Confusion about static methods in Java

Question

As I understand static, it is that static methods can be called without an instance of the object needing to exist. So instead of making an object and calling the method on that object, you can just call the method on the class.

Now, I have a class Main that has the following object: public ScribbleCanvas myCanvas;. In the ScribbleCanvas class I would like to access a method of the Main-class.

Now, since there is already an instance of Main (since this called the ScribbleCanvas), how can I access a non-static method of this class? Or perhaps the better question - where is the error in my reasoning?

Are you sure there is already an instance of the Main class? Where is your code? What have you tried? — JB Nizet
– JB Nizet, Commented Dec 22, 2013 at 13:18
could you give more details? why do you need to call a method of Main from that class? — Alessandro Roaro
– Alessandro Roaro, Commented Dec 22, 2013 at 13:18
ScribbleCanvas is the name of the class. What does that have to do with the existence of an instance? — Marko Topolnik
– Marko Topolnik, Commented Dec 22, 2013 at 13:19
can you please show some code? if the function that used ScribbleCanvas can the Main class instance, it can pass the instance to the ScribbleCanvas . if not, you can make the function "public static" , and then it will be available for every function/class. — android developer
– android developer, Commented Dec 22, 2013 at 13:22

Georgian · Accepted Answer · 2013-12-22 13:22:42Z

1

You could have a constructor or a setter for the ScribbleCanvas that takes a parameter as the current instance of Main.

ScribbleCanvas sc = new ScribbleCanvas(this);

or

sc.setMainClass(this);

And with those, you just reference a field to the parameter.

answered Dec 22, 2013 at 13:22

Georgian

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BobTheBuilder · Accepted Answer · 2013-12-22 13:16:06Z

1

You can set Main instance as a member of myCanvas and use it.

answered Dec 22, 2013 at 13:16

BobTheBuilder

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Keerthivasan · Accepted Answer · 2013-12-22 13:22:57Z

1

The below code explains how you should do it. The testInstanceMethod is been taken as an example for an instance method in Main class. This method should be accessible as well

public class ScribbleCanvas{

private Main mainObject = null;


public ScribbleCanvas(){

this.mainObject = new Main();
//Call instance method in mainObject (member instance)
this.mainObject.testInstanceMethod();
}

public void setMainObject(Main arg){
this.mainObject = arg;
}

public Main getMainObject(){
return this.mainObject;
}

}

For invoking static methods, you can directly put the Classname and invoke using DOT operator like Main.testStaticMethod() provided the method is accessible as well

Disclaimer : NOT tested / compiled

answered Dec 22, 2013 at 13:22

Keerthivasan

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Auspice · Accepted Answer · 2013-12-22 13:38:01Z

1

If I understand your question you want to:

Have an instance of the Main class, let's call it myMain.
The instance will have an instance of the ScribbleCanvas class, called myCanvas
From the ScribbleCanvas instance (myCanvas) have access to methods within myMain.

In order to do this you can:

Declare a member of type Main within ScribbleCanvas, say callingMain
Include a parameter of type Main (say paramMain) in the constructor for ScribbleCanvas
In the constructor, store paramMain in callingMain
From Main, pass in this to the constructor
Within your code, you can refer to callingMain.method()

Does this help?

answered Dec 22, 2013 at 13:38

Auspice

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