Java流式操作 Arrays.asList与Arrays.stream Java数组流操作 stream()

原文链接:https://www.cnblogs.com/baby123/p/12619872.html

Arrays.asList与Arrays.stream

java Arrays.asList与Arrays.stream

（1）Arrays.asList

　　将数组转化为list

Arrays.asList("a","ab","abc").stream()
        .filter(s->s.contains("ab"))
        .map(s->s.toUpperCase())
        .forEach(System.out::println);

 注：

　　一旦初始化元素后，集合的size就是不可变的

　　所以下面的代码会报错 java.lang.UnsupportedOperationException

List list = Arrays.asList("a","ab","abc");
list.clear();
list.remove("a");
list.add("g");

Arrays.asList源码

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

ArrayList

  private static class ArrayList<E> extends AbstractList<E>
    implements RandomAccess, java.io.Serializable
{  
    private static final long serialVersionUID = -2764017481108945198L;
    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);
    }
}

这个内部类没有add,clear,remove方法

继续找AbstractList，报错来自这

public void add(int index, E element) {
    throw new UnsupportedOperationException();
}
public E remove(int index) {

throw new UnsupportedOperationException();

}

可以在外面这样包一层ArrayList，这样就可以对数组操作了

List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
list.remove("a");
list.add("g");
list.forEach(System.out::println);

（2）Arrays.stream

将数组转换成流

Arrays.Stream 或 Stream.of 都可以将 Array 转换为 Stream

Arrays.stream(new String[]{"a","ab","abc"})
        .map(s->Arrays.asList(s))
        .flatMap(l->l.stream())
        .forEach(System.out::println);

Stream.of的源码

public static<T> Stream<T> of(T... values) {
    return Arrays.stream(values);
}

注：

　　调用了Arrays.Stream

Arrays.Stream的源码

public static <T> Stream<T> stream(T[] array) {
     return stream(array, 0, array.length);
}

原始数组

Arrays.Stream

int [] s= new int[]{1, 2, 3};
IntStream s1 = Arrays.stream(s);
s1.forEach(System.out::println);

Stream.of

Stream<int[]> s2 = Stream.of(s);
IntStream s3 = s2.flatMapToInt(Arrays::stream);
s3.forEach(System.out::println);

对于对象数组，都会调用Arrays.stream

对于原始数组，Arrays.stream更方便，它可以直接返回固定大小的IntStream