Java class and instance variables memory usage in a web application?

Answer 1

Keep a static private instance

class Foo {
    private static Foo myInstance = new Foo();

    public static void MyPretendInstanceMethod() {
        myInstance.doBar();
    }

    private void doBar() {
        // do stuff here
    }

}

Add synchronization as necessary.

Answer 2

class methods share the same memory location for all instances of the object

So do instance methods. The code for all methods of a class exists only once. The difference is that instance methods always require an instance (and its fields) for context.

If I were to create a class variable to keep track of errors, wouldn't that introduce a situation where process A could trigger an error in process B? Can instance methods even access class variables?

Yes and yes. This is why having non-final static fields is generally considered a bad thing.

Answer 3

Static methods can only access static class variables. They cannot access instance variables because static methods are not tied to any specific instance of the class.

You don't need to create a new instance of a class in order to access a static member or variable (as long as it's public). In your example, you can call methodB like this:

String bString = MyClass.methodB();

As you can see, no new instance had to be created.

But because methodA is not static, you must create an instance of the class in order to call it, like you did in your example:

MyClass myClass = new MyClass();
String aString = myClass.methodA();