数组转集合问题Arrays.asList()
1. 源码:
Arrays.class
public static <T> List<T> asList(T... a) {
return new Arrays.ArrayList(a);
}
Arrays.ArrayList.class
private static class ArrayList<E> extends AbstractList<E> implements RandomAccess, Serializable {
private static final long serialVersionUID = -2764017481108945198L;
private final E[] a;
ArrayList(E[] array) {
this.a = (Object[])Objects.requireNonNull(array);
}
...
...
}
AbstractList.class
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
public E remove(int index) {
throw new UnsupportedOperationException();
}
2. 结论:
- Arrays.asList(arr) 转为的实例化对象为Arrays中的静态内部类ArrayList,该类没有重写add() 、remove() 、clear()方法,不能对List增删,只能查改;
- Arrays.ArrayList内部类继承了AbstractList抽象类,AbstractList抽象类又实现了List接口,利用多态性,通过List接口接收asList()方法的返回结果,方法调用时,如果Arrays.ArrayList内部类进行了方法重写,则调用该重写方法,否则实际调用的是AbstractList重写List中的方法;
- 操作转换后的集合,数组也会变化,Arrays中的内部类ArrayList存放的也是数组,转换时进行的浅拷贝,只是将数组的地址值赋值给ArrayList内部类中的属性a;
- 方法参数arr数组如果使用基本数据类型,则会把它看作是一个整体保存在List底层数组中。
3. 验证代码:
@Test
public void test1(){
String[] arr = {"小轩","小刘","小王"};
List<String> asList = Arrays.asList(arr);
String oldValue = asList.set(1, "123");
System.out.println("oldvalue:" + oldValue + ",asList:" + asList);
System.out.println("改变集合影响对应的数组:" + Arrays.toString(arr));
arr[2] = "中国";
System.out.println("改变数组影响转换成的集合:" + asList);
System.out.println(Arrays.toString(arr));
int[] myArray = {1, 2, 3};
List myList = Arrays.asList(myArray);
System.out.println(myList.size());
System.out.println(myList.get(0));
int[] array = (int[]) myList.get(0);
System.out.println(array[0]);
String[] arr1 = {"小轩","小刘","小王"};
ArrayList<String> arrayList = new ArrayList<>(arr1.length);
Collections.addAll(arrayList, arr1);
arrayList.add("小王八");
}