By default, a java application appears at top left corner of the screen. You might have seen that most of the applications appear in the middle of the screen. I will tell you how to center your java application on screen.
To set JFrame in center of Just call this.setLocationRelativeTo(null); in constructor of the JFrame. If you are using Netbeans IDE, it should be after calling initComponents() method. But if you call setLocation() method of JFrame after this, your JFrame will get relocated.
In most cases this works and this is enough. if you want to get the center point of the screen, just refer to the following post:
How to Get Center Point of Screen in Java