How Selection Fits Into Program Flow

By default Java executes statements sequentially. Selection statements break this linear flow by routing execution down one path based on a boolean condition. The core AP exam insight: code after the entire if or if-else block always runs regardless of which branch executed.

One-Way Selection: the if Statement (EK 2.3.A.3)

A one-way selection provides a segment of code that runs only under a specific condition. If the condition is false, the body is entirely skipped and execution jumps to the first statement after the closing brace.

if (booleanExpression) {
    // body: executes only when booleanExpression is true
}
// always executes after the if block, true or false

Component roles

int temp = 95;
if (temp > 90) {
    System.out.println("Heat advisory");   // line A
}
System.out.println("Check complete");     // line B

if (x > 5)
    System.out.println("A");   // only this inside the if
    System.out.println("B");   // ALWAYS runs
System.out.println("C");       // ALWAYS runs

Two-Way Selection: the if-else Statement (EK 2.3.A.4)

A two-way selection provides two code paths — exactly one always executes. There is no outcome where neither branch runs, and no outcome where both branches run.

if (booleanExpression) {
    // if-body: executes when condition is true
} else {
    // else-body: executes when condition is false
}
// continues here after exactly one branch ran

// if-else: exactly one branch, always safe for mutual exclusion
int score = 55;
if (score >= 60) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

// Two separate ifs: breaks when first branch modifies tested state
int grade = 50;
if (grade < 60) { grade = 60; }    // changes grade!
if (grade < 60) {                   // now always false
    System.out.println("Fail");     // never prints
}

One-way vs. two-way comparison

Real-World Connection

Selection is the decision engine of every program. A streaming service checks if you have a subscription before playing content. A thermostat checks if temperature is below the setpoint before turning on heat. AP FRQ problems require selection logic inside methods — precise understanding of one-way vs. two-way behavior is what separates a 4 from a 5.

Summary

• One-way selection (if): 0 or 1 branches execute. Code after the block always runs. (EK 2.3.A.3)
• Two-way selection (if-else): exactly one branch always executes — never both, never neither. (EK 2.3.A.4)
• Braces essential: without them only the single following statement belongs to the if.
• Dangling else: Java pairs else with nearest unmatched if. Use braces to prevent ambiguity.
• Boolean expressions only: non-booleans in the condition cause a compile error. (EK 2.3.A.2)
• Use if-else for mutually exclusive outcomes, especially when the first branch modifies state.

int x = 8;
if (x > 5) {
    System.out.println("High");
}
System.out.println("Done");

int n = 3;
if (n > 10) {
    System.out.println("Big");
}
System.out.println("End");

int score = 55;
if (score >= 60) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

int val = 0;
if (val > 0) {
    System.out.println("Positive");
} else {
    System.out.println("Not positive");
}
System.out.println("Checked");

int x = 3;
if (x > 5)
    System.out.println("A");
    System.out.println("B");
System.out.println("C");

int age = 17;
if (age >= 18) {
    System.out.println("Adult");
} else {
    System.out.println("Minor");
}
if (age >= 16) {
    System.out.println("Can drive");
}

if (n / 2 == 0) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

if (n == 2) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

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

if (n % 2) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

int a = 10;
int b = 20;
if (a > b) {
    System.out.println("A wins");
} else {
    System.out.println("B wins");
}
if (a + b > 25) {
    System.out.println("Sum is large");
}

int x = 5;
if (x > 3) {
    System.out.println("A");
    if (x > 7) {
        System.out.println("B");
    }
    System.out.println("C");
}
System.out.println("D");

if (n == 0) {
    System.out.println("Zero");
} else {
    System.out.println("Nonzero");
}

if (n != 0) {
    System.out.println("Nonzero");
} else {
    System.out.println("Zero");
}

if (n > 0) {
    System.out.println("Nonzero");
} else {
    System.out.println("Zero");
}

int m = 12;
if (m % 3 == 0) {
    System.out.println("Divisible by 3");
}
if (m % 4 == 0) {
    System.out.println("Divisible by 4");
}
if (m % 5 == 0) {
    System.out.println("Divisible by 5");
}