5.2 Scope and Lifetime

Scope decides where a name is visible. A variable declared inside a method or block is local.

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// i cannot be used here

Method parameters are local variables too. When a method is called, argument values are copied into parameters, so changing a parameter does not automatically change the caller's variable.

A classic example is swap(): it tries to exchange two variables inside a method, but fails for primitive int values.

static void swap(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}

public static void main(String[] args) {
    int x = 3;
    int y = 8;
    swap(x, y);
    System.out.println(x + " " + y);
}

Output:

3 8

swap() only exchanges its own local copies a and b; x and y in main never change. To really update caller-visible data in Java, you usually return a value, update an array/object that was passed in, or design a small object to hold the result.

A field declared in a class can be accessed by methods in that class. If the field is static, it belongs to the class itself. Long-lived shared state can make data changes harder to trace, so prefer local variables and parameters first.

Java does not support C-style static local variables inside a method. When you need a value to persist across method calls, use a field.