[英]What is the difference between .stream() and Stream.of?
哪個是從集合中創建流的最佳方法:
final Collection<String> entities = someService.getArrayList();
entities.stream();
Stream.of(entities);
第二個不符合你的想法! 它沒有為您提供包含集合元素的流; 相反,它將為您提供一個包含單個元素的流,這是集合本身(而不是其元素)。
如果需要包含集合元素的流,則必須使用entities.stream()
。
1)
Stream<String> stream1 = entities.stream()
2)
Stream<Collection<String>> stream2 = Stream.of(entities)
所以使用1或2
Stream<String> stream3 = Stream.of("String1", "String2")
我自己一直對此感到困惑,所以我不妨把它留在這里以備將來參考:
import java.util.stream.IntStream;
import java.util.stream.Stream;
import static java.util.Arrays.*;
import static java.util.stream.Stream.*;
class Foo {
void foo() {
Stream<Foo> foo;
foo = of(new Foo(), new Foo());
// foo = stream(new Foo(), new Foo()); not possible
foo = of(new Foo[]{new Foo(), new Foo()});
foo = stream(new Foo[]{new Foo(), new Foo()});
Stream<Integer> integerStream;
integerStream = of(1, 2);
// integerStream = stream(1, 2); not possible
integerStream = of(new Integer[]{1, 2});
integerStream = stream(new Integer[]{1, 2});
Stream<int[]> intArrayStream = of(new int[]{1, 2}); // count = 1!
IntStream intStream = stream(new int[]{1, 2}); // count = 2!
}
}
我們可以看看源代碼:
/**
* Returns a sequential {@code Stream} containing a single element.
*
* @param t the single element
* @param <T> the type of stream elements
* @return a singleton sequential stream
*/
public static<T> Stream<T> of(T t) {
return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
}
/**
* Returns a sequential ordered stream whose elements are the specified values.
*
* @param <T> the type of stream elements
* @param values the elements of the new stream
* @return the new stream
*/
@SafeVarargs
@SuppressWarnings("varargs") // Creating a stream from an array is safe
public static<T> Stream<T> of(T... values) {
return Arrays.stream(values);
}
至於Stream.of()
,當輸入變量是一個數組時 ,它將調用第二個函數,並返回一個包含數組元素的流。 當輸入變量是一個列表時,它將調用第一個函數,並且您的輸入集合將被視為單個元素,而不是集合。
所以正確的用法是:
List<Integer> list = Arrays.asList(3,4,5,7,8,9);
List<Integer> listRight = list.stream().map(i -> i*i).collect(Collectors.toList());
Integer[] integer = list.toArray(new Integer[0]);
List<Integer> listRightToo = Stream.of(integer).map(i ->i*i).collect(Collectors.toList());
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