How to pass an ArrayList to a varargs method parameter?

问题

Basically I have an ArrayList of locations:

ArrayList<WorldLocation> locations = new ArrayList<WorldLocation>();

below this I call the following method:

.getMap();

the parameters in the getMap() method are:

getMap(WorldLocation... locations)

The problem I'm having is I'm not sure how to pass in the WHOLE list of locations into that method.

I've tried

.getMap(locations.toArray())

but getMap doesn't accept that because it doesn't accept Objects[].

Now if I use

.getMap(locations.get(0));

it will work perfectly... but I need to somehow pass in ALL of the locations... I could of course make keep adding locations.get(1), locations.get(2) etc. but the size of the array varies. I'm just not use to the whole concept of an ArrayList

What would be the easiest way to go about this? I feel like I'm just not thinking straight right now.

回答1:

Source article: Passing a list as argument to a vararg method

Use the toArray(T[] arr) method.

.getMap(locations.toArray(new WorldLocation[locations.size()]))

(toArray(new WorldLocation[0]) also works, but you would allocate a zero length array for no reason.)

Here's a complete example:

public static void method(String... strs) {
    for (String s : strs)
        System.out.println(s);
}

...
    List<String> strs = new ArrayList<String>();
    strs.add("hello");
    strs.add("wordld");

    method(strs.toArray(new String[strs.size()]));
    //     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...

回答2:

In Java 8:

List<WorldLocation> locations = new ArrayList<>();

.getMap(locations.stream().toArray(WorldLocation[]::new));

回答3:

A shorter version of the accepted answer using Guava:

.getMap(Iterables.toArray(locations, WorldLocation.class));

can be shortened further by statically importing toArray:

import static com.google.common.collect.toArray;
// ...

    .getMap(toArray(locations, WorldLocation.class));

回答4:

You can do:

getMap(locations.toArray(new WorldLocation[locations.size()]));

getMap(locations.toArray(new WorldLocation[0]));

getMap(new WorldLocation[locations.size()]);

@SuppressWarnings("unchecked") is needed to remove the ide warning.

来源：https://stackoverflow.com/questions/9863742/how-to-pass-an-arraylist-to-a-varargs-method-parameter

标签

java

variadic-functions