./curric/oop/class

Classes and Access

In Java, mostly everything you’ll deal with are objects, meaning that their behavior is defined by a class. Generally, objects can have information unique to them, but what they can do is the same between all objects of the same class. For example, imagine a class Car (classes are always capitalized). Any Car object you have will be able to goVrooom() (methods are in camel case, and always followed by parentheses). However, some cars may look different than others; they may have a ifferent make, model, or year. You use the. operator to access methods and fields of classes. The following code snippet may illustrate this:

System.out.println(lukesCar.year);   // 87
System.out.println(lukesCar.make);   // "Oldsmobile"
System.out.println(lukesCar.model);  // "Cutlass Sierra"
lukesCar.goVrooom();   // vroooms, albeit louder than most 
                       // as it has no muffler
System.out.println(jaransCar.year);  // 92
System.out.println(jaransCar.make);  // "Jeep"
System.out.println(jaransCar.model); // "Wrangler"
jaransCar.goVrooom();  // *quiet vrooom*

Almost all code belongs to some specific class, which is a block as described above.

class Car {

}

What goes in a class, of course, depends on what the class is for. Following the Car example from above, this class would have the methods goVrooom() and detachMuffler() and three fields year, make, and model. These are the fields and methods that are exposed to code outside the class, so they should be marked public, as should the class itself.

public class Car {
    public int year;
    public String make;
    public String model;
    
    public void goVrooom() {
        // make a noise
    }
    
    public void detachMuffler() {
        // it can't be that hard to securely mount a muffler
    }
}

Of course, there must be some state stored within the class. How does the car know how loud to vrooom? People who aren’t in the know of lukesCar don’t have access to the information of whether it has a muffler unless they try to steal it or drive the car. So there has to be a field for this that is not public but rather private.

public class Car {
    public int year;
    public String make;
    public String model;
    
    private boolean hasMuffler;
    
    public void goVrooom() {
        if (hasMuffler) {
            // make a muffled vrooom
        } else { 
            // make a loud vrooom
        }  
     
    
    public void detachMuffler() {
        hasMuffler = false;
    }
}

This idea of using public methods to control the values of private methods is called encapsulation, and it is a very common practice in Object-Oriented Programming.