Thread: Trouble clearing JTextField, getting NullPointerException

Originally Posted by Lanst83

...JDK 1.7...I can not get this field to clear...

public class TheEvent implements ActionListener, Runnable { 
 
    TheLayout gui;
    TheData data;
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    public void actionPerformed(ActionEvent event) {
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            data.setDownInt();  //<---data???
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            gui.displayDown.setText("  ");  //<---gui???       
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    }
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	   }

Your TheEvent class has instance variables gui and data that are never initialized.


One possibility that uses the organization you have set up: Make a constructor for TheEvent that takes two parameters (with types TheLayout and TheData), from which it sets values for the two instance variables.

Then in TheLayout, where you create your TheEvent object, give its constructor the arguments that it needs.


Cheers!

Z

Thank you. Now the TextField clears but it is clearing in a new window. What have I done wrong?

 public void changeText() {
 
	TheLayout gui = new  TheLayout();
	gui.setVisible(false); // I have set this to false trying to keep it from making a new window,
// but the old window still holds text
	 gui.displayDown.setText("");		
 
	}
 
 
	   public void actionPerformed(ActionEvent event) {
			 String command = event.getActionCommand();
		      if ( command == "PASS"){ 
		    	run();
 
		    	TheData data = new TheData();
		    	changeText();
		    	data.setDownInt();
 
 
 
		      }
 
		}

  TheLayout gui = new  TheLayout();

Why create a new TheLayout object? You need to get a reference to the current one that is visible so you can make changes to it. The posted code creates a new instance and makes changes it it, not to the currently viewed one.

Where is there a reference to the currently visible instance of TheLayout? How can a copy of that reference be passed to the class or method that needs to use it?

One problem I see is the code is using == to compare Strings. It should use the equals() method.

Originally Posted by Lanst83

...new window...wrong

Originally Posted by Lanst83

...I've got some thing backwards...

I will try again. You can take my word for it or not, but I'm telling you that I wouldn't make suggestions like these unless I had tested them with your code.

Maybe you can go back to your original code and try the two changes that I suggested.

I hate to repeat myself, but:

Originally Posted by Zaphod_b

Make a constructor for TheEvent that takes two parameters (with types TheLayout and TheData), from which it sets values for the two instance variables.

So, in the TheEvent.java, make the first part of TheEvent class look something like

public class TheEvent implements ActionListener, Runnable {
    TheLayout gui;
    TheData data;
 
    // Constructor sets values of gui and data to parameter values
    // passed by the calling function
    TheEvent(TheLayout g, TheData d) {
        /* Your code goes here.  It does two things */
        //Set gui  equal to the first parameter value.
        //Set data equal to the second parameter value
    }
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. // The rest of your code
.

Next: I hate to repeat myself (again) but

Originally Posted by Zaphod_b

Then in TheLayout, where you create your TheEvent object, give its constructor the arguments that it needs.

So, in TheLayout.java, the class definition could begin with something like

public class TheLayout extends JFrame {
    private static final long serialVersionUID = 1L;
 
    TheData data = new TheData();
    // Use arguments this and data to the constructor so that
    // the event object will be working on things that are
    // defined here.
    // First argument will be a reference to an object of
    // type TheLayout:  this
    // Second argument will be a reference to an object of
    // type TheData:    data
    //                 (or this.data if you like to type extra stuff)
    /* Your code for instantiating the event object goes here */
    TheEvent event = new TheEvent(First argument, Second argument);
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. // The rest of your code
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For now, leave TheData.java just as you originally posted it.


I am hopeful that these two changes can get you far enough along to start debugging.

I mean, you could re-organize the whole thing, and maybe you will decide to do things differently once you get into the real program development, but it could work OK the way you started. Having different classes for layout, event handling and data manipulation doesn't seem (to me) to be a bad idea at all.


Cheers!

Z