The easiest way to transform collection to array?

If you use it more than once or in a loop, you could define a constant

public static final Foo[] FOO = new Foo[]{};

and do the conversion it like

Foo[] foos = fooCollection.toArray(FOO);

The toArray method will take the empty array to determine the correct type of the target array and create a new array for you.

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Here's my proposal for the update:

Collection<Foo> foos = new ArrayList<Foo>();
Collection<Bar> temp = new ArrayList<Bar>();
for (Foo foo:foos) 
    temp.add(new Bar(foo));
Bar[] bars = temp.toArray(new Bar[]{});

Alternative solution to the updated question using Java 8:

Bar[] result = foos.stream()
    .map(x -> new Bar(x))
    .toArray(size -> new Bar[size]);

Where x is the collection:

Foo[] foos = x.toArray(new Foo[x.size()]);