Classes & Objects

Learning goals

Model real-world entities with classes and objects, distinguish fields from local variables, use references correctly, and apply basic object lifecycle intuition before constructors and encapsulation.

Class and object

A class is a blueprint; an object is a runtime instance.

public class BankAccount {
    String owner;      // field (instance variable)
    double balance;

    void deposit(double amount) {
        balance += amount;
    }

    void printSummary() {
        System.out.println(owner + ": $" + balance);
    }
}

BankAccount acct = new BankAccount();  // construction
acct.owner = "Ada";
acct.deposit(100);
acct.printSummary();

new allocates memory and runs the constructor (default if you define none).

Fields vs local variables

public class Demo {
    int count;  // field, default 0

    void run() {
        int count = 5;  // local, shadows field
        System.out.println(count);       // 5
        System.out.println(this.count);  // field if set
    }
}

Fields live for the object’s lifetime; locals exist only inside the block.

Reference semantics

BankAccount a = new BankAccount();
BankAccount b = a;
b.deposit(50);
System.out.println(a.balance);  // same object → 50

a and b reference the same instance. null means no instance:

BankAccount c = null;
// c.deposit(1);  // NullPointerException

Always null-check external references when failure is possible.

Static vs instance (preview)

public class Counter {
    static int globalCount = 0;
    int instanceId;

    Counter() {
        globalCount++;
        instanceId = globalCount;
    }
}

Static members belong to the class (one copy). Instance members belong to each object. Use static for utilities and shared constants sparingly.

Object lifecycle (high level)

1. Construction — new, constructor runs.
2. Use — methods read/write fields.
3. Unreachable — no references from GC roots.
4. Collected — GC frees memory (timing nondeterministic).

You rarely call System.gc() in application code.

Composition over ad-hoc fields

Model relationships by containing other objects:

public class Address {
    String city;
    String zip;
}

public class Customer {
    String name;
    Address shippingAddress;
}

Prefer composition when “has-a” fits better than “is-a” (inheritance comes later).

Records as lightweight classes (JDK 16+)

When data carrier with no mutable identity:

record Product(String sku, String name, double price) {}

Immutability by default—good for DTOs and value objects.

Naming and cohesion

• One class should have a clear responsibility (Single Responsibility Principle).
• Group behavior with the data it operates on (deposit on BankAccount, not scattered static helpers).
• Avoid god classes that know everything.

Common mistakes

1. Forgetting new — BankAccount a; a.deposit(1); compile error (uninitialized).
2. Comparing objects with == when meaning value equality.
3. Public mutable fields — leads to uncontrolled state (fix in encapsulation chapter).
4. Creating many short-lived objects in hot paths without need—GC pressure (optimize when measured).
5. Null as valid “empty” object — prefer empty collections or Optional where appropriate.

Practice checkpoint

1. Draw mentally: after Point p1 = new Point(); Point p2 = p1; p2.x = 3, what is p1.x?

   • Answer: 3 (shared reference).

2. Define class Book with title, author, method describe() returning "title by author".

   • Answer: Fields + return title + " by " + author;

3. What does new do in two words?

   • Answer: Allocates instance, invokes constructor.

4. When use a record instead of a class?

   • Answer: Immutable data bundle with generated equals/hashCode/toString.

Interview angles

• “Class vs object?” — Class is template; object is instance in memory.
• “Stack vs heap for objects?” — Object on heap; reference variable in thread stack frame.
• “Why OOP?” — Encapsulation, abstraction, reuse via inheritance/composition—not magic, but structure for large codebases.

Next: /java/learn/oop/constructors-and-encapsulation/