为什么我可以通过命令行而不是Eclipse来编译此代码？

Question

These lines of code

    System.out.println(
        Stream.of("adam", "bob", "adrian", "brian")
        .collect(
                Collectors.groupingBy(
                        x -> x.substring(0,1),        //1
                        Collectors.joining("-")))
    );

compile fine when using javac via command line, but when I try to compile in Eclipse I get this error

The method substring(int, int) is undefined for the type CharSequence

How can Eclipse interfere with the compiler? Doesn't an IDE just use javac.exe ?

I'm using Eclipse Luna 4.4.1, with Oracle jdk1.8.0_45

Answer 1

In the first place you should make sure that this isn't a syntax error. Change your code to look like this:

x -> ((String) x).substring(0,1)

Besides making sure that you use a fully Java 8 compatible version of Eclipse, you should check if you allow Eclipse to use Java 8 language features.

If you didn't specify the correct version of JRE you can change it like described here: How to change JDK version for an Eclipse project

Answer 2

The problem has been resolved in Eclipse as of 4.5 Milestone 3, most likely via bug 437444 . That change combines some bug fixes with incorporating several JLS changes made after the release of Java 8.

We could discuss whether the example was actually legal Java 8 before the spec changes (previously inference wasn't allowed to use some information from nested lambda bodies), but I hope just updating to Eclipse 4.5.1 will make everybody happy.