Text Fields, Labels and Buttons

Question

I am having some difficulty understanding GUIs and why my program won\'t run properly. Is it because I have to extend to a JFrame class? Here is a code:

import j         


        

Answer 1

Issue 1 :

 frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
 frame.setTitle("Richter Scale");
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 frame.setVisible(true); 

 JFrame frame = new JFrame();

You are trying to use an object that is not yet created. frame object is created at a later point in time. It should be as below :

 JFrame frame = new JFrame();
 frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
 frame.setTitle("Richter Scale");
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 frame.setVisible(true); 

Issue 2:

Same issue further down the code. Brother, first you need to declare an object, then only you can use it and implement functions on it. If you are not declaring the object first, how would compiler parse it and treat it as a valid variable.

 panel.add(button);
 panel.add(label);
 panel.add(rictherfield);
 panel.add(rictherlabel);
 add(panel);

 JLabel rictherlabel = new JLabel ("Ricther: ");

 final int FIELD_WIDTH = 10;
 JTextField rictherField = new JTextField(FIELD_WIDTH);
 richterField.setText("" + EARTHQUAKE_RATE);


 JButton button = new JButton("Enter");

'button', 'richterlabel' & 'richterField' are used before their declaration. As compiler does not know what dese variables mean at the time of execution, it throws up saying cannot find symbol.

First declare them and then use them. Hope you got the point.

Issue 3:

import java.awt.event.ActionListner;

ActionListner is not a class in the Event package. It should be :

import java.awt.event.ActionListener;

Looking for other issues, Meanwhile start working on this.

Answer 2

Wow... OK, first, ActionListener is spelled incorrectly as "ActionListner." Look very closely at the spelling of those words. Simple typographical errors generate syntax errors.

The rest of your problems boil down to this very simple caveat: order matters. Your order should be as follows:

1) declare and create objects; 2) declare and create all dependent objects; 3) configure objects; 4) manipulate objects.

That means that your code:

frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("Richter Scale");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);

JFrame frame = new JFrame(); 

Won't work because you're trying to mess with a frame that hasn't yet been created. Create it first, like so:

JFrame frame = new JFrame();

frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
frame.setTitle("Richter Scale");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);

Same holds true for your label and your button.

JLabel rictherlabel = new JLabel ("Ricther: ");

needs to come before

panel.add(label);