Declaring variables
java
int age; // declaration — creates the variable
age = 17; // assignment — gives it a value
int score = 95; // declaration + initialization in one step
java
x = 10; // ERROR: x hasn't been declared
Primitive data types (AP Exam)
int
java
int students = 30;
int temperature = -5;
int population = 1000000;
double
java
double gpa = 3.85;
double pi = 3.14159;
double salary = 50000.0; // still a double even though it looks "whole"
boolean
java
boolean isRaining = true;
boolean gameOver = false;
Other types to know
Naming rules and conventions
Rules (these MUST be followed or the code won't compile)
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Conventions (these SHOULD be followed for clean code)
java
// Good naming
int studentCount = 25;
double averageGrade = 88.5;
boolean isEnrolled = true;
// Bad naming (compiles but hard to read)
int x = 25;
double a = 88.5;
boolean b = true;
Constants with final
java
final double TAX_RATE = 0.08;
final int MAX_STUDENTS = 30;
TAX_RATE = 0.10; // ERROR: cannot assign a value to final variable
Default values vs. local variables
java
public static void main(String[] args) {
int x;
System.out.println(x); // ERROR: variable x might not have been initialized
}
AP Exam Tips
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Common Mistakes
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