FT NullPointerException.

[H Paul]

Please see below 3 points. Why compiler allowed point 2 and not point 1?

Point 2 leads to NPE in point 3,
Why does the compiler allowed point 2 in the first place?
I expected the compiler to give a compilation error as in point 1.


1.

public interface FtNpeInterface {
	public static Function<Integer, Integer> compositeFT= doubleFT.compose(echoFT);
	public static Function<Integer, Integer> doubleFT=(Integer t)->t*t; 
	public static Function<Integer, Integer> echoFT=(Integer t)->t; 
}

Compilation error: As I expected:

Multiple markers at this line
- Cannot reference a field before it is
defined
- Cannot reference a field before it is
defined

2. But then I solved the compilation error by using FtNpeInterface.doubleFT

public interface FtNpeInterface {
	
	public static Function<Integer, Integer> compositeFT=FtNpeInterface.doubleFT.compose(FtNpeInterface.echoFT);
	public static Function<Integer, Integer> doubleFT=(Integer t)->t*t; 
	public static Function<Integer, Integer> echoFT=(Integer t)->t; 

}

3. But then at rumtime, Caused by: java.lang.NullPointerException

FtNpeInterface.compositeFT.apply(2);

At first, I did not see why.
But tracing down from sequential timing point of view, I see why NullPointerException occured.

[Rob Spoor]

In your original code, the error message says it all:

Cannot reference a field before it is defined

If I move the first field after the other two it compiles just fine.

In your second example, you still try to access the two fields, but they haven't been initialised yet. That's why you get an NPE.

[H Paul]

Rob,

1. Thanks for NPE and the Field keyword. +1 for your Certs.

2. (A side note: On a piece of paper, the point 1 and point 2 are the same,
I thought it would be magically for the compiler to see point 1 as the point 2 "implicitly Forward Declaration"
That way, I don't have to worry about the order when in the future I can add any function composition.)

My Back to the future:
8.3.3. Forward References During Field Initialization
https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.3.3