Help with a project's plugin

I'm working on my IDEA IntelliJ Plugin with gradle and I am trying to create a button that adds a function to the current class, or a given class. I saw an example in Eclipse but couldn't find the equivalent libraries.

I've tried to seek for an equivalent library to ICompilationUnit in Eclipse but couldn't find anything

ICompilationUnit unit = compilationUnit(activeEditor());

final String messageName = inputBox("Create New Message",
"Message Name");

if (messageName == null)
return null;
String camel = camelCase(messageName);
final String methodName = "handle" + camel;

IType type = null;
try {
unit.createImport(Global.WHEN_RECEIVED_CLASS_NAME.data(), null, null);
type = unit.getType(drop(unit.getElementName(), 4));

if (!type.getSuperclassName().equals("SimpleAgent")) {
msgbox("cannot create message - unrecognized agent type");
return null;
}

IMethod mtd = method(type, methodName);

if (mtd == null) {
mtd = type
.createMethod(
"@WhenReceived(\""
+ messageName
+ "\")\npublic void "
+ methodName
+ "(){\n\t//TODO: Add Message Handling Code Here.\n\t"
+ "//you can add any parameters to the method in order to receive them within the message.\n}\n",
null, false, null);
}

CompilationUnitEditor editor = activeCompilationUnitEditor();
editor.setSelection(mtd);
My main goal is to add a button that once it's clicked it opens a text box, than it adds a function with the given text to the current class. Error if nothing has been typed.

Thank you very much in advanced.