Assignment Statements
and Input

Master how Java stores values in variables using assignment statements, and learn to capture user input with the Scanner class.

AP Computer Science A

What Is a Variable?

A variable is a named container in memory that holds a single value at a time. Think of it as a labeled box: the label is the variable's name, and the contents are its value.

Declaring Variables in Java

A declaration tells the compiler the variable's type and name. You may also initialize it with a starting value in the same statement.

// Declaration only
int age;

// Declaration with initialization
double gpa = 3.85;
String greeting = "Hello";

int

Whole numbers: \( -2, 0, 42 \)

double

Decimals: \( 3.14, -0.5 \)

boolean

True or false only

Why Assignment Matters

Almost every Java program stores, updates, and retrieves data. Without assignment statements, programs could not remember anything between lines of code.

1

Store initial values from the user or from calculations.

2

Update values as the program runs: counters, totals, flags.

3

Retrieve values to display results or make decisions.

The Assignment Statement

The assignment operator \(\texttt{=}\) copies the value on the right side into the variable on the left side. It does not mean "equals" in the math sense.

int x = 10 + 5;   // x now holds 15

Reference assignment as an analogy

Copying an object reference does not copy the mutable object. Reassigning a String reference does not alter the original reference; the apparatus is an analogy, not a literal Java heap.

Java reference analogy: a and b share a mutable object, so a mutation through b is visible through a. Reassigning t from "oak" to "elm" leaves s referring to "oak". The apparatus is an analogy, not a literal Java heap.

Re-assignment: Overwriting Values

When you assign a new value to an existing variable, the old value is destroyed. The variable only ever holds one value at a time.

int count = 3;
count = 7;       // old value 3 is gone
count = count + 1; // count is now 8

\(\texttt{=}\) versus \(\texttt{==}\)

The single equals sign and the double equals sign do completely different things in Java. Confusing them is one of the most common bugs.

	\(\texttt{=}\) (Assignment)	\(\texttt{==}\) (Comparison)
Purpose	Stores a value	Tests if two values are equal
Returns	The assigned value	A boolean (true/false)
Example	\(\texttt{x = 5}\)	\(\texttt{x == 5}\)
Used in	Statements	Conditions (if, while)

Compound Assignment Operators

Java provides shorthand operators that combine arithmetic and assignment into a single step. These are tested on the AP exam.

Shorthand	Equivalent	Result (\(\texttt{x}\) starts at 10)
\(\texttt{x += 3}\)	\(\texttt{x = x + 3}\)	13
\(\texttt{x -= 4}\)	\(\texttt{x = x - 4}\)	6
\(\texttt{x *= 2}\)	\(\texttt{x = x * 2}\)	20
\(\texttt{x /= 5}\)	\(\texttt{x = x / 5}\)	2
\(\texttt{x \%= 3}\)	\(\texttt{x = x \% 3}\)	1

Increment and Decrement

The \(\texttt{++}\) and \(\texttt{--}\) operators add or subtract 1. They are the most compact form of re-assignment.

int lives = 3;
lives++;      // lives is now 4 (same as lives = lives + 1)
lives--;      // lives is now 3 again

User Input with Scanner

The Scanner class reads input from the keyboard. You must import it and create an object before using it.

import java.util.Scanner;

Scanner input = new Scanner(System.in);

System.out.print("Enter your age: ");
int age = input.nextInt();

Key Scanner Methods

nextInt()

Reads the next integer from input

nextDouble()

Reads the next decimal number

nextLine()

Reads the entire line as a String

double price = input.nextDouble();
input.nextLine();  // consume leftover newline
String item = input.nextLine();

How to Apply Assignment

1

Identify the type of data you need to store (int, double, String, boolean).

2

Declare the variable with its type and a meaningful name.

3

Initialize the variable using \(\texttt{=}\) and an expression or literal.

4

Update as needed with \(\texttt{=}\), \(\texttt{+=}\), \(\texttt{-=}\), \(\texttt{++}\), or \(\texttt{--}\) operators.

5

Trace the variable's value after each statement to verify correctness.

