为什么不能调用已创建对象的公共方法？

Question

When I am trying to run this Java program to make a JTextField:

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

public class TextField1 extends JFrame{
  private final int WIDTH = 320; 
  private final int HEIGHT = 250; 
  private FlowLayout flow = new FlowLayout(); 
  private JTextField myOutput = new JTextField();

  public TextField1() {
    super("My TextField Example");
    setSize(WIDTH, HEIGHT); 
    setLayout(flow);
    JTextField myOutput = new JTextField(20);
    add(myOutput);
    setVisible(true); 
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
  }

  public void createGUI(){
    myOutput.setText("I am a text field showing output!");
    myOutput.setEditable(false);
  }

  public static void main(String[]args) {
 TextField1 myTextField = new TextField1(); 
 myTextField.createGUI();
 System.out.println(myTextField.myOutput.getText());
  }
}

It seems as if it does not register the createGUI() method. Can you explain why this is?

Answer 1

Inside the constructor, you are creating a new variable called myOutput , which is not the same as the instance variable declared above. JTextField myOutput should be myOutput

EDIT:

You declare a method-local variable myOutput and add it to your frame. createGUI() uses the instance variable declared at the top of the class, therefore it does not affect the TextField in the frame.

Answer 2

I am just extending the answer which was provided by blueygh2. The main problem is that :
you have not given the size of the JTextField in the class which will be 0 by default

private JTextField myOutput = new JTextField();//no size specified 

Also in constructor you are Declaring another JTextField with same name that will be eligible for Garbage collection after Object Creation :

JTextField myOutput = new JTextField(20);

Now there are 2 solutions :
1>Specify the size of JTextField in class definition like :

JTextField myOutput = new JTextField(20); // new object created for JTextField

2> Do not create a new JTextField Object in Constructor like :

myOutput = new JTextField(20);