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Call to specific class from JAR file

I want to call a Class A from JAR-1. But I have same Class in JAR-2 hence my app is calling Class A from JAR-2.
All the packages are same in both the JARs. For Eg,
com.package.classes.A in JAR-1 and com.package.classes.A in JAR-2
I want my app to call A from JAR-1.
I am using IntelliJ.(If this information helps)

Java, Load classes from JAR to use them later

I have a problem loading class from JAR file.
I can load them with URLClassLoader using a JarFile etc etc like in this answer; but if later in code I try to instantiate them with reflection:
Object obj = Class.forName(className).newInstance()
I get a ClassNotFoundException.
Can I dinamically load a class to use them later, when I need them, just like classes in ClassPath?
Thank you!

You need to provide class loader to Class.forName method - as otherwise it will look in this same class loader as your class is in.
Object obj = Class.forName("name", true, loader).newInstance() .
But you can't just load a class and then use it in your code like MyLoadedType - as here java does not know where to look for that class, unless you will ensure that your code and loaded code is in this same class loader - you can do this by running all your code from custom class loader that allows for adding new sources in runtime. (URLClassLoader allows for this but method is protected - so you need to extend it and make it public, in java 8 system class loader is also URLClassLoader - but this was changed in java 9) .
But you can operate on that code using reflections like I showed you.

Why java.lang.NoSuchMethodError when dynamically load a dex jar?

There is a interface IA class in APK file. I create a jar containing IA's implementation MA. The jar has a Global class and B class. The jar is loaded dynamically with Dexclassloader from data directory.
interface IA {
test();
}
class Global {
IA instance; //refer to MA instance.
}
class B {
....
instance.test() ; // throw NoSuchMethodError
....
}
But it's OK to call instance's test method through reflection.
Why? Surely I am not familiar with some principals of Classloader or DexClassloader. Could someone give explanation for me? Thanks

I too got a NoSuchMethod error, after doing some testing I found out that the problem was that the DEX file inside the JAR wasn't named classes.dex as it's supposed to be.
The fact that the app was able to create a new instance of your class doesn't necessarily mean that the DEX was loaded properly, I tested it with an empty JAR (i. e. removed all internal files via 7-ZIP) and was still able to instantiate my class (although I guess it would work only with a default constructor that gets zero parameters).

Importing Libraries Java

So I have two Jars I want to use in my project from here http://www.jhlabs.com/ip/filters/index.html
I added both CheckFilter and MarbleFilter to my class path. But when I do
CheckFilter();
It says I have to create a method CheckFilter()
I'm pretty sure that's the method I need to call to use that effect. But when I try any of the other methods in the library Jar it still gives me the same thing.
I have no experience with importing/using external libraries. Any help would be great.
checkFilter = new CheckFilter();
CheckFilter();
I tried above and it says I need to create a local variable checkFilter

How are you writing up the code. I will suggest to use eclipse IDE, it will make your tasks simple
If you are using eclipse. You need to do import the jar Filters.jar to your build path
which as you mentioned you downloaded from JHLabs Download page
I found Filters.jar inside dist directory.
Then you will be able to import the class or package
import com.jhlabs.image.*;
OR
import com.jhlabs.image.CheckFilter;
After importing the class or package you will be able to create object to it by
CheckFilter checkFilter = new CheckFilter();
In case you are totally new you can take help from people over IRC or chat and get going.
Someone would be able to quickly help you out
----==----==----==----==----==----==----==----==----
Read your comments and Question again.
You are totally missing the point. If you call to CheckFilter() directly without invoking new keyword, compiler will consider you are trying to access a method which is inside the class you are writing up. and give you error.
As I mentioned above. Your are trying to accessing Instance variable for the class without declaring it. Either do
CheckFilter checkFilter;
before you access checkFilter variable or directly instantiate the class the way I mentioned.
Seems to me you are missing a log of points :D

Methods don't exist without a class. That is probably the constructor to a class. Use
CheckFilter checkFilter = new CheckFilter();
instead. Then call methods on checkFilter.

From #andy-thomas in this similar question
The import statement imports classes from the jar file, not the jar file itself.
An import statement of the form:
import stdlib.*;
will import all the classes in the package stdlib.
Oracle provides this tutorial on importing.

This statement
CheckFilter();
Tries to call a method CheckFilter defined in your class, which is not the case. If this is a utility function, this may be a static method, in which case you can call it like this
ClassName.CheckFilter(); // replace ClassName with the class containing this function
If not, then you may have to instantiate an object
ClassName obj = new ClassName();
obj.CheckFilter();
or skip the object variable
new ClassName().CheckFilter(); // Not prefered
To add jar files, right click on your project in "Package Explorer", go to "Configure Build Path" and then to "Add External Jars".

Set the jar (libraries) into your classpath and use import statements in your java code to include the required Classes.

Java: Testing if a package (using its name) is present...

I would like to test if a particular package is known to the current classloader or system. Package.getPackage(String name) seems to return null thus im stuck. ClassLoader does not contain any method that does the same thus im stuck...
I d rather not load a class in the package to test if it is present, surely theres a better way.
Any pointers would be appreciated.

Package.getPackage(String name) is the right tool for the job. Make sure you've
Typed the name of the package correctly
Try Package.getPackages() to see what's available
Make sure the classloaders match - test for instance getClass().getPackage() and then Package.getPackage(String name)
The method searches for the package in the current classloader only (or System classloader if current classloader is null). If no class in the package is loaded in the current classloader the package will no be present.

There's no easy way to get information on if a package exist, unless a class has been loaded from that package. Package.getPackage does only work if a class from the named package has been loaded.
Your options are:
Load a class, or instantiate a class, from the package.
Search the classpath, and check all jars/dirs in the classpath.
The 2nd option can fail if the code also is using url classloaders.