Classes & Instances

Class

Class

A unit of code that is the basic building block of Java programs.

• In Java, nothing can exist outside of a class. A Java program is stored in a class.
• Convention: starts with capital letter, and class & file name must match
• public class <name> { ... }

• Class header
  • The first line in a class, like public class <name> {
• Two types of members in a class:
  • Fields
  • Methods
    • A program unit that represents a particular action or computation.
    • The next-smallest unit of code in Java after classes
    • The first line of a method = method header
• For getter and setter, you can use the Generate function in Intellij

class Person 
{
    String name;     // field
    void greet() {   // method
        System.out.println("Hello, World!");
    }
}

Main method

• At a minimum, a complete program requires a special method named main.
• <statement> = An executable snippet of code that represents a complete command.
  • System.out.println("Hello, world!") is a println statement
  • They are executed in order

public static void main(String[] args) {
	<statement>; // series of actions
	<statement>;
	...
	<statement>;
}

Object class

• By default, every class extends an Object class
  • Every time you try to print an object, it will call the toString() method of the object

Person p = new Person(); // creating a new instance

Component	구성 요소	Description / 설명
Field	필드	- Attributes or properties an object has (e.g., name, age, etc.) - 객체가 가지는 속성 (예: 이름, 나이 등)
Method	메서드	- Actions or behaviors the object can perform - 객체가 수행할 수 있는 동작
Constructor	생성자	- A special method called when the object is created - 체 생성 시 호출되는 특수한 메서드
Inner Class	이너 클래스	- A class defined inside another class, used when needed - 클래스 내부에 정의된 또 다른 클래스 (필요 시 사용)

🧠 Fields and methods represent the state and behavior of an object.

• state → represented by fields
  • the data an object holds
• behavior → represented by methods
  • the actions an objects can perform

Fields

• Variables that exist to hold the data the instance will have, and they represent attributes
  • Class and instance variables are both fields
  • Class vs Instance variables
• related: Local VS Class VS Instance variables

Methods

• What’s in a method
  • Method signature: method name + parameters
    • addInt(int num1, num2)
    • A good method signature lets you know what the function does & returns at first glance
  • Method body
• class methods
• method overloading

Variadic Arguments; varargs

• Allows a method to accept zero or more arguments of a specified type — like passing an array without needing to create one manually
• without this, we’ll have to overloading n times

public void printNames(String... names) {
    for (String name : names) {
        System.out.println(name);
    }
}
 
printNames(); // zero arguments
printNames("Alice");
printNames("Bob", "Charlie", "Dave");

• Conditions
  • Only one varargs parameter is allowed per method
  • It must be the last parameter in the parameter list
• Internally
  • Java converts to array

Instance

Instance

An instance is a real instance of a class.

• It has its own data stored in the attributes.
• You can call methods on the object to perform actions or change its state

• All instances are objects

Person p1 = new Person();

• p1 is a reference variable, which stores the address of the heap that has the data
• You “instantiate” with the new keyword
  • new 쓰는 건 생성자를 호출하는 것
  • new ⇒ you’re putting the object in the heap memory and putting its address in the reference variable
• Image overview
  • 즉 같은 클래스로 만든 모든 객체는 동일한 메서드 값을 공유하기 때문에 여러 번 같은 메서드를 선언해 주는 것이 아니라 한 번만 저장해 두고 필요한 경우에만 클래스 영역에 정의된 메서드를 찾아 사용할 수 있는 것입니다.
  • All instances of the same class share the same methods
    • they all reuse the same method stored in the class area