从JPanel类调用JFrame上的函数

Question

I have a login JPanel which was programmatically added inside the JFrame . The problem is I want to call a function on a JFrame from the login JPanel class using a button.

Here, I've declared the MainFrame as mainFrame in the PartLogin JPanel class:

public class PartLogin extends JPanel {

    MainFrame mainFrame;

Then, I've created a public function from the JFrame , then called it from PartLogin through a button click

mainFrame.changeLabel();

I've got an NullPointerException pointing inside the buttonclick code.

mainFrame.changeLabel();

Answer 1

A NullPointerException normally says that an object that is needed to be not null is null. Here, this is probably mainFrame . Java is not C++, so the statement is equal to:

MainFrame mainFrame = null;

You should change this so that mainFrame is not null. There are two ways:

1. assign value in declaration:

    MainFrame mainFrame = new MainFrame(); 

2. assign value in constructor:

    private MainFrame mainFrame; // don't must be private public PartLogin () { super(); // call ctor of superclass mainFrame = new MainFrame(); // ... }