Concept · Intro

Encapsulation

Bundle data with the methods that guard it, hide the internals, and expose only a safe public interface — so invariants can’t be broken.

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step 1 / 9
1/2The capsule
BankAccount
- balance: int
- pin: String
+ deposit(amt)
+ withdraw(amt)
+ getBalance()
The capsule
 1class BankAccount {
 2    private int balance;
 3    private String pin;
 4 
 5    public void deposit(int amt) { ... }
 6    public void withdraw(int amt) {
 7        if (amt > balance) throw new Error();
 8        balance -= amt;
 9    }
10    public int getBalance() { return balance; }
11}
State
−private (hidden)
✦

Here is one capsule: BankAccount. The middle rows are its data — balance and pin — each marked with a “−”. That dash means PRIVATE: locked away, unreachable from outside.