java中asList的使用

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asList概述
Java中的asList方法是数组工具类 Arrays中的一个静态方法，Arrays.asList()方法的作用是将数组或一些元素转为集合，asList方法返回值得到的集合并不是我们通常使用的List集合，asList()方法把数组转换成集合时,不能使用其修改集合相关的方法,如果使用修改集合相关的方法add/remove/clear方法会抛出java.lang.UnsupportedOperationException的异常。

1.使用asList方法返回的对象调用add/remove/clear方法

public static void main(String[] args) { 
    String[] strs={ 
   "a","b","c"}; List<String> asList= Arrays.asList(strs); System.out.println(asList.size()); //asList.add("d"); //asList.remove("b"); asList.clear(); } 

注：其get和set方法可以正常使用

public static void main(String[] args) { 
    String[] strs={ 
   "a","b","c"}; List<String> asList= Arrays.asList(strs); System.out.println(asList.size()); //asList.add("d"); //asList.remove("b"); //sList.clear(); System.out.println("asList.set() = " + asList.set(1,"d")); System.out.println("asList.get(1) = " + asList.get(1)); } 

Arrays中的asList方法源码：

 @SafeVarargs @SuppressWarnings("varargs") public static <T> List<T> asList(T... a) { 
    return new ArrayList<>(a); } 

asList方法中返回的ArrayList方法的源码：

/ * @serial include */ private static class ArrayList<E> extends AbstractList<E> implements RandomAccess, java.io.Serializable { 
    private static final long serialVersionUID = -0L; private final E[] a; ArrayList(E[] array) { 
    a = Objects.requireNonNull(array); } @Override public int size() { 
    return a.length; } @Override public Object[] toArray() { 
    return a.clone(); } @Override @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { 
    int size = size(); if (a.length < size) return Arrays.copyOf(this.a, size, (Class<? extends T[]>) a.getClass()); System.arraycopy(this.a, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } @Override public E get(int index) { 
    return a[index]; } @Override public E set(int index, E element) { 
    E oldValue = a[index]; a[index] = element; return oldValue; } @Override public int indexOf(Object o) { 
    E[] a = this.a; if (o == null) { 
    for (int i = 0; i < a.length; i++) if (a[i] == null) return i; } else { 
    for (int i = 0; i < a.length; i++) if (o.equals(a[i])) return i; } return -1; } @Override public boolean contains(Object o) { 
    return indexOf(o) != -1; } @Override public Spliterator<E> spliterator() { 
    return Spliterators.spliterator(a, Spliterator.ORDERED); } @Override public void forEach(Consumer<? super E> action) { 
    Objects.requireNonNull(action); for (E e : a) { 
    action.accept(e); } } @Override public void replaceAll(UnaryOperator<E> operator) { 
    Objects.requireNonNull(operator); E[] a = this.a; for (int i = 0; i < a.length; i++) { 
    a[i] = operator.apply(a[i]); } } @Override public void sort(Comparator<? super E> c) { 
    Arrays.sort(a, c); } } 

2.java8新特性的方式将asList返回对象转换为List对象

public static void main(String[] args) { 
    String[] strs={ 
   "a","b","c"}; //java8新特性 List<String> list = Stream.of(strs).collect(Collectors.toList()); System.out.println(list.add("d")); System.out.println(list); } 

3.将asList返回的对象转换为List对象

public static void main(String[] args) { 
    String[] strs={ 
   "a","b","c"}; List<String> list = new ArrayList<>(Arrays.asList(strs)); System.out.println(list.add("d")); System.out.println(list); }