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Methods, Constructors, Overloading and Access Levels

Lesson 3

Methods tell us what a class can do.Developers can define their own methods and Java Developers Kit has a variety of classes and each of them may contain methods. Some methods require arguments - the incoming data that have to be processed. For example, here is the code to convert a String value to a number of the type of integer:

String testStr = "5000"; int test = Integer.parseInt(testStr);

It is said that the method parseInt() has one argument of type String.

The method parseInt() is defined in the Java class Integer. Programmers could also define methods that take arguments, for example:

double calcTax(double grossIncome, String state,  
              int dependents){
 // The code implementing business logic goes here
}

Arguments are used in the method body the same way as if they were declared as local variables, but their values will be provided by the caller of the method, for example:

double myTax = calcTax(45000.00, "NJ", 2);

The 45000.00, "NJ", and 2 are provided by the caller of the method calcTax().

Special Methods: Constructors

One of the most widely used ways of creating an instance of a class is the operator new, for example:

Tax t = new Tax();

During the process of the class instantiation a special method called constructor will be called automatically. In our example parentheses after the word Tax mean that a constructor with no arguments will be called by the Java Virtual Machine (JVM).

Constructors have the following characteristics:

  • They are called only once when the class is being instantiated.
  • They must have the same name as the class itself.
  • They do not return a value and you do not have to specify the keyword void.

A class can have more than one constructor (see the section "Method Overloading" below).

If you do not create a constructor for the class, Java helps you by using a so called default no-argument constructor. That's why the Java compiler does not complain about the statement new Tax(), even though we have not defined any constructor for the class Tax.

Constructors are usually used to assign initial values to member variables of the class, for example:

class Tax {
  double grossIncome;   // member variables
  String state;
  int dependents;
   
  Tax (double gi, String st, int depen){
    grossIncome = gi;  // member variable initialization
    state = st;
    dependents=depen;
  }
  ...
}

If a constructor with arguments has been defined in a class, you can no longer use a default no-argument constructor - you have to write write one.

Besides the operator new, you can create an instance of the class by loading the class first and then calling the method newInstance() which will also executes constructor's code:

Class.forName("Tax").newInstance();

The Keyword this

The keyword this is useful when you need to refer to an instance of the class from its method. Let's consider an example:
class Tax {
   double grossIncome;
   Tax(double grossIncome){
     this.grossIncome = grossIncome;
   }
}

The keyword this helps avoid name conflicts, for example this.grossIncome refers to a member variable grossIncome, while the grossIncome on the right refers to the argument's value.

Let's look at another example - say you have some class with a method verifyTax( Tax t) that needs an instance of class Tax as an argument. This is how you can call it from the class Tax:

class Tax {
  void myMethod(){
    ...
    SomeOtherClass s = new SomeOtherClass();
    s.verifyTax(this);
  }
}

Method Overloading

If a class has more than one method with the same name, but with different argument lists, it's called method overloading. For example, the method print() could be called with different types of arguments. Actually there are multiple overloaded versions of the method print(), it's just easier to remember one method print(), than printString(), printInt(), etc.

Constructors also can be overloaded.

The next version of our class Tax will have two overloaded constructors: one with 3 arguments (income, state and dependents), and another one with 2 (income and state). If the 2-argument constructor will be used when the class is instantiated, the default value of 1 dependent is assumed.

class Tax {
  double grossIncome;
  String state;
  int dependents;
   
  Tax (double gi, String st, int depen){
    grossIncome = gi;
    state = st;
    dependents=depen;
  }

  Tax (double gi, String st){
    grossIncome = gi;
    state  = st;
    dependents=1;   // Default value 
  }
  ...
}

Only one constructor will be used when a class is instantiated, based on the provided argument list. The example below uses the 3-arguments constructor:

Tax t = new Tax( 50000.00, "NJ", 2);

The following example uses 2-argument constructor:

Tax t = new Tax( 50000.00, "NJ");

The keyword super

The keyword super is used to refer to the superclass of an object, for example it could be used to call a constructor of the superclass:
class  NJTax extends Tax{
   NJTax(double income, String state){
    super(income, state, 1);
    ...
  }
}

The keyword super could also be used to call any method in the superclass. You do not usually use it, because methods of the superclass are available just by specifying their names. Having an overridden method could come in handy - one in the current class and the other one in the superclass. If for any reason you want to call a particular method from a superclass, even though there is a method with the same signature in the subclass, do it as follows:

super.myMethod();

Access Levels

Java classes, methods and member variables could have public, private, protected and package access levels, for example:
public class Tax {
   private double grossIncome; 
   private String state;
   private int dependents;
   protected double calcTax(…) {…}
}

The keyword public means that this element (class, method or a variable) could be accessed from any other Java class.

The keyword protected makes the element "visible" either from the current class, or from it's subclasses.

The keyword private is the most restrictive one and makes the member variable or a method available only within the current class. For example, our class Tax may need some additional internal method that could be called from the method calcTax(). Descendents of the class Tax do not need to know about it and this method should be declared as private.

If you do not specify any access level, a default (package) access level is used.

One of the main features of object-oriented languages is encapsulation, which is an ability to hide and protect its elements. The classes should expose to their possible users only the necessary methods, i.e. alcTax(). The methods that the class Tax exposes to its prospective clients could be called Application Program Interface (API).

Do you really know what exactly happens under the hood when you start you car's engine? Do you really want to know? That ignition key slot and the "Check oil" signal are example of your car's API.

If you are not sure which access level to give to methods or variables, just make them all private. If at the later development stage another class needs to access them, you can always change it, but this is a simple way to protect all the internal of your application from misuse. Think of it this way: "I want to sell my class Tax to various accounting firms across the country. If software developers of these firms will incorporate this class into existing systems - what's the minimum number of methods that they must know about to be able to calculate a tax"? If car designers would not ask themselves a similar question, you'd need to press hundreds of buttons just to start the engine.

More Stories By Yakov Fain

Yakov Fain is a co-founder of two software companies: Farata Systems and SuranceBay. He authored several technical books and lots of articles on software development. Yakov is Java Champion (https://java-champions.java.net). He leads leads Princeton Java Users Group. Two of Yakov's books will go in print this year: "Enterprise Web Development" (O'Reilly) and "Java For Kids" (No Starch Press).

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