Core Java. One dimensional array. Swap Function. Usage of Null. Everything work just question.

I have a case, I need to rewrite data from one array to another at the way that all integer below zero were at first places.

Input : OriginalVector[] = {1, 2, 3, 4,-9, 54, 5, -23} ;
           NewVector[] =       {0, 0 , 0 , 0 , 0 , 0 , 0, 0} ;
OutPut : OriginalVector[] = {1, 2, 3, 4,-9, 54, 5, -23} ;
              NewVector[] =       {-9, -23 , 1 , 2 , 3 , 4 , 54, 5} ;

So the code below works fine, however, I've few questions, please help me to figure out.
1. Why I cannot give "OriginalVector[]" array variables SIZE to finalize it size. (   OriginalVector[SIZE] ) ?
2. Why I cannot make NewVector[] = null ; - eclipse gives me compile error
Exception in thread "main" java.lang.NullPointerException
      at TaskCh.TaskCh.main(TaskCh011N245.java:26)

3. Are the any standard implementation of swap function is java ,  like std:swap in Cpp ?
Please take a look on the code below ! Looking forward for reply from you dear Experts ! thank you in advance !


package TaskCh;



public class TaskCh {


	@SuppressWarnings("null")
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		
		final int SIZE = 8 ; 
		
		int OriginalVector[] = {1, 2, 3, 4,-9, 54, 5, -23} ;
	 	int NewVector[] = {0, 0 , 0 , 0 , 0 , 0 , 0, 0} ;
	//	int NewVector[] = null ; 
		int i, j , k;
		
		for ( i = 0 , j = 0 , k = SIZE -1 ; i < SIZE ; ++i){
			
			if ( OriginalVector[i] < 0){
				NewVector[j++] = OriginalVector[i];
			}
			else {
				NewVector[k--] = OriginalVector[i];
			}
		}
		
		for ( i = k + 1 , j = SIZE - 1 ; i < j ; ++i, --j){
			
			NewVector[i] = NewVector[i] +  NewVector[j];
			NewVector[j] = NewVector[i] - NewVector[j]; 
			NewVector[i] = NewVector[i] - NewVector[j]; 
		}
		
		for ( int m = 0 ; m < SIZE ; m ++ ){
			System.out.println(NewVector[m]); 
		}
		
		
	}

}

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SunnyXAsked:
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mccarlConnect With a Mentor IT Business Systems Analyst / Software DeveloperCommented:
1. Why I cannot give "OriginalVector[]" array variables SIZE to finalize it size. (   OriginalVector[SIZE] ) ?

You can't because you have already basically defined the number of elements by including 8 values in the literal array that follows it. So for example, what you could do for NewVector is just this...

int NewVector[SIZE];

or even this...

int NewVector[OriginalVector.length];


2. Why I cannot make NewVector[] = null

Because, null means no array at all. This is different to an empty array (which is what you have defined or what I have shown just above). And because when you use null, the array doesn't even exist yet, the NullPointerException is the result of you trying to put a value into something that doesn't exist.


3. Are the any standard implementation of swap function is java

Not that I know of. However, i would got about this problem a bit different where you would need to swap anything. If you want to try and think of another way, think about doing 2 iterations of the original vector... the first looks for negative numbers and fills the new vector as it finds them. Then the second pass looks for positive numbers and keeps filling the new vector.

(Yes, I being a little bit vague because I don't want to just give you the full solution but rather get you thinking about it. But definitely ask questions if you don't get my idea)
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CEHJConnect With a Mentor Commented:
1
 
SunnyXAuthor Commented:
I confirm that all my questions aren't academic ! I've experience to work with knowledge exchange communities and I know that if I start to ask question providing, for example,  real ( full long ) code, the speed of answer drops dramatically ! Therefore ( and this is obvious ) It's more effective to parse the question to the simplest form of smaller once  and remains only to core of the problem.
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SunnyXAuthor Commented:
Thx you dear experts for your help !
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SunnyXAuthor Commented:
mccarl , sorry but

int NewVector[OriginalVector.length];

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isn't work compile error please take a look on print screen

https://gyazo.com/0fd86638fc2c118bc036cb7fdf1bf8f9

I tried modification static / final still compile error
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