初始化对象时，实例变量是否始终未初始化？

Question

The following code produces NullPointerException -

public class MyClass {

    static MyClass instance= new MyClass(); // line 3
    static Boolean have_Instance = true;
    Boolean inst_Avail=have_Instance; // line 5

    Boolean isInstAvail(){
        return inst_Avail;
    }

    public static void main(String[] args) {
        System.out.println(instance.isInstAvail() ? "Instance is there.":""); // gives java.lang.NullPointerException
    }

}

If I move line 3 to after line 5 , it runs fine. How does the order matter here? Shouldn't instantiation of a class set iVar values everytime?

Answer 1

When an object is created on line 3 the class has not finished initializing yet, and the have_instance variable has its default value, null . This value is assigned to the inst_Avail member variable of the object, so the value returned by instance.isInstAvail() in the main method will be null .

An easy way to fix this is swapping lines 3 and 4, so have_instance already has a value when the object is created. Or you could declare have_instance as boolean instead of Boolean , so it will have the value false and not null . This would make the program print nothing though.

Or maybe you could rethink what you're trying to do. It's rarely a good idea to create instances of a class before the class has finished initializing, especially if the class is not "final" (ie may have subclasses).

Answer 2

The order of fields matter if you initialize these fields either directly by setting their values or using a static initializer block. They are executed in order. So you couldn't do a forward reference:

private int someInt = 10 + otherInt;
private int otherInt = 22;

This won't work, because the fields are initialized in order of their textual declaration. If you have two static initializers, they will be executed in order as well:

static { System.out.println("first"); }
static { System.out.println("second"); }

So in your case, you initialize instance before have_instance , so the latter is still null (default value for non-primitives). The JVM will create a MyClass object to be assigned to instance and initialize its fields, ie assign the value of have_instance to inst_Avail which will be set to null as well.

Some readings:

• Object initialization in Java

Answer 3

This code is really weird and I dont see use-cases for this, but this would do the fix:

public class MyClass {

    static MyClass instance; // line 3
    static Boolean have_Instance = true;
    Boolean inst_Avail=have_Instance; // line 5

    Boolean isInstAvail(){
        return inst_Avail;
    }

    public static void main(String[] args) {
        instance = new MyClass();
        System.out.println(instance.isInstAvail() ? "Instance is there.":""); // gives     java.lang.NullPointerException
    }

}

Answer 4

另一个解决方案可能是将inst_Avail设置为静态，以便在加载类时，实例化此变量：

 private static  Boolean inst_Avail=have_Instance;

Answer 5

you are assigning static variable value to non-static variable

ie

Boolean inst_Avail=have_Instance;

either make it static or

assign inst_Avail = true in the constructor