public class Main {
    public static void main(String[] args) {
        Account acc = new Account("Asha", 1000);
        acc.withdraw(300);
        System.out.println(acc.getOwner() + ": " + acc.getBalance());
        try {
            acc.withdraw(5000);
        } catch (IllegalArgumentException e) {
            System.out.println("rejected: " + e.getMessage());
        }
        System.out.println("balance still " + acc.getBalance());
    }
}

class Account {
    private final String owner;
    private long balance;                  // hidden: no one can set it to -1

    Account(String owner, long opening) {
        if (owner == null || owner.isBlank()) throw new IllegalArgumentException("owner required");
        this.owner = owner;
        this.balance = opening;
    }

    String getOwner() { return owner; }
    long getBalance() { return balance; }  // read-only: no setBalance()

    void withdraw(long amount) {
        if (amount <= 0 || amount > balance)
            throw new IllegalArgumentException("invalid amount " + amount);
        balance -= amount;
    }
}

Asha: 700
rejected: invalid amount 5000
balance still 700

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> input = new ArrayList<>(List.of("math", "physics"));
        Leaky leaky = new Leaky(input);
        Safe safe = new Safe(input);

        input.add("hacked");                 // caller mutates its own list
        leaky.getSubjects().clear();         // caller mutates via the getter

        System.out.println("leaky: " + leaky.getSubjects());
        System.out.println("safe:  " + safe.getSubjects());
        try {
            safe.getSubjects().add("x");
        } catch (UnsupportedOperationException e) {
            System.out.println("safe getter is read-only");
        }
    }
}

class Leaky {
    private final List<String> subjects;
    Leaky(List<String> subjects) { this.subjects = subjects; }   // stores caller's list
    List<String> getSubjects() { return subjects; }              // hands out internals
}

class Safe {
    private final List<String> subjects;
    Safe(List<String> subjects) { this.subjects = List.copyOf(subjects); }   // copy in
    List<String> getSubjects() { return subjects; }                          // already unmodifiable
}

leaky: []
safe:  [math, physics]
safe getter is read-only