Aim for FiveComputer Science A

Unit 1 · Topic 1.2

1.2 Variables and Data Types

Every value in Java has a type, and the type decides what values are allowed and what you can do with them. This topic introduces the three primitive types you'll use all year, int, double and boolean, and the idea of a variable as a named place to store a value.

Key terms

  • data type
  • primitive type
  • reference type
  • int
  • double
  • boolean
  • variable

What a data type is

A data type is two things together: a set of allowed values and the operations that work on them. An int can hold 42 and can be added, subtracted and so on; a boolean can only be true or false, and you combine booleans with logic instead of arithmetic.

Java sorts types into two groups. Primitive types are the simple built-in types that hold a single value directly. Reference types are everything else: String, Scanner, ArrayList and any class you or someone else writes. A reference type describes objects, and you'll learn how they're stored in 1.12.

The three primitive types in this course

TypeHoldsExamples
intWhole numbers (integers), positive, negative or zero0, -12, 2026
doubleReal numbers, which can have a decimal part3.14, -0.5, 7.0
booleanOne of two valuestrue, false

Variables

A variable is a named storage location. When you declare one, you give its type and its name, and you usually give it a starting value too:

int score = 0; score = 15; double temperature = 71.5; boolean isRaining = true;

Each variable keeps its type forever, but its value can change while the program runs. After the second line, score holds 15, not 0. A variable of a primitive type stores the value itself.

Naming rules: a name starts with a letter, has no spaces, and can't be a Java keyword like int or class. Names are case-sensitive, so score and Score are different variables. Java programmers write multi-word names in camelCase, like stepsToday, and pick names that say what the value means.

Choosing the right type

Questions often describe some data and ask which type fits best. Ask yourself what the value looks like:

  • Something you count, like a number of students, points or steps: int.
  • A measurement that can have a fractional part, like a price, distance, temperature or average: double.
  • A yes-or-no fact, like whether a door is locked or a game is over: boolean.
  • Text, like a name or a sentence: String, which is a reference type, not a primitive.

Not on the exam

Java has five more primitive types: long, short, byte, float and char. You won't be tested on any of them, so stick with int, double and boolean.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1

    Picking types for a fitness app

    A fitness app tracks the number of steps a user takes today, the distance they walked in miles, whether they reached their daily goal, and their name. Declare a variable for each, then record that the goal has now been met and print everything. One correct answer:int stepsToday = 8250; double milesWalked = 3.6; boolean goalMet = false; String userName = "Maya"; goalMet = true; System.out.println(userName + " " + stepsToday + " " + milesWalked + " " + goalMet);

    Show the solution
    1. Step 1: Steps are counted one at a time and are always whole numbers, so stepsToday is an int.
    2. Step 2: Distance can be 3.6 miles, which has a decimal part, so milesWalked is a double.
    3. Step 3: Reaching the goal is yes or no, so goalMet is a boolean. It starts as false, and the fifth line changes it to true. Notice there's no type on that line: you only state the type when you declare the variable.
    4. Step 4: A name is text, so userName is a String, a reference type.
    5. Step 5: The + signs join the values into one string with spaces between them (more on that in 1.3 and 1.15).

    Answer: Use int for steps, double for miles, boolean for the goal and String for the name. The code prints Maya 8250 3.6 true.

  2. Example 2

    Spotting a type mismatch

    Will this declaration compile?int count = 3.5;

    Show the solution
    1. Step 1: The variable is declared as an int, so it can only hold whole numbers.
    2. Step 2: The value 3.5 is a double. Storing it in an int would lose the .5, and Java won't do that silently.
    3. Step 3: The compiler reports an error ("possible lossy conversion from double to int"), so this is a syntax error and the program won't run.

    Answer: No. It doesn't compile, because a double value can't be stored in an int variable without a cast (see 1.5).

Common mistakes

  • Using int for a value that can have a decimal part, like a price or an average. The decimal part will be lost or the code won't compile.
  • Putting the type in front of a variable every time you use it. int score = 0; declares it; after that, write score = 15;. Writing int score = 15; again in the same block is a compile error.
  • Treating String as a primitive type. It's a reference type (a class), which is why it has methods like length().

On the exam

  • Expect questions that describe data and ask for the most appropriate type, or that ask which declaration compiles. Match counts to int, measurements to double and yes-or-no facts to boolean.
  • On free-response questions, declare each local variable with the right type before you use it, and reuse the names the question gives you.

Connected topics

Videos

  • AP Computer Science A - Topic 1.2 – Variables and DataTypes

    Tim Gallagher Computer ScienceWatch on YouTube (opens in a new tab)

  • Variables and Primitive Data Types (Part 1)

    CodeHSWatch on YouTube (opens in a new tab)

  • Primitive and Reference (Object) Types in Memory (Java Tutorial)

    Bill BarnumWatch on YouTube (opens in a new tab)

  • Variables and Primitive Data Types (Part 2)

    CodeHSWatch on YouTube (opens in a new tab)

  • Primitive Types and Reference Types in Java

    Neso AcademyWatch on YouTube (opens in a new tab)

Check yourself

4 questions on 1.2 Variables and Data Types. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of the following declarations compiles without error?

Question 2 of 4

A program needs to store whether a library book is currently checked out. Which variable declaration is the best choice?

Question 3 of 4

Which of the following variables holds a reference to an object rather than a primitive value?

Question 4 of 4

Consider the following code segment.int points = 12; points = 20; double rate = points; System.out.println(points + " " + rate);What is printed as a result of executing the code segment?

0 of 4 answered