Why does the order in which Netbeans loads Java Libraries cause errors sometimes? and how do I avoid it?

Question

Basic background data: Windows 7, Netbeans 8.0.2, LWJGL 2.9.3, Slick Util. The rest of the code is my own.

So, periodically, I was having what I thought was a bug, that caused my program to run within the IDE, but would fail when I used "Clean and Build" on the project.

Today, when it happened again, I decided to get to the bottom of it. So, taking a copy of the whole project folder, that was working and otherwise identical, and replacing 1 file at a time, and testing it, I was able to narrow it down to the /nbproject/project.properties file.

Every other file could be replaced and the problem persisted. Replace ONLY this file, and the problem went away. So then I loaded up both files and compared them side by side (using the Netbeans "Diff to..." feature) and narrowed it down to the following lines:

javac.classpath=\
${libs.LWJGL-2.9.3.classpath}:\
${libs.0-Slick_Util.classpath}:\
${libs.0-Loaders_v03.classpath}:\
${libs.0-Text2D_v03.classpath}:\
${libs.0-Foundation_v04.classpath}:\
${libs.0-Abstracts_v04.classpath}

Now this code, looks like it is telling the compiler the ORDER in which the libraries should be loaded (I may be wrong).

With that in mind, I decided to test it. I copied this block from the working file, to the non-working file, and it worked just fine. The working code btw is this:

javac.classpath=\
${libs.LWJGL-2.9.3.classpath}:\
${libs.0-Slick_Util.classpath}:\
${libs.0-Foundation_v04.classpath}:\
${libs.0-Abstracts_v04.classpath}:\
${libs.0-Loaders_v03.classpath}:\
${libs.0-Text2D_v03.classpath}

The only differences, you may notice, is the ORDER. Continuing my test, I went into the non-working project's Library properties page, and simply re-ordered the libraries to match the working list. PROBLEM WENT AWAY! If I simply moved the Foundation and Abstracts libraries down in the list, the PROBLEM CAME BACK!

I must have tried a dozen different order combinations, and got about 2/3 that failed, and 1/3 that worked. The ones that worked all involved Foundation and/or Abstracts to be near the top.

Why is this happening? How can I know what order my libraries need to be loaded to avoid the error?

The error btw is this:

F:\Dropbox\2-Documents\4-Java Programming\Library\0-LoadingScreen_v04-Copy\src\A_Library\Test_LoadingScreen.java:94: error: cannot find symbol
    Lib_Foundation                  .setConfigLocation(configLocation);
  symbol:   method setConfigLocation(String)
  location: class Lib_Foundation

Any information that can help me avoid this problem in the future will be appreciated.

Answer 1

It would appear that either “Loaders_v03” or “Text2D_v03” contains its own version of Foundation, including an incompatible Lib_Foundation class. A classpath is searched in order, so your current solution—reordering javac.classpath —will always work, assuming NetBeans doesn't mess with it the next time you make any change to your project.

Whether that will break Loaders and Text2D depends upon how well Foundation adheres to object-oriented design: public classes and their public members are never supposed to be changed or removed in successive versions. (That is why 20-year-old code written for Java 1.1 will still compile in Java 8.)