Implementing Private Class Data in Java

Private Class Data: A pattern that protects internal state by grouping it behind a narrow public API instead of exposing the state structure directly.

In Java, a simple implementation often uses a private immutable state holder.

 1public final class BankAccount {
 2    private record State(String id, Money balance, AccountStatus status) {}
 3
 4    private State state;
 5
 6    public BankAccount(String id, Money openingBalance) {
 7        this.state = new State(id, openingBalance, AccountStatus.OPEN);
 8    }
 9
10    public Money balance() {
11        return state.balance();
12    }
13
14    public void deposit(Money amount) {
15        state = new State(state.id(), state.balance().add(amount), state.status());
16    }
17}

Clients do not see the state shape directly. They see operations and selected read access.

When This Helps

This pattern is useful when:

• multiple fields must move together consistently
• state should not be mutated piecemeal
• internal representation may change later

Review Rule

If the internal data structure is part of the class’s invariants, hide the structure and expose behavior or carefully chosen queries instead.