How can I append text in JTextArea from another class

[chikaman]

i have two classes A and B, i defined a TextArea myTextArea in A and try to append text to it from B but it is not showing anything in the textArea. below i have some parts of the program. Some one help me please.
Class A

Class A extends javax.swing.JFrame{
private JTextArea myTextArea;
A( ){
myTextArea = new JtextArea;
}
public void setArea(String mess){
myTextArea.Append(mess);
}
}

Class B
Class B{
A newA;
B( ){
newA = new A;
}
//i have this line below which is supposed to append text in //myTextArea in class A
newA.setArea(“message”);
}

[PhHein]

newA references a different instance of A. You need a reference to the instance of A that is shown on screen.

[chikaman]

how can i reference the instance of A shown on screen

[PhHein]

Either you pass the reference to B or you show the one in B.

[chikaman]

could you please show me a code example

[LittleRave]

Either write a constructor like this:

B(A newA) {
this.newA = newA;
}

But this is kind of stupid to call and always put the A class into the constructor.

Or do it the elegant way with a singleton and write your classes like that:

Class A extends javax.swing.JFrame{
private static final A INSTANCE = null;

public static void A getInstance() {
if (INSTANCE == null) {
INSTANCE = new A();
}
return A;
}

private A() {
myTextArea = new JtextArea;
}

public void setArea(String mess){
myTextArea.Append(mess);
}
}

Class B
Class B{
A newA;
B( ){
newA = A.getInstance();
}
//i have this line below which is supposed to append text in //myTextArea in class A
newA.setArea(“message”);
}

With this solution you always get the A class you entered the text into no matter where in your program you call the A.getInstance().
But only use this if you need one object of the A class in the whole program, you CANNOT create a second one.

[PhHein]

public class C{
 A a = new A();
 B b = null;
 public C(){
   b = new B(a);
   a.setVisible(true);
 }
}


public class B{
 A newA;
 B(A a){
  newA =a;
 }
 public void doSomething(){
  newA.setArea("BlaBla")
 }

}

[PhHein]

@LittleRave: that is far from elegant!

Being Scott Densmore : Why Singletons are Evil
Singletons are Evil – Renaissance Developer

[chikaman]

thank you very much. i will just implement it immediately

[chikaman]

i have my main method in A, so do have to instanciate C in A

[PhHein]

No, if your main is in A it should look like this:

public class A{
   B b = null;
   public static void main(String agrs[]){
    A a = new A();
    b = new B(a);
    a.setVisible(true);
   }
}

B stays as it was in my last post.

[chikaman]

thanks to all of u. you guys are great. i just did it using PhHein's method

[PhHein]

Cool, so did you understand what was going wrong when you first tried?

[LittleRave]

lol ... It depends on what you're trying to achieve and how you use it.
I recently wrote a multiplayer game with some of my fellow-students and I needed about 6 classes only for the keyboard input, but since every class was connected to another i had to pass around lots of ugly data in every single constructor, plus I couldn't get a single interface, I had to create the classes hiding behind the interface somewhere outside and pass it inside. The singleton helped with all that and since there's NEVER a second keyboard instance used, it was a lot more elegant than the normal way with passing the objects around.

But you're right, in this case it would be better to use the normal way with a constructor passing the reference. Especially since its only two classes with the main in one of it. Since our softwareproject is finished I always tend to think way too complex *lol*.