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How to call java classes of a compiled jar file within android studio?

Hello StackOverflow folks,
I have a jar file of java classes. I added this jar file to my android studio project under folder /libs. Now, what I want to do is use those classes within the jar file in MainActiviy.java. I just do not know how.
Some details:
My jar file is named: zombi.jar.
The class within the jar file to call is named: COMBI.class
I tried the following:
In MainActivity.java, I wrote:
// declared class variable
private COMBI mCOMBI;
Then, in OnCreate method, I wrote:
mCOMBI = new COMBI();
//to start calling method COMBIStart to launch the command-line system
mCOMBI.COMBIStart();
I actually called the classes as I would in normal Java. I think Android uses special java code that looks like java, but I don't know how to use them.
I could not get the code to work.
Can you help me?

I'm going to assume you've already written your import statement for the jar class. If you already put the jar file in the /lib folder, Android Studio should update your build.gradle file. Check to see if you have a link to the jar file in your dependencies{...}
If not, you can add it manually.

Just like in normal java you have to add an import to the correct package at the top of your file for everything that's not in the same package the file is in.
It should look something like this:
import android.view.View;
but instead of android.view.view it would be a reference to the class in the jar you're trying to add

Add a new class to an existing JAR File(which contains source code)

I'll try to illustrate the problem as simple as I can.
I have a JAR file, which I extracted using Winrar. (The jar file contains an open source android library).
I want to modify this JAR file by adding a new class to the library.
So here are my steps:
First, I created a class using Eclipse and set the package name same as the android's library package name.
Second, I copied this java File to the folder of the other java files in the library.
Third, I tried to compile the JAVA file via the CMD using javac.
The path of the new java file and the other .JAVA and .CLASS files of the library is: C:\com\example\core\
The name of the new java file would be: "MyNewClass.java"
The command I run via the CMD is: javac C:\com\example\core\MyNewClass.java
But, during the compilation I get many errors saying: Cannot find symbols.
I've been looking up for a solution of this problem but couldn't figure how to solve it and make the new JAR File having another class that I created seperately.
What am I missing?

As per earlier comments:
Rather than trying to modify the JAR, you can get access to the full source code of the Universal Image Loader library by cloning the repository using git or hitting "Download ZIP" on the righthand side of the page you linked.
Once you have the source, import the library in your IDE. From there on you'll be able to build the whole thing from scratch, make any adjustments/modifications you like, etc.

Your classpath might be wrong or there might be some mistake in package name.
When a Java program is being compiled the compiler it creates a list of all the identifiers in use. If it can't find what an identifier refers to it cannot complete the compilation. This is what the cannot find symbol error message is saying, it doesn't have enough information to piece together what the Java code wants to execute.

Try:
javac -cp com/* C:\com\example\core\MyNewClass.java
That should make the compiler aware of all the other classes under com/...

Java: silly newbie issue about path used in import statement

I'm working through the example here:
http://www.vogella.com/articles/JavaPDF/article.html
In my file, I've got:
package com.mycompanyname.mydirectory;
import com.mycompanyname.OneOfMyClasses;
import com.itextpdf.text.Document;
...
public class MyClass {
...
}
Everything is working fine. What I don't understand is that since I just copied the import statement directly from the link above for the iText portion -- why does com.itextpdf.text.Document work?
I mean, if I look in directory com.mycompanyname I can see OneOfMyClasses.java there.
But in the com directly, there is no itextpdf directory (although maybe my user doesn't have permission to see it(?)).
Hoping someone can help me understand what I'm missing here. Doesn't the import point to a specific directory that I should be able to see the class? Is there a different com directory somewhere that iText is using, and com.itextpdf.text points to there? (if so, where's the directory located)?
I installed the jar file for iText in the lib folder as per usual, and made sure it was included in the classpath.

Those classes are inside a JAR file that is added to the classpath:
Create a new Java project "de.vogella.itext.write" with the package "de.vogella.itext.write". Create a folder "lib" and put the iText library (jar file) into this folder. Add the jar to your classpath.
import statements will look inside whatever directory trees are in the classpath, which includes the current directory at compilation time (tipically the src/ directory in your project) as well as any directory specified through environment variable or JVM startup parameter. See this about the classpath.
EDIT
You do need the imports whenever you use classes across packages. Every public class/interface you define is in a package. If whatever you are referencing belongs to another package, you need to import it.
JARs are zip files that contain directories and files inside. It's the same as plain directories and files, only packed.

It comes from the iText dependency (jar) you added in an earlier step.

Not necessarily - you could also import from libraries, etc.
In fact, Java will try to search through the classpath. Here is some helpful documentation.

That class is most probably imported in a JAR library. Inside such JAR file, the class files are kept in exact package/folder structure as you use when importing them.

How to use classes from the .jar lib added to netbeans project?

Can anybody help me out in this?
what I have is a abc.jar file with me. It contains ABC.class file inside it. I added the jar file to netbeans project Libraries. But I am getting an error when I write
ABC a=new ABC();
Error :
"can not find symbol class ABC"
any help?
Edited :
also I am able to see the structure of ABC class when I click on the ABC.class file inside the library.

Remember to import it with an import statement. You should probably read the documentation that follows the jar file. Hopefully there's an example of usage - mostly there is.

The answer :
I found out that if the jar has only default package you won't be able to import classes inside it. You need to have packages other than default inside the jar to import it. You can only import non default packages.
the thing with default packages works fins in eclipse, but not in netbeans.

Create a jar file from the java code

i want to create a jar file from java program i looked at some examples Java code to create a JAR file but it didnt impressed me as this will not create the proper package structure my original command was
jar -cfv formBuilder.jar .\com\accenture\* .\com\wysiwyg\util\XmlUtil.class .\com\wysiwyg\exception\ApplicationException.class .\com\wysiwyg\constants\*.class .\com\wysiwyg\util\FormBuilderUtill.class .\com\wysiwyg\util\SaveFormOnLocalUtil.class .\com\wysiwyg\logger\LogInfo.class .\com\wysiwyg\factory\Validation.class
now i want to do the same using java code but without ant, and proper package structures should be created, is this feasible?

Use java.util.jar package for this work.