.stream() vs Stream.of

问题:

Which is the best way to create a stream out of a collection:

    final Collection<String> entities = someService.getArrayList();

1. entities.stream();

2. Stream.of(entities);

回答1:

The second one does not do what you think it does! It does not give you a stream with the elements of the collection; instead, it will give you a stream with a single element, which is the collection itself (not its elements).

If you need to have a stream containing the elements of the collection, then you must use entities.stream().

回答2:

1)

Stream<String> stream1 = entities.stream()

2)

Stream<Collection<String>> stream2 = Stream.of(entities)

So use 1, or for 2

Stream<String> stream3 = Stream.of("String1", "String2")

回答3:

I myself keep getting confused about this so I might as well leave this here for future reference:

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!
    }
}