Topic 3.8 — Scope and Access

Goal: explain variable scope (where a variable can be used) and access control (what code can access fields/methods), including the roles of private and public.

The big idea

Scope is about where a variable exists and can be used (local vs instance vs class). Access is about who is allowed to use a class’s fields/methods (private vs public).

Scope levels (fast)

Kind	Where declared	Where usable
Local	inside a method / block	only inside that block
Instance	inside a class (not static)	in instance methods (for that object)
Class	inside a class with `static`	in static methods (class-wide)

Local scope example

Variables declared inside an if or loop only exist inside that block.

public void demo(int x) {
  if (x > 0) {
    int y = x * 2;     // y exists only inside this if-block
    // System.out.println(y); // ✅ here
  }
  // System.out.println(y);   // ❌ y not in scope here
}

Instance variables: scope across methods

Instance variables are declared in the class and can be used by instance methods.

public class Counter {
  private int value;     // instance variable

  public void inc() { value++; }
  public int get() { return value; }
}

Every Counter object has its own value.

Shadowing (name collision)

If a local variable has the same name as an instance variable, the local one “wins” inside that scope.

private int score;

public void setScore(int score) {
  score = score;   // ❌ sets parameter to itself
}

You’ll fix this with this.score = score; in Topic 3.9.

Access control: `private` vs `public`

Modifier	Who can access?	Typical use
`private`	only inside the same class	fields (protect state)
`public`	any code can access	methods (the “interface”)

Why make fields private?

Prevents other code from changing data in unsafe ways.
Forces updates to go through methods (so you can validate).
Makes the class easier to maintain (you can change internals later).

// Better design
private int age;

public void setAge(int a) {
  if (a >= 0) age = a;
}

Quick self-check

What does “scope” mean?
Where can a local variable be used?
What does private prevent?
Why do we usually make fields private?
What is shadowing? (Why is it confusing?)

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