How about File.isAbsolute():

File file = new File(path);
if (file.isAbsolute()) {
    ...
}

Nope.

There are some underlying FileSystem classes (that's Java 7, but they exist prior to it as well) that expose isAbsolute(), but they're not public - so you shouldn't use them, and even if you did your code would be full of reflection junk - and only the "correct" OS ones are included in the JRE, so you'd have to code around them anyway.

Here are the Java 7 implementations of isAbsolute(...) to get you started. Note that File.getPrefixLength() is package-private.

Win32FileSystem:

public boolean isAbsolute(File f) 
{
        int pl = f.getPrefixLength();
        return (((pl == 2) && (f.getPath().charAt(0) == slash))
                || (pl == 3));
}

UnixFileSystem:

public boolean isAbsolute(File f) 
{
        return (f.getPrefixLength() != 0);
}

Try with this:

import java.io.IOException;
import java.nio.file.Path;
import java.nio.file.Paths;

public class Main {
    public static void main(String[] args) throws IOException {
        Path path = Paths.get("myFile.txt");
        Path absolutePath = path.toAbsolutePath();

        System.out.println(absolutePath.toString());
    }
}

If I get your problem right, you could do something like this:

File a = new File("/some/abs/path");
File parentFolder = new File(a.getParent());
File b = new File(parentFolder, "../some/relative/path");
String absolute = b.getCanonicalPath(); // may throw IOException