If setResizable(false) is called before setVisible the frame slides to the down left corner of the screen.

To reproduce use:

import javax.swing.*;
import java.awt.*;

public class JFramePositionTest {

    public static void main (String args[]){
        new JFramePositionTest();
    }

    public JFramePositionTest(){
        JFrame aFrame = new JFrame();
        aFrame.setSize(300,300);
        aFrame.setResizable(false);
        aFrame.setVisible(true);
        //aFrame.setResizable(false); //COMMENT ABOVE setResizable(false) method and CALL THE CODE HERE THEN THERE'S NO PROBLEM
        aFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

    }

}