5.1.1 Accessing Members

Inside of a class, we can access instance variables and call instance methods of the class directly by name. Here's an example that expands upon our Pendulum:

class Pendulum { 
    ... 
    void resetEverything() { 
        cycles = 0; 
        mass = 1.0; 
        ... 
        float startingPosition = position( 0.0 ); 
    } 
    ... 
} 

Other classes access members of an object through a reference, using the C-style dot notation:

class TextBook { 
    ... 
    void showPendulum() { 
        Pendulum bob = new Pendulum(); 
        ... 
        int i = bob.cycles; 
        bob.resetEverything(); 
        bob.mass = 1.01; 
        ... 
    } 
    ... 
} 

Here we have created a second class, TextBook, that uses a Pendulum object. It creates an instance in showPendulum() and then invokes methods and accesses variables of the object through the reference bob.

Several factors affect whether class members can be accessed from outside the class. You can use the visibility modifiers, public, private, and protected to restrict access; classes can also be placed into packages that affect their scope. The private modifier, for example, designates a variable or method for use only by other members inside the class itself. In the previous example, we could change the declaration of our variable cycles to private:

class Pendulum { 
    ... 
    private int cycles; 
    ... 

Now we can't access cycles from TextBook:

class TextBook { 
    ... 
    void showPendulum() { 
        ... 
        int i = bob.cycles;         // Compile time error 

If we need to access cycles, we might add a getCycles() method to the Pendulum class. We'll look at access modifiers and how they affect the scope of variables and methods in detail later.

5.1.2 Static Members

Instance variables and methods are associated with and accessed through a particular object. In contrast, members that are declared with the static modifier live in the class and are shared by all instances of the class. Variables declared with the static modifier are called static variables or class variables; similarly, these kinds of methods are called static methods or class methods.

We can add a static variable to our Pendulum example:

class Pendulum { 
    ... 
    static float gravAccel = 9.80; 
    ... 

We have declared the new float variable gravAccel as static. That means if we change its value in any instance of a Pendulum, the value changes for all Pendulum objects, as shown in Figure 5.2.

Figure 5.2: Static variables shared by all instances of a class

Static members can be accessed like instance members. Inside our Pendulum class, we can refer to gravAccel by name, like an instance variable:

class Pendulum { 
    ... 
    float getWeight () { 
        return mass * gravAccel; 
    } 
    ... 
} 

However, since static members exist in the class itself, independent of any instance, we can also access them directly through the class. We don't need a Pendulum object to set the variable gravAccel; instead we can use the class name as a reference:

Pendulum.gravAccel = 8.76; 

This changes the value of gravAccel for any current or future instances. Why would we want to change the value of gravAccel? Well, perhaps we want to explore how pendulums would work on different planets. Static variables are also very useful for other kinds of data shared among classes at run-time. For instance, you can create methods to register your objects so that they can communicate, or you can count references to them.

We can use static variables to define constant values. In this case, we use the static modifier along with the final modifier. So, if we cared only about pendulums under the influence of the Earth's gravitational pull, we could change Pendulum as follows:

class Pendulum { 
    ... 
    static final float EARTH_G = 9.80; 
    ... 

We have followed a common convention and named our constant with capital letters; C programmers should recognize the capitalization convention, which resembles C #define statements. Now the value of EARTH_G is a constant; it can be accessed by any instance of Pendulum (or anywhere, for that matter), but its value can't be changed at run-time.

It's important to use the combination of static and final only for things that are really constant. That's because the compiler is allowed to "inline" static and final values within classes that reference them. This is probably okay for constants like PI, which aren't likely to change for a while, but may not be ideal for other kinds of identifiers.

Static members are useful as flags and identifiers, which can be accessed from anywhere. They are especially useful for values needed in the construction of an instance itself. In our example, we might declare a number of static values to represent various kinds of Pendulum objects:

class Pendulum { 
    ... 
    static int SIMPLE = 0, ONE_SPRING = 1, TWO_SPRING = 2; 
    ... 

We might then use these flags in a method that sets the type of a Pendulum or, more likely, in a special constructor, as we'll discuss shortly:

Pendulum pendy = new Pendulum(); 
pendy.setType( Pendulum.ONE_SPRING ); 

Remember, inside the Pendulum class, we can use static members directly by name as well:

class Pendulum { 
    ... 
    void resetEverything() { 
        setType ( SIMPLE ); 
        ... 
    } 
    ... 
}