 90 because the first condition was false.

Switch Statements

When you’re comparing a single variable against exact values (not ranges), switch is cleaner:

switch (dayOfWeek) {
    case 1: System.out.println("Monday"); break;
    case 2: System.out.println("Tuesday"); break;
    case 3: System.out.println("Wednesday"); break;
    case 4: System.out.println("Thursday"); break;
    case 5: System.out.println("Friday"); break;
    case 6:
    case 7: System.out.println("Weekend!"); break;
    default: System.out.println("Invalid day");
}

Common Pitfall: Forgetting break causes fall-through — Java continues executing the next case. Cases 6 and 7 above use intentional fall-through (both print “Weekend!”), but accidental fall-through is a frequent bug.

Common Mistakes

1. Using = instead of ==:

if (x = 5) { ... }   // COMPILER ERROR — assignment, not comparison
if (x == 5) { ... }  // Correct — comparison

2. Comparing Strings with ==:

if (name == "Alice") { ... }     // WRONG — compares references
if (name.equals("Alice")) { ... } // CORRECT — compares content

3. Omitting braces:

if (x > 0)
    System.out.println("positive");
    System.out.println("done");  // This ALWAYS runs! Not part of the if.

Always use braces, even for single-line bodies. It prevents bugs and makes code clearer.

Summary

Conditionals control which code runs based on boolean expressions. Use if-else-if for range checks and complex conditions. Use switch for discrete value matching. Always use braces. Compare strings with .equals(), never ==.