Incompatible types: bad return type in lambda expression?

Question

Given the following code:

/**
 * Prints the grid with hint numbers.
 */
private void printGridHints() {
  minesweeperGrid.forEach((k, v) -> {
    v.stream().forEach(
        square -> square.isMineLocatedHere() ? System.out.print("*") : System.out.print(square
            .getNumSurroundingMines()));
    System.out.println();
  });
}

My compiler is giving me the following error:

error: incompatible types: bad return type in lambda expression

square -> square.isMineLocatedHere() ? System.out.print("*") : System.out.print(square
                                                                                ^

missing return value

I am running Gradle version 2.2, and I have JDK 8u31 installed. What's interesting that Eclipse does not show any compiler errors, even after I clean and rebuild my project, but when I run gradle build on the command line, I get this compiler error.

Why am I getting this error, and how do I fix it?

Solution

You can't have void as type for the second and third expression in a ternary expression. I.e., you can't do

.... ? System.out.print(...) : System.out.print(...)
                  ^^^^^                   ^^^^^

(If Eclipse says otherwise, it's a bug.) Use an if statement instead:

minesweeperGrid.forEach((k, v) -> {
    v.stream().forEach(
        square -> {
            if (square.isMineLocatedHere())
                System.out.println("*");
            else
                System.out.println(square.getNumSurroundingMines());
        })
  });

Or break it down as follows:

minesweeperGrid.forEach((k, v) -> {
    v.stream().forEach(
        square -> {
            System.out.println(square.isMineLocatedHere()
                    ? "*" : square.getNumSurroundingMines())
        })
  });