java8 stream接口终端操作 toArray操作

2023年9月29日 156次阅读来源: 葵花下的獾

对于中间操作和终端操作的定义，请看《JAVA8 stream接口中间操作和终端操作》，这篇主要讲述的是stream的toArray操作，我们先看下函数的定义以及实现

	Object[] toArray();
	<A> A[] toArray(IntFunction<A[]> generator);

	@Override
    @SuppressWarnings("unchecked")
    public final <A> A[] toArray(IntFunction<A[]> generator) {
        // Since A has no relation to U (not possible to declare that A is an upper bound of U)
        // there will be no static type checking.
        // Therefore use a raw type and assume A == U rather than propagating the separation of A and U
        // throughout the code-base.
        // The runtime type of U is never checked for equality with the component type of the runtime type of A[].
        // Runtime checking will be performed when an element is stored in A[], thus if A is not a
        // super type of U an ArrayStoreException will be thrown.
        @SuppressWarnings("rawtypes")
        IntFunction rawGenerator = (IntFunction) generator;
        return (A[]) Nodes.flatten(evaluateToArrayNode(rawGenerator), rawGenerator)
                              .asArray(rawGenerator);
    }

    @Override
    public final Object[] toArray() {
        return toArray(Object[]::new);
    }

可以看到，返回Object[]，没有传参数的toArray，在具体的实现中，也是调用了

public final <A> A[] toArray(IntFunction<A[]> generator)

重载的toArray的实现，传入了一个Object的数组

下面我们通过案例，看下具体的使用

List<String> strs = Arrays.asList("a", "b", "c");
        String[] dd = strs.stream().toArray(str -> new String[strs.size()]);
        String[] dd1 = strs.stream().toArray(String[]::new);
        Object[] obj = strs.stream().toArray();

        String[] dd2 = strs.toArray(new String[strs.size()]);
        Object[] obj1 = strs.toArray();

可以看到，前三个，是调用的stream的toArray的函数，以及一些用法，后面的两个，是直接调用的List接口的toArray函数，List接口里的，只是顺带提了一下，用法就是这样，请大家自己get吧