Understanding problem regarding operated streams in Java-CodePudding

Let's say we have the following code

Stream<Integer> ints = Stream.of(1, 2, 3);
    
ints.peek(System.out::println);
    
ints.forEach(System.out::println);

and I run it, I'll get an exception:

Exception in thread "main" java.lang.IllegalStateException:
stream has already been operated upon or closed

But why?

peek is an intermediate operation, so I thought, that it won't run/start the stream itself? Only forEach does this, so why has the stream already been operated when reaching forEach?

I also thought, that the peek method would be discarded, because it returns a new Stream that I do not consider. Like in

String str = "hello world";
str.toUppercase();
str.charAt(0); // <-- h not H

CodePudding user response：

peek is an intermediate operation, which means it performs an action and produces a new Stream. But you're reusing the same Stream, therefore getting an IllegalStateException.

If you write the code like this, it would work as intended:

Stream<Integer> ints = Stream.of(1, 2, 3);
    
Stream<Integer> ints1 = ints.peek(System.out::println);
    
ints1.forEach(System.out::println);

Which is basically a verbose equivalent of the chain:

Stream.of(1, 2, 3)                 // ints
    .peek(System.out::println)     // ints1
    .forEach(System.out::println);