Unit 1.4

Assignment Statements and Input

AP Computer Science A

What we'll learn: How to store values in variables and get input from users

Variable Initialization

Every variable must be assigned a value before it can be used in an expression.

A variable is initialized the first time it is assigned a value.

int x; // Declaration only - not initialized! // System.out.println(x); // ERROR! Can't use x yet int y = 5; // Declaration AND initialization System.out.println(y); // OK! y has a value x = 10; // Now x is initialized System.out.println(x); // OK! x now has a value

The Assignment Operator (=)

The assignment operator = allows a program to initialize or change the values stored in a variable.

The value of the expression on the right is stored in the variable on the left.

int score = 85; // Initialize score to 85 score = 90; // Change score to 90 score = score + 5; // Use current value in expression // score is now 95 String name = "Alice"; // Initialize name name = "Bob"; // Change name to "Bob"

Expression Evaluation

During execution, an expression is evaluated to produce a single value.

The value of an expression has a type based on the evaluation.

int result = 3 + 4 * 2; // Evaluates to 11 (not 14!) double average = (85 + 92 + 78) / 3.0; // 85.0 boolean passed = average >= 70; // true String greeting = "Hello" + " " + "World"; // "Hello World"

Reading Input with Scanner

Input can come in various forms: tactile, audio, visual, or text.

The Scanner class is one way to obtain text input from the keyboard.

import java.util.Scanner; Scanner input = new Scanner(System.in); System.out.print("Enter your name: "); String name = input.nextLine(); System.out.print("Enter your age: "); int age = input.nextInt(); System.out.print("Enter your GPA: "); double gpa = input.nextDouble(); System.out.println("Hello " + name + "!");

Summary & Key Points

  • Variables must be initialized before use
  • Assignment operator = stores right-side value in left-side variable
  • Expressions are evaluated to produce a single typed value
  • Reference types can be assigned objects or null
  • Scanner class helps read text input from keyboard
Practice: Try writing code that declares variables, assigns values using expressions, and reads user input with Scanner!
1 / ?