This. метод() и объект. setContentPane() - Java

import javax.swing.*;
import java.awt.GridLayout;
import java.awt.BorderLayout;
 
public class Calculator {
    // Declaration of all calculator's components.
    JPanel windowContent;
    JTextField displayField;
    JButton button0;
    JButton button1;
    JButton button2;
    JButton button3;
    JButton button4;
    JButton button5;
    JButton button6;
    JButton button7;
    JButton button8;
    JButton button9;
    JButton buttonPoint;
    JButton buttonEqual;
    JPanel p1;
 
      // Constructor  creates the components in memory
    // and adds the to the frame using combination of 
    // Borderlayout and Gridlayout
    Calculator(){
      windowContent= new JPanel();
        
    // Set the layout manager for this panel
      BorderLayout bl = new BorderLayout(); 
      windowContent.setLayout(bl);
 
     // Create the display field and place it in the   
     // North area of the window
      displayField = new JTextField(30);
      windowContent.add("North",displayField);
     
     // Create buttons using constructor of the  
     // class JButton that takes the label of the 
     // button as a parameter 
      button0=new JButton("0");
      button1=new JButton("1");
      button2=new JButton("2");
      button3=new JButton("3");
      button4=new JButton("4");
      button5=new JButton("5");
      button6=new JButton("6");
          button7=new JButton("7");
          button8=new JButton("8");
          button9=new JButton("9");
          buttonPoint = new JButton(".");
          buttonEqual=new JButton("=");
                                
         // Create the panel with the GridLayout 
         // that will contain 12 buttons - 10 numeric  
         // ones, and  buttons with the point and the 
         // equal sign   
          p1 = new JPanel();
          GridLayout gl =new GridLayout(4,3); 
          p1.setLayout(gl);
         //  Add window controls to the panel p1
          p1.add(button1);
          p1.add(button2);
          p1.add(button3);
          p1.add(button4);
          p1.add(button5);
          p1.add(button6);
          p1.add(button7);
          p1.add(button8);
          p1.add(button9);
          p1.add(button0);
          p1.add(buttonPoint);
          p1.add(buttonEqual);
 
         // Add the panel p1 to the center area 
         // of the window         
            windowContent.add("Center",p1);
         //Create the frame and set its content pane                   
            JFrame frame = new JFrame("Calculator");
            frame.setContentPane(windowContent);
         // set the size of the window to be big enough   
         // to accomodate all controls
            frame.pack(); 
         // Finally, display the window
          frame.setVisible(true);
          }
 
          public static void main(String[] args) {
              Calculator calc = new Calculator();
          }
      }

frame.setContentPane(windowContent);

 windowContent
из
 windowContent = new JPanel();

p1
из
p1 = new JPanel();

this.setLayout(gb);

// Set the GridBagLayout for the window’s content pane
GridBagLayout gb = new GridBagLayout();
this.setLayout(gb);

JPanel blabla = new JPanel();
GridBagLayout gb = new GridBagLayout();
blabla.setLayout(gb);