CODE EXAMPLE FOR JAVA

JFrame change outline

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

public class BorderFrameExample extends JFrame 
{
   JLabel l = new JLabel("Welcome!", JLabel.CENTER);
   
   public BorderFrameExample() {
      //add label to frame
      add(l, BorderLayout.CENTER);
      //remove the decoration
      setUndecorated(true);
      //set border
      getRootPane().setBorder(
         BorderFactory.createMatteBorder(4, 4, 4, 4, Color.BLUE)
      );
      setSize(250,250);
      setVisible(true);
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   }
   public static void main(String[] args) {
      new BorderFrameExample();
   }
}