Variables and the Primitive Types

Where you are: Week 0 review > Variables and the primitive types

Try This First

A friend who knows Python says: “I can write x = 5 and later x = 'hello' and it works.” Decide whether the same two lines work in Java before reading on.

Before You Start

Check each box you can do from memory. A box you cannot check yet is not a problem; it points you to a quick refresher, not a grade.

• I can name the file the JVM actually runs (not the .java source file).
• I can explain what happens if you change a .java file and run it without recompiling.

See How a Java Program Compiles and Runs if either box is unclear.

What You Need To Walk In With

Walk into the next class able to state these:

• A variable is a labeled box that holds one value of one declared type, and that type cannot change.
• The four CS1 primitive types are int (whole numbers), double (decimals), boolean (true or false), and char (one character). String holds text and is not a primitive.
• = means gets, not equals. The right side is computed first, then stored into the name on the left.
• A local variable has no default value. Assign a value before you read it.
• 'A' in single quotes is a char; "A" in double quotes is a String. They are different types.

You should be able to: classify a literal as int, double, char, String, or illegal; trace a short sequence of assignments and state the exact output.

How It Works

Writing int age = 20; does three things at once: it reserves memory for a whole number, labels that memory age, and stores 20 in it. The type label is permanent; the value inside can change.

int age = 20;
double gpa = 3.75;
boolean enrolled = true;
char section = 'A';
String name = "Alex";

= means gets

int x = 5;   // x gets 5
x = x + 1;   // x gets the old value of x plus 1, so x is now 6

Read = as gets. The right side is evaluated, and the result is stored on the left. It is a command, not a claim that the two sides are equal.

Declaration without a value

int count;
System.out.println(count);   // compile error: variable count might not have been initialized

A local variable starts with no value. Assign one (int count = 0;) before you read it.

A constant that does not change

final double TAX_RATE = 0.08;

final marks a value that must not be reassigned. The convention names constants in all capitals.

Worked Example: Predict, Then Check

int x = 10;
x = x + 5;
System.out.println("x: " + x);
char grade = 'B';
System.out.println("grade: " + grade);

Predict both lines of output before reading on.

x: 15
grade: B

A Common Mistake

Using double quotes for a single character is a frequent error: char letter = "A"; does not compile, because "A" is a String and a String does not fit in a char box. Single quotes make a char ('A'); double quotes make a String ("A"). When a compiler message mentions a char literal or a String literal, check the quote style first. (Source: BJP (Reges and Stepp), Ch 2; JLS 25, §3.10.4 and §3.10.5.)

Go Deeper (optional)

For the curious: a char is really a 16-bit number underneath. char c = 'A'; System.out.println((int) c); prints 65, the code point for A. And int stops near two billion because 32 binary digits can represent only so many values, the same place-value idea as base ten, with base two instead. (This connects to how the real number system is modeled in computing.)

Check Yourself

Close the notes and answer each one from memory, then reveal it. Pulling an idea back from memory is one of the strongest ways to make it stick.