java - NullPointerException | `this` inside enum constructor causing NPE

Question

public class Test {
    public static void main(String[] args) {
        Platform1 p1=Platform1.FACEBOOK; //giving NullPointerException.
        Platform2 p2=Platform2.FACEBOOK; //NO NPE why?
    }
}

---

enum Platform1{
    FACEBOOK,YOUTUBE,INSTAGRAM;
    Platform1(){
        initialize(this);
    };
    public void initialize(Platform1 platform){
        switch (platform) {
        //platform is not constructed yet,so getting `NPE`.
        //ie. we doing something like -> switch (null) causing NPE.Fine!
        case FACEBOOK:
            System.out.println("THIS IS FACEBOOK");
            break;
        default:
            break;
        }
    }
}

---

enum Platform2{
    FACEBOOK("fb"),YOUTUBE("yt"),INSTAGRAM("ig");
    private String displayName;
    Platform2(String displayName){
        this.displayName=displayName;
        initialize(this);
    };  
    public void initialize(Platform2 platform){
        switch (platform.displayName) {
        //platform not constructed,even No `NPE` & able to access its properties.
        //switch (null.displayName) -> No Exception Why?
        case "fb":
            System.out.println("THIS IS FACEBOOK");
            break;
        default:
            break;
        }
    }
}

Can anyone explain me why there is NullPointerException in Platform1 but not in Platform2. How in the second case we are able to access the enum object and its properties, even before the object is constructed?

Answer 1

Exactly. Just as @PeterS mentioned using enum before it has been properly constructed is causing NPE, because values() method is being called on un-constructed enum.

One more point, I would like to add here that Platform1 and Platform2 both are trying to use unconstructed enum in switch() but NPE is only in Platform1. Reason behind this is as follows :-

 public void initialize(Platform1 platform){
        switch (platform) {

Above piece of code from Platform1 enum is using platform enum object in switch where internally $SwitchMap$Platform1[] array is used and to initialize this array values() method is utilized, thus you get NPE. But in Platform2, switch (platform.displayName) is comparison on displayName which is already initialized and a string comparison occurs thus no NPE.

Following are fragments of decompiled code :-

Platform1

 static final int $SwitchMap$Platform1[] =
            new int[Platform1.values().length];

Platform2

switch ((str = platform.displayName).hashCode())
    {
    case 3260: 
      if (str.equals("fb")) {