Final

In variables

• In Java, constants (상수) are typically declared using the final keyword.
  • You can’t change the value again (재할당이 불가능)

Final Variables (Constants)

• constants are usually uppercase

final double PI = 3.141592;
// PI = 3.15 ->  error

Final Instance Variables (Object Fields)

You need to indicate that the variable is never null (used in spring)

public class Test {
    final int number; // must be initialized in constructor
 
    public Test(int number) {
        this.number = number; // ✅ allowed: initialized once
    }
}

Final References (Objects)

• final prevents reassigning the reference, not modifying the object itself.
• You can still change fields or call methods on the object.

final Person p1 = new Person("김신의");
p1.setName("문종모"); // ✅ allowed
System.out.println(p1.getName()); // 문종모
 
// p1 = new Person("다른 사람"); // ❌ Error: final reference cannot be reassigned

Practices in classes

• you can make an attribute public final and not private because you can’t change it anyways even though it’s public
• if it’s public, you don’t need a getter function anymore too
• best practice: Use final and static together for constants

public class CodeitConstants {
    public static final double PI = 3.141592653589793;
    public static final double EULERS_NUMBER = 2.718281828459045;
    public static final String THIS_IS_HOW_TO_NAME_CONSTANT_VARIABLE = "Hello";
 
    public static void main(String[] args) {
        System.out.println(CodeitConstants.PI + CodeitConstants.EULERS_NUMBER);
    }
}

In methods

• It will make the method non-overridable

class A {
    public final void show() {
        System.out.println("A's show");
    }
}
 
class B extends A {
    // ❌ This will cause a compile-time error!
    public void show() {
        System.out.println("B's show");
    }
}
 

In classes

• It will make the class non-inheritable
• When we want to stop overriding, we can use final methods

public final class A
{
	public final void show()
	{
	}
}
 
// ❌ This will cause a compile-time error!
public final class B extends A ==> Error!
{
	public final void show()
	{
	}
}