Unexpected interaction with String and File

I have following pieces of code:

    if (e.getSource() == theView.addButton) {

            System.out.println("Add Button clicked");
            theView.setBotTextArea("Adding category...");

            File directory = new File(theModel.getDirectory() + theView.getCategoryNameInput());

            boolean isDirectoryCreated = directory.mkdir();

            if(isDirectoryCreated) { 

                System.out.println("Created new directory in: " + directory);

            } else if (directory.exists()) { 

                System.out.println("Category already exists!");

            }               

        }

This is part of the ActionListener's ActionPerformed() method.

private File directory = new File("C:/Users/Lotix/Desktop/TestFolder/");
public File getDirectory() { 

    return directory;

}

What i expect this method to do is to create a subfolder in the chosen directory. However, for some reason unknown to me, it creates completly another folder on my desktop instead of TestFolder.

I tried theModel.getDirectory().toString() and manipulating the variable but to no avail. The solution i came up with is to simply add forward slash between theModel.getDirectory() and theView.getCategoryNameInput() such as this:

File directory = new File(theModel.getDirectory() + "/" + theView.getCategoryNameInput());

However, when i concatenate File variable with another String it works perfectly fine.

What gives?