Thread: scope - quick question

hi all,

i am looking at some very simple beginner swing applications and i have a quick question. in the examples given, certain components are created within the constructor of the class:

class MyClass{
...
 
MyClass(){
JButton but = new JButton("Test");
...
}
}

It looks like the scope of the but reference variable is limited to the constructor. After the MyClass object is created and the constructor has finished executing, there is nothing left pointing to the JButton object, and it is subject to garbage collection. So it seems that the resources allocated by the JButton object would be freed and the button wouldn't work anymore?

Am I missing something here?

I guess another way of thinking about the same question is: what happens when you add the button to the content pane? Is another reference to the JButton object created somewhere?

when you add the button to the content pane? Is another reference to the JButton object created

Yes, when you add a component to a container it has a reference to the component. You can have many references to the same object.

Hmm, I guess I'm just confused as to why you wouldn't make it like this:

class MyClass{
JButton but;
...
 
MyClass(){
but = new JButton("Test");
...
}
}

What is the advantage to making the reference variables local to the constructor?

Originally Posted by bbr201

What is the advantage to making the reference variables local to the constructor?

If you don't need a class member variable reference, you shouldn't provide one. So you only need the class variable if you have to reference it from outside the scope where it is created. For a JButton, you would probably create it, then add a listener to it, then add it to a JPanel. You might not need to directly access it again, so you might not need a class-scope reference to it. The JPanel and the listener would have their own references to it, so it would not be garbage collected.

If it turns out that you do need to access the JButton (e.g. to change the text, or disable it), you can then move the variable declaration to class scope so it becomes accessible in other methods.

In general, try to keep variable declarations as close to their point of use as possible and don't extend their scope unnecessarily.