Enum NullPointerException

Question

Could anyone explain why

public class Testabut{
    enum ThreeColors{RED, BLUE, green;
        public void woowoo(){
        System.out.println("woo");}
    }
     ThreeColors color;



    class Innerclass{
        Innerclass(){
             color.woowoo();
        }
    }

generates a null pointer exception at the invocation of woowoo() ? The instance of color should be reachable, no?

Answer 1

Because color is not initialized and it's default value is null .
Initialize it like

ThreeColors color = ThreeColors.RED;  //Or any other value

Answer 2

Your color variable is null. You have to initialize it to use it.

Answer 3

The instance of color should be reachable, no?

There is no instance, color is null by default, because it is not initialized.

Answer 4

All instance variables are initialized with a value. If you do not provide a value, the variable will be assigned the default value for the type. For non-primitive types, the default value is null .

Currently, your code is equivalent to:

ThreeColors color = null;

So when you use it, of course you get a NPE. Instead, try something like this:

ThreeColors color = ThreeColors.RED;

Answer 5

You have to initialize color . Try color = ThreeColors.RED; or color = ThreeColors.BLUE; or color = ThreeColors.green; !

Answer 6

您没有初始化color变量。

Answer 7

更改为此（或您喜欢的颜色）：

ThreeColors color = ThreeColors.RED;