从单独方法中声明的 actionPerformed 访问数组

Question

Apologies for the noob question beforehand. Just trying to wrap my head around this.

My question revolves around the following code(summarized):

public class GUI extends JFrame implements ActionListener, MouseMotionListener {

  public static void main(String[] args) {
    GUI ig = new GUI();
  }

  GUI() {
    // code that sets several jpanels/buttons/labels and creates a GUI
    Object items[] = new Object[1000];
  }

  public void actionPerformed(ActionEvent e) {
    if (e.getSource() = X) {
      // I am trying to somehow call the array of object items here, but have had
      // little success.
    }
  }
}

It doesn't seem like I can have actionPerformed receive an array as a method because it is pre-defined as void in the library. There has to be a way to pass in the array into the method, I'm just at a loss on finding out how currently.

I know I can declare items right before main to make it visible to actionPerformed, but I'm wondering if there's a way to call it if its declared inside GUI constructor instead?

Answer 1

Variables exists only in the block they are declared in. So when you write

GUI()
{
    Object items[] = ...
}

you can use it only between these brackets. This is called "variable scope". You can move the declaration of this variable outside the constructor and make it a class field. This means any method and constructor in your class can access it (in respect to static vs non-static access). So in your case it might look like this:

public class GUI
{
    Object items[]; // declare it here, outside the constructor

    GUI()
    {
        // access "items" here
    }

    public void actionPerformed(ActionEvent e)
    {
        // also access "items" here
    }
}

So the steps you do are:

• Declare the variable as a field at the class level, not the constructor level.
• Initialize the field in your constructor with the values you want to set.
• Read/access the field in your actionPerformed() method as you want to.