如何使用 readline 在 kotlin 中输入 null 或 EOF

Question

I'm doing beginner level Kotlin and I'm trying to do a multi line input and I found this code:

var originalTexts: MutableList<String> = mutableListOf()
while (true) {
    val originalText = readLine()
    if (originalText == null) break
    originalTexts.add(originalText)
}

The problem is I dont know how to input null or EOF in the input stream in console using readLine(). I tried ctrl+z or ctrl+d but is not working. Beginner here please help guys.

Edit: I'm using Intellij in Windows OS

Answer 1

A null return from readLine() indicates an end-of-file condition — basically, there can be no more input.

But whether and how that can happen depends on how you're running the program:

• If you're running it in a terminal window, then you can signal end-of-file with a special key combination, which depends on the platform:

  • On macOS and Linux: Ctrl + D
  • On Windows: Ctrl + X then Return

• If you're running it with the input redirected from a file (eg by appending <filename to the command), then you'll get an end-of-file condition after reading all the content of the file.

• Some IDEs may provide their own combinations. For example:

  • IntelliJ on macOS: Cmd + D . ( Ctrl + D starts the debugger instead.)
  • IntelliJ on Windows: Ctrl + D .

• Some IDEs may provide no way at all.

• Some IDEs (eg online) don't support user input, and so readLine() will always return null.

---

Due to the confusion that this can cause, Kotlin 1.6 added a readln() function, which never returns null: instead, it throws a RuntimeException if end-of-file is reached, or on platforms such as Kotlin/JS which don't support user input in this way.

(That may make it slightly easier to write beginner programs, but IMHO it's merely turning a compile-time issue into a hidden run-time one, which seems like a backward step for a language as clear and safe as Kotlin…)