Question

In my class B, I want to be able to use methods from my class A. This is what I have tried.

1. Imported class A
2. initialized it – classA a = new classA(null, null);

3. The added to my methods where I want to use a class A method

   public void getAMethod() {
   a.getTestValue;
   }

My questions are:

1. Is this the correct way to do it?
2. Is every time I execute class B, is it always creating a new instance of class A?
3. If class A is opened and I run class B, does class B get the current values of variables in class A, or does it get the class A defaults because of new classA(null, null)?

My issues are:

1. I am getting 2 different values for the same variable.

Example: Class A –

initialize = boolean test = false

When class A dialog opens , set test = true

Create method

public boolean getTestValue() {
   return test;
}

From within class A when I run getTestValue, it is always true
From class B when getTestValue is called, it always returns false (even with the class A dialog open)

It seems like that class b is making a new instance of class A every time, so it only sees the default values of class A. When class B calls class A, I need to get the current values of class A, not the defaults

EDIT
I changed the method in Class A to static and it fixed the issue I was having. This was a big help to me – Thanks to all for the suggestions!

Answer 1

Okay, there’s a fair amount of ground to cover, but I’ll try to be brief and answer each question in turn.

1. Is this the correct way to do it?

You don’t typically need to import classes in a small program. If ClassA and ClassB are inside the same package, or they both do not have a package assigned, but reside in the same directory, then no importing is required.

As far as wrapping ClassA methods in ClassB methods, no, no. You can access a ClassA static method from within ClassB at any time by doing;

ClassA.myMethodInClassA();

If myMethodInClassA(); is not static, then you need to call it on an instance of ClassA so, (still in ClassB) you would do;

ClassA myA = new ClassA();
myA.myMethodInClassA();

Which brings me on to static vs non static fields. If your fields initialize and test are marked as static, such as;

public static boolean initialize = false;

Then they will always have the same value across all instances of ClassA and can be access from anywhere with;

if(ClassA.initialize) {
    //something
}

If they are not marked as static, then again (much like the non-static methods) you need an instance of ClassA with which to query the fields such as;

ClassA myA = new ClassA();
if(myA.initialize) {
    //something
}

And in this case, each instance of ClassA has its own copy of initialize, so the value of initialize depends on which instance you are querying.

2. Is every time I execute class B, is it always creating a new instance of class A?

If you call a constructor for ClassA inside the constructor for ClassB then the answer is yes, in other words if you do;

public ClassB() {
    //constructor for ClassB
    ClassA c = new ClassA();
}

Then yes, you will create a new ClassA every time you create a new ClassB.

3. If class A is opened and I run class B, does class B get the current values of variables in class A, or does it get the class A defaults because of new classA(null, null)?

If by ‘opened’ you mean you ran java ClassA and then ran java ClassB in your command prompt, then what you have there is two separate programs, two instances of the JVM running in parallel. These two programs have nothing to do with one another, and cannot communicate with each other in any way unless you specifically implement something.

If by ‘opened’ you mean constructed, such as new ClassA(); then we’re talking about class vs instance members again.

I ranted on about static vs non-static fields and methods in my answer to question 1, which covers this ground. Overall I would advise you to read some docs, such as:

http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

TLDR: Go read this link ^^