List of Names to Uppercase

Question

Accepted Answer

map() is a Stream operation that transforms each element into a new value using a given function, and applying String::toUpperCase converts every string in a stream to its uppercase form.

Key Points:
• map() applies a one-to-one transformation function to each element, producing a new stream of the mapped type.
• String::toUpperCase is a method reference that's equivalent to the lambda name -> name.toUpperCase().
• Because streams don't modify the source, the original list of names remains unchanged after mapping.
• Method references are preferred over equivalent lambdas here since they're more concise and equally readable.
• collect(Collectors.toList()) materializes the transformed stream back into a concrete list.

Example:
Given the list ["Alice", "Bob", "Charlie"], mapping with String::toUpperCase produces a new list ["ALICE", "BOB", "CHARLIE"] while leaving the original list untouched.

Code Example:
```
List names = Arrays.asList("Alice", "Bob", "Charlie");
List upperNames = names.stream()
        .map(String::toUpperCase)
        .collect(Collectors.toList());
```

Interview Tip:
A concise interview answer is:

"I use map(String::toUpperCase) to transform each element of the stream into its uppercase form and collect the results into a new list, leaving the original list unchanged since streams don't mutate their source."