Run two streams and make an object from these

Question

I have two Lists

List<String> a = Arrays.asList( "1" , "2" , "3" );
List<String> b = Arrays.asList( "a" , "b" , "c" );

and a class

class Obj{
   String a;
   String b;
   public Obj(String a, String b){
       this.a=a;
       this.b=b;
   }
}

I have to convert every element 2 lists into an object of above class. Resulting into a list of object of Obj in something like this -

[ Obj("1","a") , Obj("2","a") , Obj("3","a") ]

I know i can do it simply by running a loop.

List<Obj> arrayList = new ArrayList<>();

for( int i=0; i<allAttributes.size(); i++){
    arrayList.add( new Obj(a[i],b[i]) );
}

Wanted to know if it can be done in Java 8 some elegant way.

no problem just curious if can be done in better way in java8 — rishabhjainps, May 15 '19 at 12:21

Ravindra Ranwala · Accepted Answer · 2019-05-15T12:11:48.377

3

Given that a and b lists are of same size, this should do the trick,

List<Obj> objList = IntStream.range(0, a.size())
    .mapToObj(i -> new Obj(a.get(i), b.get(i)))
    .collect(Collectors.toList());

edited May 15 '19 at 12:11

answered May 15 '19 at 12:04

Ravindra Ranwala

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This will be break if `b.size< a.size` – Hadi J May 15 '19 at 12:08
I assume both the arrays are of same size. – Ravindra Ranwala May 15 '19 at 12:10
2

both arrays are of same size for my use case. Thanks. – rishabhjainps May 15 '19 at 12:12
Can we also some how add object only if b.get(i) is not empty string? – rishabhjainps May 15 '19 at 12:13
@nekkel add a `filter(i -> !b.get(i).isEmpty())` before mapping. – Naman May 15 '19 at 12:32

sfiss · Answer 2 · 2019-05-15T12:25:59.610

There are a few ways to implement a Zip-Operation for lists and streams (see this post for example), but you could use the existing method com.google.common.collect.Streams.zip in Guava Streams. That way, your code could look like this:

final List<String> a = Arrays.asList("1", "2", "3");
final List<String> b = Arrays.asList("a", "b", "c");
List<Obj> objs = Streams.zip(a.stream(), b.stream(), (pa, pb) -> new Obj(pa, pb))
            .collect(Collectors.toList());

You can further filter the stream afterwards, e.g.:

List<Obj> objs = Streams.zip(a.stream(), b.stream(), (pa, pb) -> new Obj(pa, pb))
            .filter(obj -> StringUtils.isNotEmpty(obj.b))
            .collect(Collectors.toList());

Run two streams and make an object from these

2 Answers2