AP CSA Unit 1 | Data & Variables

Unit 1: Data & Variables

This unit builds the foundation: how Java stores values, evaluates expressions, and prints results. Students must be fluent with integer division, modulus, assignment, casting, and String concatenation.

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Common AP CSA Unit 1 Mistakes

Why this comes first: AP CSA questions often ask students to predict what code does, not just write new code. These examples target the mistakes that show up again and again with primitive types, expressions, assignment, casting, and Strings.

Mistake #1: Assignment Is Not Algebra

In Java, = means “store the value on the right into the variable on the left.” It does not mean both sides are permanently equal.

public class Main {
    public static void main(String[] args) {
        int x = 5;
        x = x + 3;
        System.out.println(x);
    }
}

Mistake #2: Integer Division Truncates

If both operands are int values, Java gives an int result and cuts off the decimal part.

public class Main {
    public static void main(String[] args) {
        System.out.println(7 / 2);
        System.out.println(7 / 2.0);
    }
}

3
3.5

Mistake #3: Remainder Is Not Percent

The % operator gives the remainder after division. It is not a percent sign.

public class Main {
    public static void main(String[] args) {
        System.out.println(17 % 5);
        System.out.println(20 % 4);
        System.out.println(7 % 2);
    }
}

2
0
1

Mistake #4: String Concatenation Changes +

Before Java reaches a String, + does math. Once a String is involved, + joins text.

public class Main {
    public static void main(String[] args) {
        System.out.println(2 + 3 + " meters");
        System.out.println("meters: " + 2 + 3);
        System.out.println("meters: " + (2 + 3));
    }
}

5 meters
meters: 23
meters: 5

Mistake #5: Primitive Types vs. Object Types

int, double, and boolean are primitive types. String is an object type.

public class Main {
    public static void main(String[] args) {
        int age = 16;
        double height = 1.75;
        boolean passed = true;
        String name = "Emma";

        System.out.println(name.length());
    }
}

Mistake #6: Casting Truncates

Casting a double to an int cuts off the decimal part. It does not round.

public class Main {
    public static void main(String[] args) {
        double value = 9.8;
        int whole = (int) value;
        System.out.println(whole);
    }
}

Mistake #7: Order of Operations Still Matters

Java follows standard operator precedence. Multiplication, division, and modulus happen before addition and subtraction.

public class Main {
    public static void main(String[] args) {
        int answer = 2 + 3 * 4;
        System.out.println(answer);
    }
}

Mistake #8: Compound Assignment Updates in Order

Java executes statements one at a time from top to bottom.

public class Main {
    public static void main(String[] args) {
        int score = 90;
        score += 5;
        score *= 2;
        System.out.println(score);
    }
}

Mistake #9: Primitive Variables Store Separate Copies

Copying an int value into another variable does not link the variables together.

public class Main {
    public static void main(String[] args) {
        int x = 4;
        int y = x;
        y = y + 3;
        System.out.println(x);
        System.out.println(y);
    }
}

4
7

Mistake #10: Mixed int and double Arithmetic

If either operand is a double, Java promotes the calculation to a decimal result.

public class Main {
    public static void main(String[] args) {
        int a = 5;
        double b = 2;
        System.out.println(a / b);
    }
}

Unit 1 AP Exam Challenge

This combines primitive types, assignment, mixed arithmetic, and String concatenation.

public class Main {
    public static void main(String[] args) {
        int x = 10;
        double y = 3;

        System.out.println(x / y);

        x += 2;

        System.out.println(x);
        System.out.println("Result: " + x + " " + y);
    }
}

3.3333333333333335
12
Result: 12 3.0

Primitive Types and Variables

These examples connect the common mistakes above to the core Unit 1 skills: using arithmetic, casting, and variables in realistic calculations.

int a = 17 / 5;
int b = 17 % 5;
System.out.println(a + " " + b);

int correct = 17;
int total = 20;
double pct = (double) correct / total;
System.out.println(pct);

double v0 = 20.0;
double g = 9.8;
double t = 2.0;
double y = v0 * t - 0.5 * g * t * t;
System.out.println(y);

Try It in Java

Run and modify the examples below. Add print statements, change values, and test edge cases.

Code Tracing Drop-Down Practice

Debugging Practice

AP-Style Multiple Choice

FRQ Practice

Write a method public static double height(double v0, double t) that returns y = v0*t - 0.5*9.8*t*t. Then call it for t = 0, 1, 2, 3 and print the results.

Physics / Real-World Mini Project

Build a vertical motion calculator. Store initial velocity, time, and gravity as variables, calculate height, and print a clear sentence explaining the result.