Encapsulation in Java — ICSE Class 10 Computer Applications

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Pehli line: har object ne apna id rakha → 1 aur 2. Doosri line: count static hai, isliye p.count aur q.count dono usi ek copy ko dekh rahe hain → 2 aur 2. Teesri line: p.id badalne se q.id par koi asar nahi pada, kyunki woh alag variable hai.

Meera scored 88
Marks must be between 0 and 100
After invalid set : 88
After valid set   : 95

150 ko setter ne rok diya, isliye marks 88 hi raha. 95 valid tha, isliye woh andar gaya. Agar marks public hota toh koi bhi s.marks = 150; likh kar yeh jaanch chakma de deta — yahi hai encapsulation ka asli faayda.

Encapsulation is the wrapping up of data and the methods that operate on that data into a single unit, namely a class. Data hiding is the part of encapsulation that keeps the data out of reach from outside the class, achieved by declaring the data members private and providing public methods to work with them. In short: encapsulation binds them together, data hiding controls who may reach them.

The three access specifiers are private, protected and public. In increasing order of visibility: private (same class only), then protected (same class, same package and derived classes), then public (everywhere). If default access is also to be placed, it sits between private and protected.

A class variable is declared with the static keyword and only one copy exists, shared by every object of the class; changing it through one object changes it for all. An instance variable is declared without static and every object gets its own separate copy, so changing it in one object leaves the others untouched. A class variable belongs to the class; an instance variable belongs to the object.

Inside test, n = 20
Data member n = 10
From main, n = 10

Local n ne data member ko chhupa diya, isliye khaali n ne 20 diya. this.n ne data member ka 10 diya. Local variable method ke saath khatam ho gaya, isliye main() mein 10 hi bacha.

U1.java:6: error: variable sum might not have been initialized
        sum = sum + 5;
              ^
1 error

sum ek local variable hai, aur local variables ko default value nahi milti. Line 6 par uski value padhi ja rahi hai jab usmein kuch rakha hi nahi gaya. Sudhaar: int sum = 0; likhiye. Agar sum data member hota toh use apne aap 0 mil jaata aur koi error na aati.

Making data members private prevents any code outside the class from reading or changing them directly, so the object cannot be put into an invalid state by accident. Making methods public provides controlled doorways to that data: the method can check the value before storing it, or format it before returning it. The class therefore decides what may be done to its own data, rather than trusting every other class to be careful.

200 50 9 5
200 77 9 5

classVar dono baar 200 — ek hi copy sabke liye. instanceVar 50 aur 77 — alag-alag copies. Argument aur local dono baar wahi, kyunki har call unhein naye sire se banata hai.

class E1
{
    public static void main(String args[])
    {
        Account a = new Account(500);
        a.deposit(250);
        System.out.println("Balance = " + a.getBalance());
        a.deposit(-100);
        System.out.println("Balance = " + a.getBalance());
    }
}

class Account
{
    private int balance;

    Account(int b)
    {
        balance = b;
    }

    public int getBalance()
    {
        return balance;
    }

    public void deposit(int amt)
    {
        if (amt > 0)
            balance = balance + amt;
        else
            System.out.println("Deposit must be positive");
    }
}

Output:
Balance = 750
Deposit must be positive
Balance = 750

Dhyaan dijiye ki getBalance() ek pure (accessor) method hai aur deposit() impure (mutator), kyunki woh object ki state badalta hai.

The program does not compile. The compiler reports that the member has private access in that class, because a private member is visible only inside the class that declares it. The fix is to add a public setter method inside the class, for example public void setMember(int m) { member = m; }, and call obj.setMember(5); instead. Simply changing the member to public would also compile, but it defeats data hiding and is not the expected answer.

(i) Class variable — declared inside the class with the static keyword; one shared copy. (ii) Instance variable — declared inside the class without static; each object has its own copy. (iii) Argument variable — declared in the brackets of a method or constructor; exists only for the duration of that call. (iv) Local variable — declared inside the body of a method or block; exists only within that block and receives no default value.