Java toArray() 方法 的使用

// 无参版本
Object[] toArray();

// 泛型版本（推荐使用）
<T> T[] toArray(T[] a);

List<String> list = List.of("Java", "Python", "C++");
Object[] arr = list.toArray();

System.out.println(Arrays.toString(arr));
// 输出: [Java, Python, C++]

String[] strArr = (String[]) list.toArray(); // 会报错 ❌

class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.String; ([Ljava.lang.Object; and [Ljava.lang.String; are in module java.base of loader 'bootstrap')

List<String> list = List.of("Java", "Python", "C++");
String[] arr = list.toArray(new String[0]);

System.out.println(Arrays.toString(arr));
// 输出: [Java, Python, C++]

T[] array = list.toArray(new T[0]);

String[] arr = list.toArray(String[]::new);

public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // 如果传入数组太小，则创建一个新的同类型数组
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

// 推荐写法
Integer[] numArray = numbers.toArray(new Integer[0]);

System.out.println(Arrays.toString(numArray));
// 输出: [1, 2, 3, 4, 5]

Set<String> set = new HashSet<>(Arrays.asList("A", "B", "C"));
String[] arr = set.toArray(new String[0]);

System.out.println(Arrays.toString(arr));
// 输出: [A, B, C]

List<String> list = List.of("Java", "Kotlin", "Scala");

// 使用方法引用创建新数组
String[] arr = list.toArray(String[]::new);

System.out.println(Arrays.toString(arr));
// 输出: [Java, Kotlin, Scala]

List<String> list = List.of("A", "B", "C");

// 数组大小刚好
String[] arr1 = list.toArray(new String[3]);
System.out.println(Arrays.toString(arr1)); // [A, B, C]

// 数组大小过小
String[] arr2 = list.toArray(new String[1]); // 会创建新数组
System.out.println(Arrays.toString(arr2)); // [A, B, C]

// 数组大小过大
String[] arr3 = list.toArray(new String[5]);
System.out.println(Arrays.toString(arr3)); // [A, B, C, null, null]

String[] arr = {"A", "B", "C"};
List<String> list = Arrays.asList(arr);
System.out.println(list); // [A, B, C]

List<String> list = new ArrayList<>(Arrays.asList(arr));
list.add("D"); // ✅

list.toArray(new String[list.size()]);

list.toArray(new String[0]);