直接在Java中设置ArrayList的值

Question

Setting a list of values for a Java ArrayList works:

Integer[] a = {1,2,3,4,5,6,7,8,9};
ArrayList<Integer> possibleValues2 = new ArrayList<Integer>(Arrays.asList(a));

However, the following doesn't work and has the error "Illegal start of type" as well as other. Why not? Since the first line in the first code block is simply assignment, shouldn't it not have an effect?

ArrayList<Integer> possibleValues2 = new ArrayList<Integer>(Arrays.asList({1,2,3,4,5,6,7,8,9}));

Answer 1

You should use either the vararg version of Arrays.asList , eg

ArrayList<Integer> possibleValues2 =
    new ArrayList<Integer>(Arrays.asList(1,2,3,4,5,6,7,8,9));

or explicitly create an array parameter, eg

ArrayList<Integer> possibleValues2 =
    new ArrayList<Integer>(Arrays.asList(new Integer[]{1,2,3,4,5,6,7,8,9}));

Answer 2

A strange and little used idiom,

List<Integer> ints = new ArrayList<Integer>() {{add(1); add(2); add(3);}}

This is creating an anonymous class that extends ArrayList (outer brackets), and then implements the instance initializer (inner brackets) and calls List.add() there.

A good interview question if nothing else.

Answer 3

Another option is to use Guava ("Google collections"), which has a Lists.newArrayList(...) method.

Your code would be something like

ArrayList<Integer> possibleValues2 = Lists.newArrayList(1,2,3,4,...);

Answer 4

来自Java 7 SE文档：

List<Integer> possibleValues2 = Arrays.asList(1,2,3,4,5,6,7,8,9);