如何将存储在字符串数组中的特定字符串打印到字符串数组的 ArrayList 中？

Question

所以，根据问题，我在 Java 中有这段代码：

public class example {
    static ArrayList<String[]> test = new ArrayList<String[]>();
    private String[] a = {"this", "is,", "a test"};
    private String[] b = {"Look", "a three-headed", "monkey"};

    public void fillTest() {
        test.add(a);
        test.add(b);
        // so far so good, I checked this method
        // with a System.out.print and it works
    }

    // later in the code I have a method that try
    // to take the arrayList test and copy it into
    // a String[] named temp. In my vision temp
    // should than be accessed randomly by the
    // method itself and the content printed out
    // from temp should be removed from test -
    // that's why I'm using an ArrayList

    public void stuff() {
        // some stuff
        // runtime error happens here:
        String[] temp = test.toArray(new String[test.size()]);
        // other stuff that never made it to runtime
    }
}

问题是虽然编译器对此没有任何反对意见，但在运行时我收到此错误：

Exception in thread "main" java.lang.ArrayStoreException: arraycopy: element type mismatch: can not cast one of the elements of java.lang.Object[] to the type of the destination array, java.lang.String

我不明白背后的原因 - 在我看来我是在要求它用字符串填充字符串数组，那么为什么会出错？

Answer 1

您正在尝试将元素为 String 数组的 List 转换为元素为 String 的数组。这不起作用，因为 String 的数组不是 String.

相反，您可以将 List 个数组转换为 String 个二维数组：

String[][] temp = test.toArray(new String[test.size()][]);

如果想把List的String数组的所有元素放在一个String的数组中，就得做一些处理。使用 Streams 可以完成：

String[] temp = test.stream().flatMap(Arrays::stream).toArray(String[]::new);

Answer 2

您可以使用 Stream.flatMap 方法展平此 字符串数组列表 并在单个字符串数组上获取流。然后你可以得到一个包含这个流的元素的数组：

List<String[]> test = Arrays.asList(
        new String[]{"this", "is,", "a test"},
        new String[]{"Look", "a three-headed", "monkey"});

String[] temp = test
        // return Stream<String[]>
        .stream()
        // return Stream<String>
        .flatMap(arr -> Arrays.stream(arr))
        // return an array of a specified size
        .toArray(size -> new String[size]);

System.out.println(Arrays.toString(temp));
// [this, is,, a test, Look, a three-headed, monkey]

^{另请参阅：}

如何将存储在字符串数组中的特定字符串打印到字符串数组的 ArrayList 中？

How to print a specific String stored into an array of String into an ArrayList of Strings Arrays?

java

arrays

arraylist

toarray

arraystoreexception