Question

I’m having trouble adding a .jar file I downloaded for my Java project. This is really the first time I’ve used eclipse, so please bear with me and for some reason (I have no clue why), I just find it somewhat confusing.

I know that in order reference different class files you simply need to create a class library and add it to the build path. From there, all which needs to be done (unless I’m misunderstanding this for whatever reason) is use the “import” keyword to import whatever .jar, .java, or .class/.interface file necessary into the project.

I’ve tried that with my .jar. I have it referenced in the build path (all I did was just copy the jar to the project directory, and then use the build path option to add it externally), but when ever try to call the object “Delegator”, which obviously is a part of the .jar file, it won’t read.

Am I missing something here? Seriously, anyone who knows the answer to this – you’re relieving a mother of a headache. And before anyone asks – yes, I’ve searched this one to death. I’ve found similar questions, but nothing which quite hit what I was looking for. Either that, or I really just lack the common sense.

Answer 1

There are several possible reasons, for the question hasn’t mentioned the specific failure, and where it has occurred. The following is a list of possible reasons I could think of, but this may not be exhaustive:

• You can import a class, in a different package only if the class is public. The only exception is when you are using the class in the same package. If the class is an inner class marked as private, then you’re well and truly out of luck. The Delegator class in question might not be public, and that’s why you may be unable to use it. This issue ought to be caught by the compiler.
• The directory structure within the JAR might not match your package import statements in your classes. This might not be necessary, for Eclipse ought to provide possible fixes, but it is better to verify that nevertheless. Again, the compiler should complain if this is the case.
• If the issue is at runtime, then, it is most likely that the JAR is not available in the runtime classpath. You’ll need to configure the Runtime configuration, to add the JAR to the runtime classpath. Refer to the Eclipse documentation on run configurations, if you need to know how to change the runtime classpath.

Note:

1. Exporting the build classpath entries would matter to other projects that depend on the pertinent project; unexported entries will have to be re-imported if required in other projects. This would not apply to a run configuration.

Update

Every Java application needs a main(String[] args] method to start execution. This is the entrypoint for the application. From the comment, it appears that the main method is in a different class. If so, the said class ought to be used to start the application. In Eclipse, a “Run configuration” might be used for the class that lacks this entrypoint, resulting in the described error. One can rectify this by creating a new Run configuration for the class with the said entrypoint. This may be done by one of the following:

• editing the existing Run configuration to use the desired Class (the one with the main method). See the above link, in the third bullet point. Edit the value of the class to be launched.
• creating a new Run configuration for the desired Class. Usually, you’ll need to traverse to the desired class, and run your application (using the Alt+Shift+X+J shortcut) from the said class.