AP Computer Science A - Unit 3 Review: Boolean Expressions and Conditional Statements

1. Boolean Expressions

• A boolean expression evaluates to either true or false.
• Common use: inside an if statement.
• Example:

java if (3 > 2) { // Executes because expression is true }

2. Relational Operators

• Used to compare two values:

• == : equals (used with primitive types)

• != : not equal
• > : greater than
• < : less than
• >= : greater than or equal to
• <= : less than or equal to

• Important:

• == is not the same as = (assignment).

• == is only for primitives. Use .equals() for object comparisons.

3. Conditional (Logical) Operators

• Combine boolean expressions:

• && : AND (both must be true)

• || : OR (either can be true)

• Short-Circuiting:

• In &&, if the first condition is false, the second is not evaluated.

• In ||, if the first condition is true, the second is not evaluated.
• Avoids errors, e.g., dividing by zero.

4. If / Else If / Else Statements

• Allow decision-making based on boolean expressions.
• Syntax:

java if (condition) { // Executes if condition is true } else if (anotherCondition) { // Executes if above is false and this is true } else { // Executes if all above are false } * Rules:

• if and else if need a boolean expression.
• else does not take a boolean expression.
• Only one block is executed in an if-else if-else chain.

5. Code Tracing Example

int x = 5;
if (x < 10) {
    System.out.println("A"); // Runs
}
if (x < 4) {
    System.out.println("B"); // Skipped
}
if (x < 7) {
    System.out.println("C"); // Runs
}

6. Nested If-Else Statements

• You can place if-else structures inside one another.
• Use indentation and comments to keep them clear.
• Example:

int x = 5, y = 3;
if (x < 10) {
    if (y != 3) {
        System.out.println("A");
    } else if (y <= 5) {
        System.out.println("B"); // Runs
    } else {
        System.out.println("C");
    }
} else {
    if (y < 2) {
        System.out.println("D");
    } else if (y == 3) {
        System.out.println("E");
    }
}

7. Common Errors

• Semicolon after if, else if, or else:

• Breaks the structure; block executes regardless.

• Adding condition to else:

• Invalid: else (x < 5) → causes a compile error.

• Multiple statements without curly braces:

• Only the first statement is considered inside the if.

• Use {} for clarity and correctness.

8. One-Line Conditionals

• If only one line of code is needed, curly braces {} can be omitted.
• Example:

if (x > 5)
    System.out.println("X is large");

Summary:

• Master relational and conditional operators.
• Understand the flow of if-else chains.
• Use short-circuit logic to avoid runtime errors.
• Trace and debug code to confirm logic correctness.
• Avoid common syntax errors by understanding Java rules.

Practice writing and tracing if-else structures to prepare for AP CSA exam questions!