Scope and Storage

Scope determines where a variable can be accessed. Understanding scope helps you reason about variables inside and outside methods.

Local variables

Variables defined inside a method are local variables. They can only be accessed inside that method.

static void swap(int a, int b) {
    int temporary = a;
    a = b;
    b = temporary;
    System.out.println("inside swap(): " + a + " " + b);
}

public static void main(String[] args) {
    int a = 1;
    int b = 2;
    System.out.println("Before: " + a + " " + b);
    swap(a, b);
    System.out.println("After: " + a + " " + b);
}

Output:

Before: 1 2
inside swap(): 2 1
After: 1 2

static variables

A local variable exists only inside its method. When several static methods need to read and update the same value, declare the variable outside those methods and mark it static.

A static variable has one value for the duration of the program. After one static method changes it, another static method reads the updated value.

static int score = 0;

static void addScore(int points) {
    score += points;
}

static int getScore() {
    return score;
}

public static void main(String[] args) {
    addScore(10);
    addScore(5);
    System.out.println(getScore());
}

Output:

15

Use a static variable for state maintained by several static methods. Prefer focused methods for updates instead of assigning to it from arbitrary locations.