Stream: Predict the Output

Intermediate ops are lazy: no terminal operation means `map` never executes, no values are produced.

Streams are single-use: reusing one after a terminal operation throws `IllegalStateException: stream has already been operated upon or closed`.

`count()` on a SIZED stream skips `peek` in Java 9+, so it prints nothing and returns 3.

Five indices mapped to "a" and reduced by concatenation from an empty start give the string `"aaaaa"`.

`limit(5)` gives five "x"s from the infinite generator; `forEachOrdered` keeps them in order despite parallel, so "x" prints five times in sequence.

Duplicate key in two-argument `toMap` throws `IllegalStateException`; add a merge function as the third argument to resolve it.

`sorted()` orders the data, but parallel `forEach` consumes it unordered; use `forEachOrdered` to preserve the sorted order in output.