在另一个类中调用一个方法

Question

I have a class called Ball, and i want to call a method called update in a class called MagicBallImage. Below is the method isVisible() in the Ball class, from which I am trying to call the method update, but I am getting an error saying:

Ball.java:58: non-static method update() cannot be referenced from a static context. 
MagicBallImage.update();

Ball Class

public boolean isVisible()
{
  if (magicBallState != 1)
  {
    return true;
    MagicBallImage.update();
  }
}

Anyone know how to solve this?

Answer 1

It's telling you that you're trying to call a non-static method without a class instance. You either need an instance of MagicBallImage to call the method on, or you need to convert the method to static .

Answer 2

 if (magicBallState != 1)
  {
    return true;
    MagicBallImage.update();
  }

1. you're returning before you call update, so it will have no effect

2. update() is not a static method , so you can't call it from a static context. You can simply call update() or this.update() if you're already in object scope, or call o.update() where o is your object.

Answer 3

This error tells you that you cannot call update without specifying an instance of MagicBallImage .

Two things could have happened:

• You have forgotten to make the MagicBallImage.update() method static , or
• You need to make an instance of MagicBallImage that you created somewhere using the new operator available to your isVisible() method.

Note 1: The way it is coded now, MagicBallImage.update() is not accessible, because it comes after the return statement.

Note 2: It is very undesirable for a getter isVisible to have side effects, such as updating something in an instance of another class. It is a nearly 100% indication that something is wrong with your design.

Answer 4

If you want to call this method like this, the method has to be declared static . If it's not you have to declare an instance of MagicBallImage to access update() :

MagicBallImage mbi = new MagicBallImage();
mbi.update();

Answer 5

使update（）方法为静态，或创建MagicBallImage的实例并调用update（）