Access Specifiers and Variable Scope in Java (ICSE Class 10)

Understand Access Specifiers and Variable Scope Programs

class Person {
    private int age = 10;
    int getAge() { return age; }
    void setAge(int a) { if (a > 0) age = a; }
}

Access Specifiers and Variable Scope Programs: key facts

Visibility rules

private	same class only
protected	same class, same package, sub classes
public	all classes

Order from narrowest to widest: private, protected, public.

Four kinds of variables

Local	inside a method or block
Argument	in the method header
Instance	in the class, one per object
Class	static, one per class

Only local variables have no default value.

Controlled access

• •Make variables private
• •Provide public getters
• •Provide public setters with checks

This is encapsulation (data hiding).

Name clash

Plain name: nearest variable (local or argument) | this.name: the instance variable

Same name twice in one scope is an error.

How to answer Access Specifiers and Variable Scope Programs questions

1. 1Find which class each line of code is written in.
2. 2For each member used, check its specifier against the class that uses it.
3. 3For variables, mark the block in which each is declared and check each use falls inside that block.
4. 4When names clash, use the nearest declaration for the plain name and the instance variable for this.name.
5. 5If a rule is broken, name the compile-time error and give the fix (getter, setter or a wider specifier).

Access Specifiers and Variable Scope Programs in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Access Specifiers and Variable Scope Programs

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Private < Protected < Public

  Each step to the right lets more classes in: only me, my family and neighbours, everyone.

  Use it when: Visibility questions.

• Brace is the boundary

  A local variable lives only between the braces where it is declared.

  Use it when: Scope errors.

• this points at my own

  this.x is the object's variable; plain x is the closest x.

  Use it when: Same-name variables.

Access Specifiers and Variable Scope Programs: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

class Acct {
    private int bal = 100;
    public int getBal() { return bal; }
}
public class Test {
    public static void main(String[] args) {
        Acct a = new Acct();
        System.out.println(a.getBal());
        System.out.println(a.bal);
    }
}

class Sc {
    int x = 10;
    void show(int x) {
        int z = x * 2;
        System.out.println(x + " " + z + " " + this.x);
    }
}
// in main
new Sc().show(4);

class Shop {
    static int shops = 0;
    int stock;
    void sell(int qty) {
        int left = stock - qty;
        stock = left;
    }
}
public class Test {
    public static void main(String[] args) {
        Shop s = new Shop();
        s.stock = 20;
        s.sell(5);
        System.out.println(s.stock + " " + Shop.shops);
    }
}

int a = 5;
for (int i = 0; i < 3; i++) {
    int t = i * a;
    a = a + 1;
}
System.out.println(a);

Where students lose marks in Access Specifiers and Variable Scope Programs

The slips examiners see most often, and what to write instead.

• ✗Mistake: Using a loop variable after the loop

  A variable declared in the for header lives only within the loop.

  Wrong working

  for (int i = 0; i < 3; i++) { } System.out.println(i);

  Right working

  Declare int i before the loop if it is needed afterwards.

  ✓Do this: Declare outside the loop.

  Usually costs: 1 mark

• ✗Mistake: Calling protected or private members "available everywhere"

  Only public members are available to every class.

  Wrong working

  protected: accessible everywhere.

  Right working

  protected: same class, same package and sub classes.

  ✓Do this: Learn the three rules in order.

  Usually costs: 1 mark

• ✗Mistake: Forgetting this. when a parameter has the same name

  The plain name is the parameter, so the instance variable is not changed.

  Wrong working

  void set(int x) { x = x; }

  Right working

  void set(int x) { this.x = x; }

  ✓Do this: Write this.x on the left.

  Usually costs: 1 mark

Practice questions on Access Specifiers and Variable Scope Programs

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which access specifier allows access to a member only inside its own class?
   (a) public (b) protected (c) private (d) static

   Hint

   Most restricted.

   Answer

   (c) private

2. Q2 · 1 mark · Very short answerBoard level

   What name is given to a variable declared inside a method?

   Hint

   Its scope is that method only.

   Answer

   A local variable.

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   class T {
       int n = 3;
       void f() {
           int n = 7;
           System.out.println(n + " " + this.n);
       }
   }
   // in main
   new T().f();

   Hint

   Plain n is the nearest declaration.

   Answer

   7 3

4. Q4 · 3 marks · Short answerHarder board level

   Given class Person with private int age = 10; and the methods getAge() and setAge(int a) { if (a > 0) age = a; }, write the output of:
   Person p = new Person();
   p.setAge(-5);
   System.out.println(p.getAge());
   p.setAge(12);
   System.out.println(p.getAge());
   Also state why age is made private.

   Hint

   The setter checks before assigning.

   Answer

   10
   12. age is private so that other classes cannot give it an invalid value directly; the setter checks that the value is positive (data hiding).

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): A private variable of a class cannot be accessed directly from another class.
   Reason (R): The private specifier limits access to the class in which the member is declared.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Does R define private?

   Answer

   (a) Both are true and R explains A.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which specifier gives access to every class?Show answer

   public

2. 2.What is a getter?Show answer

   A public method that returns the value of a private variable.

3. 3.What is the scope of a local variable?Show answer

   The method or block in which it is declared.

4. 4.What does this.x refer to?Show answer

   The instance variable x of the current object.

5. 5.Which variable has one copy for the whole class?Show answer

   A class (static) variable.

6. 6.What must be done to a local variable before it is read?Show answer

   It must be assigned a value; the compiler gives it no default.

Access Specifiers and Variable Scope Programs: frequently asked questions