Instance and Class Members

When you declare a member variable such as aFloat in MyClass:
class MyClass {
    float aFloat;
}
you declare an instance variable. Every time you create an instance of a class, the runtime system creates one copy of each the class's instance variables for the instance. You can access an object's instance variables from an object as described in Using Objects.
Instance variables are in constrast to class variables (which you declare using the static modifier). The runtime system allocates class variables once per class regardless of the number of instances created of that class. The system allocates memory for class variables the first time it encounters the class. All instances share the same copy of the class's class variables. You can access class variables through an instance or through the class itself.
Methods are similar: your classes can have instance methods and class methods. Instance methods operate on the current object's instance variables but also have access to the class variables. Class methods, on the other hand, cannot access the instance variables declared within the class (unless it creates a new object and accesses them through the object). Also, class methods can be invoked on the class, you don't need an instance to call a class method.
By default, unless otherwise specified, a member declared within a class is an instance member. The class defined below has one instance variable--an integer named x--and two instance methods--x() and setX()--that let other objects set and query the value of x:
class AnIntegerNamedX {
    int x;
    public int x() {
        return x;
    }
    public void setX(int newX) {
        x = newX;
    }
}
Every time you instantiate a new object from a class, you get a new copy of each of the class's instance variables. These copies are associated with the new object. So, every time you instantiate a new AnIntegerNamedX object from the class, you get a new copy of x that is associated with the new AnIntegerNamedX object.
All instances of a class share the same implementation of an instance method; all instances of AnIntegerNamedX share the same implementation of x() and setX(). Note that both methods, x() and setX(), refer to the object's instance variable x by name. "But", you ask, "if all instances of AnIntegerNamedX share the same implementation of x() and setX() isn't this ambiguous?" The answer is no. Within an instance method, the name of an instance variable refers to the current object's instance variable (assuming that the instance variable isn't hidden by a method parameter). So, within x() and setX(), x is equivalent to this.x.
Objects outside of AnIntegerNamedX that wish to access x must do so through a particular instance of AnIntegerNamedX. Suppose that this code snippet was in another object's method. It creates two different objects of type AnIntegerNamedX, sets their x values to different values, then displays them:
. . .
AnIntegerNamedX myX = new AnIntegerNamedX();
AnIntegerNamedX anotherX = new AnIntegerNamedX();
myX.setX(1);
anotherX.x = 2;
System.out.println("myX.x = " + myX.x());
System.out.println("anotherX.x = " + anotherX.x());
. . .
Notice that the code used setX() to set the x value for myX but just assigned a value to anotherX.x directly. Either way, the code is manipulating two different copies of x: the one contained in the myX object and the one contained in the anotherX object. The output produced by this code snippet is:
myX.x = 1
anotherX.x = 2
showing that each instance of the class AnIntegerNamedX has its own copy of the instance variable x and each x has a different value.
You can, when declaring a member variable, specify that the variable is a class rather than an instance variable. Similarly, you can specify that a method is a class method rather than an instance method. The system creates a single copy of a class variable the first time it encounters the class in which the variable is defined. All instances of that class share the same copy of the class variable. Class methods can only operate on class variables--they cannot access the instance variables defined in the class.
To specify that a member variable is a class variable, use the static keyword. For example, let's change the AnIntegerNamedX class such that its x variable is now a class variable:
class AnIntegerNamedX {
    static int x;
    public int x() {
        return x;
    }
    public void setX(int newX) {
        x = newX;
    }
}
Now the exact same code snippet from before that creates two instances of AnIntegerNamedX, sets their x values, and then displays them produces this, different, output.
myX.x = 2
anotherX.x = 2
The output is different because x is now a class variable so there is only one copy of the variable and it is shared by all instances of AnIntegerNamedX including myX and anotherX. When you invoke setX() on either instance, you change the value of x for all instances of AnIntegerNamedX.
You use class variables for items that you need only one copy of and which must be accessible by all objects inheriting from the class in which the variable is declared. For example, class variables are often used with final to define constants (this is more memory efficient as constants can't change so you really only need one copy).
Similarly, when declaring a method, you can specify that method to be a class method rather than an instance method. Class methods can only operate on class variables and cannot access the instance variables defined in the class.
To specify that a method is a class method, use the static keyword in the method declaration. Let's change the AnIntegerNamedX class such that its member variable x is once again an instance variable, and its two methods are now class methods:
class AnIntegerNamedX {
    private int x;
    static public int x() {
        return x;
    }
    static public void setX(int newX) {
        x = newX;
    }
}
When you try to compile this version of AnIntegerNamedX, you will get compiler errors:
AnIntegerNamedX.java:4: Can't make a static reference to nonstatic variable x in class AnIntegerNamedX.
        return x;
               ^ 
AnIntegerNamedX.java:7: Can't make a static reference to nonstatic variable x in class AnIntegerNamedX.
        x = newX;
        ^
2 errors
This is because class methods cannot access instance variables unless the method created an instance of AnIntegerNamedX first and accessed the variable through it.
Let's fix AnIntegerNamedX by making its x variable a class variable:
class AnIntegerNamedX {
    static private int x;
    static public int x() {
        return x;
    }
    static public void setX(int newX) {
        x = newX;
    }
}
Now the class will compile and the same code snippet from before that creates two instances of AnIntegerNamedX, sets their x values, and then prints the x values produces this output:
myX.x = 2
anotherX.x = 2
Again, changing x through myX also changes it for other instances of AnIntegerNamedX.
Another difference between instance members and class members is that class members are accessible from the class itself. You don't need to instantiate a class to access its class members. Let's rewrite the code snippet from before to access x() and setX() directly from the AnIntegerNamedX class:
. . .
AnIntegerNamedX.setX(1);
System.out.println("AnIntegerNamedX.x = " + AnIntegerNamedX.x());
. . .
Notice that you no longer have to create myX and anotherX. You can set x and retrieve x directly from the AnIntegerNamedX class. You cannot do this with instance members, you can only invoke instance methods from an object and can only access instance variables from an object. You can access class variables and methods either from an instance of the class or from the class itself.

Objects, Classes, and Interfaces