Java - Encapsulation

Encapsulation

Access Modifiers

• public: visible everywhere
• protected: visible in same package and subclasses
• (package-private): no modifier; visible in same package
• private: visible only within the declaring class

Getters/Setters with Validation

public class BankAccount {
  private int balance; // cents

  public synchronized void deposit(int cents) {
    if (cents <= 0) throw new IllegalArgumentException("positive only");
    balance += cents;
  }
  public synchronized int getBalance() { return balance; }
}

Immutability

Make state unchangeable after construction to simplify reasoning and enable thread-safety.

public final class Address {
  private final String city;
  private final java.util.List lines;
  public Address(String city, java.util.List lines) {
    this.city = city;
    this.lines = java.util.List.copyOf(lines); // defensive copy
  }
  public String getCity() { return city; }
  public java.util.List getLines() { return lines; } // unmodifiable
}

Defensive Copies

Never expose internal mutable state directly.

class User {
  private final java.util.Date registeredAt;
  User(java.util.Date d) { this.registeredAt = new java.util.Date(d.getTime()); }
  public java.util.Date getRegisteredAt() { return new java.util.Date(registeredAt.getTime()); }
}

Try it

1. Expose an internal List safely by returning List.copyOf instead of the mutable list.
2. Add validation to a setter and test that invalid values throw exceptions.