Iterate through multiple collections in the same “for” loop?

Question

I wonder if there\'s such a way to iterate thru multiple collections with the extended for each loop in java.

So something like:

for (Object element          


        

Answer 1

You can do exactly this with Guava's Iterables.concat():

for (Foo element : Iterables.concat(collection1, collection2)) {
    foo.frob();
}

Answer 2

With plain Java 8 and without any additional libraries:

public static <T> Iterable<T> compositeIterable(Collection<? extends T>... collections)
{
    Stream<T> compositeStream = Stream.of(collections).flatMap(c-> c.stream());
    return compositeStream::iterator;
}

Then you can use it as:

for (Foo element : MyClass.compositeIterable(collection1, collection2)) {
    foo.frob();
}

Answer 3

Collection<Foo> collection1 = ...
Collection<Foo> collection2 = ...
Collection<Foo> collection3 = ...
...

Collection<Foo> all = ...
all.addAll(collection1);
all.addAll(collection2);
all.addAll(collection3);
...

for(Foo element : all)
{

}

Answer 4

If your lists have the same length, just use the raw for loop:

Object[] aNum = {1, 2}; 
Object[] aStr = {"1", "2"}; 

for (int i = 0; i < aNum.length; i++) {
    doSomeThing(aNum[i]);
    doSomeThing(aStr[i]);
}

It works!