静态，非静态以及在Java Tapestry中单独的类之间进行调用

Question

In my Java Tapestry application, I have a class called NachrichtenBubble.java which tests to see if there are any messages to announce, and therefor if a particular element should be shown. It looks like this:

public boolean ShowBubble() {
    int n = getHowManyNachrichten();
    if (n == 0) {
        return false;
    }
    return true;
}

Now, in a different place in my application, I want to determine whether to show or hide an element based on how many messages there are. Since this is already done in NachrichtenBubble.ShowBubble(), I'd like to be able to call ShowBubble() from my separate class (let's call it OtherClass.java) and act on the result.

If I put this into my OtherClass.java, I get an "Cannot make a static reference to the non-static method ShowBubble() from the type NachrichtenBubble" error:

public boolean ShowNachrichten() {
    boolean m = NachrichtenBubble.ShowBubble();
    return m;
}

I have read through the answers to other static/non-static method questions on here, which has been educational, but I have been unable to apply them successfully to this problem. I have tried to create a new Context method in NachrichtenBubble.java and called that from OtherClass.java, but it's not working.

Should I persist with attempting a Context-based solution, or is there something else that I could try to get a useable result from NachrichtenBubble.ShowBubble() into OtherClass.ShowNachrichten()?

Answer 1

Your call to NachrichtenBubble is static, in other words you don't have an instance of the class. The method ShowBubble is an instance method though. You could change the ShowBubble method to be static but only if the method it calls (getHowManyNachrichten()) also can be made static and doesn't use any instance fields, which seems unlikely considering the name.

To put it differently, without a specific NachrichtenBubble you (probably) don't know how many Nachrichten there are, so you can't ask for that information.

Alternatively, you could let your OtherClass know of this instance of NachrichtenBubble.

On another note, I would probably not re-use a method like that. Especially since you can replace the entire method by getHowManyNachrichten() > 0

Answer 2

This is completely unrelated to Tapestry, being instead a pure Java and Object-Oriented Programming question instead. You can either change ShowBubble() and getHowManyNachrichten() to be static methods or, better yet, in ShowNarichten, create a NarichtenBubble field named narichtenBubble and call narichtenBubble.showBubble() instead. By the way, your method names are so out of the Java conventions that it makes it hard to read and undestand it.