Access modifiers (접근 제어자)

Control who can access a class, field, method, or constructor

• can only use ONCE, unlike non-access modifiers

Types

The further, larger scope (public > protected > default > private)

• public
  • same package, same class
• protected
  • can be accessed by the subclass, regardless of the package
  • can be accessed by same package
  • Used a lot in studying design patterns
• default (no keyword)
• private
  • a private variable is only accessible through the current class

Chart

MEMORIZE THIS BRO

	Public	Default	Protected	Private
Same class (the one we’re in)	✅	✅	✅	✅
Same package subclass	✅	✅	✅	❌
Same package non-subclass	✅	✅	✅	❌
Different package subclass	✅	❌	✅	❌
Different package non-subclass	✅	❌	❌	❌

• default is available in the same class & same package
  • subclass ⇒ extends
  • package ⇒ basically just folder

Simple example

• Dangerous stuff can happen if you give too much access to an object, so you need to restrict its access
  • you can have a getter and setter to get & change your values
    • Encapsulation

public class Person
{
	String name;
	private int age; // can only be accessed here
 
	public void setAge(int newAge)
	{
		if (newAge >= 0) // this method allows us to not set negative age
		{
			age = newAge;
		}
	}
	public int getAge()
	{
		return age;
	}
}

Example 1

Same package

package package1;
 
public class Parent {
    private int a = 1;
    int b = 2;            // default
    protected int c = 3;
    public int d = 4;
 
    public void printAll() {
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
        System.out.println(d);
    }
}
 
class Test {
    public static void main(String[] args) {
        Parent p = new Parent();
        // System.out.println(p.a); // 에러 발생: private 접근 불가
        System.out.println(p.b);    // default: 같은 패키지 접근 가능
        System.out.println(p.c);    // protected: 같은 패키지 접근 가능
        System.out.println(p.d);    // public: 어디서든 접근 가능
    }
}

Different package

package package2;
import package1.Parent;
 
class Child extends Parent {
    public void print() {
        // System.out.println(a); // private: 접근 불가
        // System.out.println(b); // default: 접근 불가 (다른 패키지)
        System.out.println(c);    // protected: 상속 관계라 접근 가능
        System.out.println(d);    // public: 접근 가능
    }
}
 
public class Test2 {
    public static void main(String[] args) {
        Parent p = new Parent();
        // System.out.println(p.a); // private: 접근 불가
        // System.out.println(p.b); // default: 접근 불가
        // System.out.println(p.c); // protected: 같은 패키지+상속만 허용
        System.out.println(p.d);    // public: 접근 가능
    }
}