Java – How to use classes from .jar files

jarjava

I read the Java tutorials on Sun for JAR files, but I still can't find a solution for my problem. I need to use a class from a jar file called jtwitter.jar, I downloaded the file, and tried executing it (I found out yesterday that .jar files can be executed by double clicking on them) and Vista gave me an error saying "Failed to load Main-Class Manifest attribute from [path]/jtwitter.jar".

The guy who coded the .jar file wants me to import it, but where do I store the .jar file to import it in my code? I tried putting both the .jar file and my .java file in the same directory, didn't work.

The file I'm trying to work for is here: http://www.winterwell.com/software/jtwitter.php

I'm using JCreator LE.

Best Answer

Let's say we need to use the class Classname that is contained in the jar file org.example.jar

And your source is in the file mysource.java Like this:

import org.example.Classname;

public class mysource {
    public static void main(String[] argv) {
    ......
   }
}

First, as you see, in your code you have to import the classes. To do that you need import org.example.Classname;

Second, when you compile the source, you have to reference the jar file.

Please note the difference in using : and ; while compiling

If you are under a unix like operating system:
```
javac -cp '.:org.example.jar' mysource.java
```

If you are under windows:

javac -cp .;org.example.jar mysource.java

After this, you obtain the bytecode file mysource.class

Now you can run this :

If you are under a unix like operating system:
```
java -cp '.:org.example.jar' mysource
```
If you are under windows:
```
java -cp .;org.example.jar mysource
```

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Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

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new ArrayList<>(Arrays.asList(array));

Best Answer

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