İç İçe Sınıflar — static, Inner, Anonim ve Yerel

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public final class Account implements Iterable<Movement> {
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    private final String iban;
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    private final List<Movement> movements;
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    private Account(Builder builder) {
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        this.iban = builder.iban;
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        this.movements = List.copyOf(builder.movements);
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    }
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    public String iban() {
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        return iban;
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    }
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    @Override
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    public Iterator<Movement> iterator() {
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        return new MovementIterator();
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    }
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    /** Static: builds an account before any account exists. */
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    public static final class Builder {
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        private String iban;
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        private final List<Movement> movements = new ArrayList<>();
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        public Builder iban(String iban) {
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            this.iban = iban;
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            return this;
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        }
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        public Builder movement(Movement movement) {
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            movements.add(movement);
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            return this;
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        }
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        public Account build() {
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            return new Account(this);
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        }
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    }
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    /** Inner: walks this account's own movements and never outlives it. */
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    private final class MovementIterator implements Iterator<Movement> {
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        private int next;
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        @Override
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        public boolean hasNext() {
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            return next < movements.size();
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        }
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        @Override
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        public Movement next() {
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            return movements.get(next++);
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        }
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    }
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}

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public final class StatementReminder implements Runnable {
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    /** Static nested on purpose: the scheduler keeps it for days, so it must not hold an Account. */
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    public static final class Payload {
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        private final String iban;
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        public Payload(String iban) {
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            this.iban = iban;
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        }
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        public String iban() {
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            return iban;
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        }
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    }
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    private final Payload payload;
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    private final Notifier notifier;
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    public StatementReminder(Payload payload, Notifier notifier) {
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        this.payload = payload;
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        this.notifier = notifier;
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    }
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    @Override
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    public void run() {
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        notifier.send(payload.iban(), "Aylık ekstreniz hazır.");
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    }
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}

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public class AccountScreen {
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    private final ScheduledExecutorService scheduler;
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    private final Notifier notifier;
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    public AccountScreen(ScheduledExecutorService scheduler, Notifier notifier) {
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        this.scheduler = scheduler;
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        this.notifier = notifier;
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    }
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    public void scheduleStatementReminder(Account account) {
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        var payload = new StatementReminder.Payload(account.iban());
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        scheduler.schedule(new StatementReminder(payload, notifier), 30, TimeUnit.DAYS);
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    }
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}