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How does java find .class files of java standard library or libraries I buy

I came across this question:
What is a classpath and how do I set it?
and the first answer explainns what classpath is and how to set it:
.....First, let's suppose that MyClass is something you built as part of
your project, and it is in a directory in your project called output.
The .class file would be at
output/org/javaguy/coolframework/MyClass.class (along with every other
file in that package). In order to get to that file, your path would
simply need to contain the folder 'output', not the whole package
structure, since your import statement provides all that information
to the VM.
Now let's suppose that you bundle CoolFramework up into a .jar file,
and put that CoolFramework.jar into a lib directory in your project.
You would now need to put lib/CoolFramework.jar into your classpath.
The VM will look inside the jar file for the org/javaguy/coolframework
part, and find your class.
So, classpaths contain:
JAR files, and Paths to the top of package hierarchies....
but if java only looks for classes in directories specified by CLASSPATH variable how does java find classes from libraries that are part of JRE?

Can I put a Java class with no package in a different directory?

I inherited all these classes that have no package, and the only way I can make this work is to put all these in the root directory where I run my class program. The issue is that this pollutes the directory and it's not a clean installation. Since these classes were already compiled, I cannot change the package. Is there a way to put these classes in a directory and have Java find them in the classpath?

Since you have them only as bytecode, you are using them more or less like a libray. So why not generate a jar file out of them?
jar cf jar-file-name input-files-or-dirs-with-wildcard
Then you can put this jar-file wherever you want, you just need to add it to your classpath and your root-directory is no longer polluted.

Adding jar in classpath using regexp

I have a question around addition of specific jars from a specific folder.
Scenario:
I have a lib folder which contains a number of jar files. Due to some issue, I have to give preference to a jar file over others.
Refer this issue: http://bugzilla.slf4j.org/show_bug.cgi?id=327
Now am trying to put a specific jar file to be loaded first here log4j-1.2.17.jar
One option is I simply hardcode this in my app script to make sure this jar is always added firstly in the classpath.
Problem in the approach:
Since the project is maven based, there is a chance that log4j version changes. And in that cases I am prone to same issue again, as I have to go and change the version in the script too.
Other option (for which I am searching the solution), there is some possibility like:
Example command (Expected, but this is not working though):
java -cp <path to my app>/lib/log4j*:<path to my app>/lib/* MainClass
The advantage I see here, even if log4j version changes, I will not have to change the script.
Need inputs how can I achieve approach 2?
Thanks

According to this: http://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html
Class path entries can contain the basename wildcard character , which is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR. For example, the class path entry foo/ specifies all JAR files in the directory named foo. A classpath entry consisting simply of * expands to a list of all the jar files in the current directory.
So "*" is equivalent to all *.jar files in a directory. Maybe you could put the log4j in a lib/log directory? The following is working for me under Windows:
java -cp "lib\*;lib\log\*" MainClass

What files are used for classpath?

I have some classes in Eclipse which cannot be resolved to a type. I know that classes can be in .class, .jar, .par, .zip files. Are there any other file types that I have to look for? Or is there anything eclipse how I could make Eclipse recognize the classes?
In my general understanding, once I have found these files and added them to the classpath, the Eclipse should be able to recognize them.

Actually my colleague gave me an answer:
If the class cannot be found or resolved to a type within Eclipse, this is only the problem with build path. And to the build path, the files should be added of type as I have said: class, jar, par, zip; no other suffix is accepted.

Maybe I am misunderstanding your question, so please excuse me if I am. Type resolution errors are often due to incorrect imports or misspellings. Ill give an example:
public class Foo extends Component{
//If you forgot to import java.awt.*; You would receive the error
"Component cannot be resolved to a type"
}
Also if you misspelled an extension you would get the same error.When you create a class in eclipse you need to create a new file for that class.
You should also post your code so we know exactly what you're talking about. I don't think your error has anything to do with the classpath. :)

Class path entry could be jar, zip or directory (source)
set CLASSPATH=classpath1;classpath2...
Class paths to the .jar, .zip or .class files. Each classpath should end with a filename or directory depending on what you are setting the class path to:
For a .jar or .zip file that contains .class files, the class path ends with the name of the .zip or .jar file.
For .class files in an unnamed package, the class path ends with the directory that contains the .class files.
For .class files in a named package, the class path ends with the directory that contains the "root" package (the first package in the full package name).
Multiple path entries are separated by semi-colons. With the set command, it's important to omit spaces from around the equals sign (=).
The default class path is the current directory. Setting the CLASSPATH variable or using the -classpath command-line option overrides that default, so if you want to include the current directory in the search path, you must include "." in the new settings.
Classpath entries that are neither directories nor archives (.zip or .jar files) nor * are ignored.

Java NoClassDefFoundError even when Jars in same folder

I have created a simple Java program (1 java file that contains the main() ), and I've included all Jar files in the same directory as the .class file. It is giving the NoClassDefFoundError message.
I've tried updating the Classpath to point to the directory, and I've also set "-cp ." to suggest that it look in the same directory as the .class file. However, the program still says it can't find the class def.
Any thoughts on what I should do?

Adding a folder tells java to look in that folder for .class files.
You can't reference .jar files via a folder name... Each .jar file's path must be listed on the CLASS_PATH explicitly.
this question's answer may be useful

When you try running a class from command line then a NoClassDefFound exception usualy means there is something wrong with your classpath.
You have explicitly define the classpath. You can do this in a few ways but the following way is the least prone to error:
Open a command shell and do the following:
1.) set classpath = {path to class files};{path to jars}
2.) java com.example.mainclass
Note: Even if your classes path and jar path is the same you need to specify them explicitly.
Note: If you have more then one jars place them in a folder say lib and add it to the classpath like: {path}/lib/* This will include all of the jar otherwise you have to specify them individually.
References: https://javarevisited.blogspot.com/2011/01/how-classpath-work-in-java.html

Import the following package:
Import java.lang.NoClassDefFoundError;