Java Encapsulation

lecture
java
oop
encapsulation

Introduced in 2026-03-05-object-oriented-programming-i (Lecture, Week 2).

Classes and objects

  • A class describes the contents of the objects that belong to it: an aggregate of data fields (properties) plus the operations (methods) defined on them.
  • An object is an element (instance) of a class; objects have the behaviours of their class. The object is the actual component of a running program, while the class specifies how instances are created and how they behave.

Encapsulation

Encapsulation is the process of bundling code (methods/member functions) and data (member variables) together into a single unit — a class. It restricts direct access to some of an object’s components, preventing the accidental modification of data.

public class Person {
    private String name;
    private int age;
    private String email;
    private String[] phoneNumber;

    public Person(String name, int age, String email, String[] phoneNumber) {
        this.name = name;
        this.age = age;
        this.email = email;
        this.phoneNumber = phoneNumber;
    }

    public String getName() { return name; }
    public int getAge() { return age; }
    public String getEmail() { return email; }
    public String[] getPhoneNumber() { return phoneNumber; }
}

Constructors

A constructor is a special method invoked when an object of the class is created (via new):

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public static void main(String[] args) {
        Person person = new Person("John", 20);
        System.out.println("The name of the person is " + person.name);
    }
}

If a class has no constructor, the Java compiler automatically creates a default constructor at runtime.

Notes on constructors:

  • Constructors are invoked implicitly when objects are instantiated.
  • The constructor’s name MUST be the same as the class.
  • A constructor must not have a return type.
  • A constructor can be overloaded but cannot be overridden.

Access modifiers

Access modifiers set the accessibility (visibility) of classes, interfaces, properties, methods, constructors, and data members:

Modifier Accessible from
private Only within the declaring class
public Anywhere
protected Same package, plus subclasses (see java-inheritance)
(default, no keyword) Only within the same package

Attempting to access a private field from outside its class is a compile error (X has private access in Y).

Getters and setters

Getter and setter methods provide controlled access to an object’s private fields, keeping its internal representation hidden from the outside world:

  • Getters (accessors) retrieve the value of a private field from outside the class.
  • Setters (mutators) set/update the value of a private field from outside the class.
public class BankAccount {
    private double balance;
    private String accountNumber;

    public BankAccount(String accountNumber) {
        this.accountNumber = accountNumber;
    }

    public double getBalance() { return balance; }
    public void setBalance(double balance) { this.balance = balance; }
    public String getAccountNumber() { return accountNumber; }
}

The static keyword

static is a non-access modifier for methods and attributes:

  • Static methods/attributes can be accessed without creating an object of the class.
  • Useful in memory management; can be applied to variables, methods, blocks, and nested classes.

Static variables (class variables) are shared among all instances of a class — useful for constants and shared properties, e.g. public static int totalAccounts = 0;.

Static methods can be called without creating an instance of the class, but cannot access non-static (instance) variables or methods directly:

class Bank {
    static double interestRate = 5.0;
    static double calculateInterest(double balance, int years) {
        return (balance * interestRate * years) / 100;
    }
}

Class invariants

A class invariant specifies a condition (or set of conditions) that should always be true throughout the life of an object — used to ensure a system remains in a valid state. A class invariant:

  1. Must be established after the class constructor.
  2. May be assumed as a precondition of each method (excluding the constructor).
  3. Must be established after each method call.
class Counter {
    private int count;
    public Counter() { count = 0; }
    public void increment() { count = count + 1; }
}
// Invariant: count >= 0

Invariants complement preconditions (what must be true before a method executes) and postconditions (what must be true after) — together these define a contract for how a method should behave.

Protecting invariants

A specification can claim an invariant while the implementation still allows it to be broken. Two common leaks:

  1. Public fields — a directly-mutable field lets any caller bypass the class entirely and violate the invariant. Fix: make the field private.
  2. Returning an internal reference — a getter that returns its internal mutable collection/object directly hands the caller a way to mutate it from outside, bypassing any checks the class’s own methods perform. Fix: return a defensive copy:
private List<String> files;

public List<String> getFiles() {
    return new ArrayList<>(files); // copy, not the internal reference
}

Preserving the invariant may also require adding preconditions to methods that could otherwise violate it (e.g. rejecting an addition that wouldn’t satisfy the invariant), or, when a method is likely to be called by code outside your control, defensively throwing an exception (e.g. IllegalArgumentException/IllegalStateException) rather than relying purely on the precondition contract (see java-specification — Defensive programming).

Method signatures

A method signature is a method’s unique identifier: its name plus its parameter types. The return type and parameter names do not count towards the signature, and a signature MUST be unique within a class:

public void print() { ... }          // signature: print()
public void print(int parameter) { ... } // signature: print(int)