Merge two list together, replace null value of list which is null-CodePudding

I want merging 2 list with these conditions

List<int> A = 1,1,1,nul,null,null,null,null,null
List<int> B = null,null,null,2,2,2,null,null,null

The result i want after merging

List<int> C = 1,1,1,2,2,2,null,null,null

where null value in list A will replace with value in list B, Also in case there will have a case like: 1 , null , 1, null, i try to use for loop but i cost a lot of performance i want proper way to do it

for(int i = 0; i <A.size; i  ) 
{
   for(int j=0 ;j <B.size; j  )

}

CodePudding user response：

2 loops as you wrote it would mean your code runs the inner bit A * B times, which you don't want. You want to run it just 'A' times, assuming A and B are equal in size, or max(A.size(), B.size()) times if not.

var out = new ArrayList<Integer>();
for (int i = 0; i < a.size(); i  ) {
  Integer v = a.get(0);
  if (v != null) {
    out.add(v);
    break;
  }
  out.add(b.get(0));
}

CodePudding user response：

From my understanding of the question.

Case 1: If both Lists are of equal size then you can use Java stream API two write a consistent code without any loop.

List<Integer> C = IntStream.range(0, A.size())
                .mapToObj(i -> {
                    Integer a = A.get(i);
                    Integer b = A.get(i);
                    if (a == null && b != null) {
                        return b;
                    } else if (a != null && b == null) {
                        return a;
                    } else {
                        // assuming when both the value are present
                        // you want to return from list A
                        return a;
                    }
                })
                .collect(Collectors.toList());

Case 2: If both lists can be of unequal size.

List<Integer> C = IntStream.range(0, Math.max(A.size(), B.size()))
                .mapToObj(i -> {
                    Integer a = i < A.size() ? A.get(i) : null;
                    Integer b = i < B.size() ? B.get(i) : null;

                    if (a == null && b != null) {
                        return b;
                    } else if (a != null && b == null) {
                        return a;
                    } else {
                        // assuming when both the value are present
                        // you want to return from list A
                        return a;
                    }
                })
                .collect(Collectors.toList());