How do I include an external .class file to use it in my Java code?

Question

A basic question: I'm using the command line to compile and run my java code (javac and java commands respectively). Our professor provided us with a .class file we can use in our future projects. The class filename is U.class, and our professor told us which methods are in included in this class so we can invoke them in our own code. My question is, how can I tell Java I want to use this .class file in my code and how would I type it in the command line so to compile this (using javac)? The following is how my source code looks like so far:

import U; // This line marks an error.

public class DemoU
{
    public static void main(String[] args)
    {
        U u = new U();

        String[] myJavaFiles = u.system("dir");
        u.displayArray(myJavaFiles);
    }
}

I've placed both files, DemoU.java and U.class, in the same folder, and I try to compile everything using javac DemoU.java.

"-cp <class search path of directories and zip/jar files>" — MadProgrammer
– MadProgrammer, Commented Sep 15, 2015 at 2:52
remove import, you don't need it as both classes in the same folder — Iłya Bursov
– Iłya Bursov, Commented Sep 15, 2015 at 2:57
Ask your professor to provide that shared library as a JAR, instead of simply a .class file. Then just put that JAR in your classpath when you are building/executing. Done!. Simply providing a class file can be tricky, coz you don't even know which package that class is supposed to be! You need to put it in a correct directory structure if it is not in default package. — Adrian Shum
– Adrian Shum, Commented Sep 17, 2015 at 3:14

shan1024 · Accepted Answer · 2015-09-15 03:16:18Z

4

Remove import U;. You don't need it because both files are in the same location. That is what causing the problem.

edited Sep 15, 2015 at 3:16

answered Sep 15, 2015 at 3:04

shan1024

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1 Comment

Erwin Bolwidt Over a year ago

The OP says "I've placed both files, DemoU.java and U.class, in the same folder, and I try to compile everything using javac DemoU.java." and you're suggesting that he do exactly what he already does. That doesn't seem very helpful.

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