Bug 4240 Can't edit class path entry (1GKEPIF)
There is no way to edit an existing entry for a library on the Java Build Path project properties page (other than editing the .classpath file directly). NOTES: EG (9/22/2001 11:42:11 AM) this is particularly annoying for variables. we should have an Edit button when a variable is selected.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/NewContainerDialog.java
public void dialogFieldChanged(DialogField field) {
			doStatusLineUpdate();
		}
	}
	
	protected void doStatusLineUpdate() {
		checkIfPathValid();
		updateStatus(fContainerFieldStatus);
	}		
	
	protected void checkIfPathValid() {
		fFolder= null;
		
		String pathStr= fContainerDialogField.getText();
		if (pathStr.length() == 0) {
			fContainerFieldStatus.setError(NewWizardMessages.getString("NewContainerDialog.error.enterpath")); 
			return;
		}
		IPath path= fCurrProject.getFullPath().append(pathStr);
		IWorkspace workspace= fCurrProject.getWorkspace();
		
		IStatus pathValidation= workspace.validatePath(path.toString(), IResource.FOLDER);