扩展数组以传递给varargs

Question

Given this function signature:

public static void test(final String... args) {}

I would like to fit a string and an array of strings into the args :

test("a", new String[]{"b", "c"});

But it is not possible because the second argument is not expanded.

So is it possible to expand an array to fit into varargs?

If that is not possible, what is the shortest way to construct a concatenated string array given one string and a string array? Eg:

String a = "a";
String[] b = new String[]{"b", "c"};
String[] c = // To get: "a", "b", "c"

Thank you.

Answer 1

I don't know any other shorter ways but this is pretty clean to me. With plain java ( without ant libraries )

String a = "a";
String[] b = new String[] { "b", "c" };
String[] c = new String[b.length + 1];
c[0]=a;
System.arraycopy(b, 0, c, 1, b.length);

That will work no matter what is the size of b

Answer 2

You can use Guava's ObjectArrays.concat(T, T[]) method:

String a = "a";
String[] b = new String[]{"b", "c"};
String[] c = ObjectArrays.concat(a, b);

Notice the order of arguments. Invoking with (b, a) will also work, but that will append the element to the array rather than prepend (which is what you seem to want). This internally uses System.arraycopy() method.

Answer 3

您可以创建一个单独的数组，并首先将String “a”放入该数组，然后使用System.arraycopy()将数组b []复制到数组中，然后将新数组传递给该方法。

Answer 4

You can use StringBuilder :

StringBuilder sb = new StringBuilder();
sb.append(a);
for(String s : b) {
    sb.append(s);
}

And then you can convert it back to ArrayList or array of Strings...

Answer 5

Shortest way which comes to my mind (BUT not clean code ):

test((a + "," + Arrays.toString(b)).replaceAll("[\\[\\] ]", "").split(","));

Test:

public static void main(String[] args) {
    String a = "a";
    String[] b = new String[] {"b", "c"};

    System.out.println(Arrays.toString(
            (a + "," + Arrays.toString(b)).replaceAll("[\\[\\] ]", "").split(",")));

}

Output:

[a, b, c]