Example 1: Simple Assignment

Trace the value of each variable after every line.

int a = 5;
int b = 10;
a = b;
b = 20;

Two collaborators work on one shared model

Two collaborators adjust one blue articulated model while a second model stands apart.

Java assignment copies a value. For primitive variables this is the primitive value; for object variables it is a reference to an object.

Assignment copies a value

Primitive variables x and y separate after reassignment; String references a and b point to different immutable strings.

After x is changed to 9, y keeps its copied value of 42. For String variables, assignment copies a reference: reassigning a to "world" leaves b referring to the unchanged "hello" object.

Example 2: Swapping Two Variables

You cannot swap two variables directly. A temporary variable is required to hold one value during the swap.

int x = 3, y = 7;

int temp = x;   // temp = 3
x = y;          // x = 7
y = temp;       // y = 3

Example 3: Input and Calculation

A complete program that reads two numbers from the user, calculates their average, and prints the result.

import java.util.Scanner;

public class Average {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    System.out.print("Enter num1: ");
    double num1 = sc.nextDouble();

    System.out.print("Enter num2: ");
    double num2 = sc.nextDouble();

    double avg = (num1 + num2) / 2.0;
    System.out.println("Average: " + avg);
  }
}

Common Mistakes

✕ Using an uninitialized variable

Declaring \(\texttt{int x;}\) then using \(\texttt{x}\) before assigning a value causes a compile error.

✕ Confusing \(\texttt{=}\) and \(\texttt{==}\)

Writing \(\texttt{if (x = 5)}\) in Java causes a compile error because the assignment returns an \(\texttt{int}\), not a \(\texttt{boolean}\). Use \(\texttt{==}\) for comparison.

✕ Integer division surprise

\(\texttt{int / int}\) truncates the decimal. \(7 / 2\) gives \(3\), not 3.5. Use a double operand.

Question 1 of 5

What is the value of \(\texttt{y}\) after the following code runs?

int x = 4; int y = x + 3; x = 10;

When \(\texttt{y = x + 3}\) executes, \(\texttt{x}\) is \(4\), so \(\texttt{y}\) becomes \(7\). Changing \(\texttt{x}\) later to \(10\) does not affect \(\texttt{y}\)'s stored value.

Question 2 of 5

Which line of code correctly declares and initializes a double variable?

The correct syntax is \(\texttt{type name = value;}\). Option (c) follows this pattern. Option (b) uses \(\texttt{int}\), which cannot hold a decimal. Option (d) uses \(\texttt{==}\) (comparison), not \(\texttt{=}\) (assignment).

Question 3 of 5

What does \(\texttt{x \%\%= 4}\) do if \(\texttt{x}\) is currently \(15\)?

The \(\texttt{\%=}\) operator assigns the remainder of division. Since \( 15 \div 4 = 3 \) remainder \( 3 \), \(\texttt{x}\) becomes \(3\).

Question 4 of 5

To swap the values of variables \(\texttt{a}\) and \(\texttt{b}\), what is the minimum number of assignment statements needed?

You need three assignments: (1) save one value in a temp variable, (2) overwrite the first with the second, (3) overwrite the second with temp. With only 2, you lose one original value.

Question 5 of 5

What is printed by this code?

int n = 7; n += 3; n *= 2; System.out.println(n);

Step by step: \(\texttt{n}\) starts at \(7\). After \(\texttt{n += 3}\), \(\texttt{n}\) is \(10\). After \(\texttt{n *= 2}\), \(\texttt{n}\) is \(20\). Compound operators execute left to right.

Assignment Statementsand Input

What Is a Variable?

Declaring Variables in Java

Why Assignment Matters

The Assignment Statement

Reference assignment as an analogy

Re-assignment: Overwriting Values

\(\texttt{=}\) versus \(\texttt{==}\)

Compound Assignment Operators

Increment and Decrement

User Input with Scanner

Key Scanner Methods

How to Apply Assignment

Example 1: Simple Assignment

Two collaborators work on one shared model

Assignment copies a value

Example 2: Swapping Two Variables

Example 3: Input and Calculation

Common Mistakes

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Assignment Statements
and Input