Understanding a Java Program

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This article presents a small Java program which can be run from the console. It computes the distance between two points on a plane. You do not need to understand the structure and meaning of the program just yet; we will get to that soon. Also, because the program is intended as a simple introduction, it has some room for improvement, and later in the module we will show some of these improvements. But let's not get too far ahead of ourselves!

The Distance Class: Intent, Source, and UseEdit

This class is named Distance, so using your favorite editor or Java IDE, first create a file named Distance.java, then copy the source below, paste it into the file and save the file.

  Code listing 2.1: Distance.java
 1  public class Distance {
 2    private java.awt.Point point0, point1;
 4    public Distance(int x0, int y0, int x1, int y1) {
 5      point0 = new java.awt.Point(x0, y0);
 6      point1 = new java.awt.Point(x1, y1);
 7    }
 9    public void printDistance() {
10      System.out.println("Distance between " + point0 + " and " + point1
11                      + " is " + point0.distance(point1));
12    }
14    public static void main(String[] args) {
15      Distance dist = new Distance(
16               intValue(args[0]), intValue(args[1]),
17               intValue(args[2]), intValue(args[3]));
18      dist.printDistance();
19    }
21    private static int intValue(String data) {
22      return Integer.parseInt(data);
23    }
24  }

At this point, you may wish to review the source to see how much you might be able to understand. While perhaps not being the most literate of programming languages, someone with understanding of other procedural languages such as C, or other object oriented languages such as C++ or C#, will be able to understand most if not all of the sample program.

Once you save the file, compile the program:

  Compilation command
$ javac Distance.java

(If the javac command fails, review the installation instructions.)

To run the program, you supply it with the x and y coordinates of two points on a plane separated by a space. For this version of Distance, only integer points are supported. The command sequence is java Distance <x0> <y0> <x1> <y1> to compute the distance between the points (x0, y0) and (x1, y1).

  If you get a java.lang.NumberFormatException exception, some arguments are not a number. If you get a java.lang.ArrayIndexOutOfBoundsException exception, you did not provide enough numbers.

Here are two examples:

  Output for the distance between the points (0, 3) and (4, 0)
$ java Distance 0 3 4 0
Distance between java.awt.Point[x=0,y=3] and java.awt.Point[x=4,y=0] is 5.0
  Output for the distance between the points (-4, 5) and (11, 19)
$ java Distance -4 5 11 19
Distance between java.awt.Point[x=-4,y=5] and java.awt.Point[x=11,y=19] is 20.518284528683193

We'll explain this strange looking output, and also show how to improve it, later.

Detailed Program Structure and OverviewEdit

As promised, we will now provide a detailed description of this Java program. We will discuss the syntax and structure of the program and the meaning of that structure.

Introduction to Java SyntaxEdit

public class Distance {
   private java.awt.Point point0, point1;

   public Distance(int x0, int y0, int x1, int y1) {
     point0 = new java.awt.Point(x0, y0);
     point1 = new java.awt.Point(x1, y1);

   public void printDistance() {
     System.out.println("Distance between " + point0 + " and " + point1
                     + " is " + point0.distance(point1));

   public static void main(String[] args) {
     Distance dist = new Distance(
              intValue(args[0]), intValue(args[1]),
              intValue(args[2]), intValue(args[3]));

   private static int intValue(String data) {
     return Integer.parseInt(data);
Figure 2.1: Basic Java syntax.
For a further treatment of the syntax elements of Java, see also Syntax.

The syntax of a Java class is the characters, symbols and their structure used to code the class. Java programs consist of a sequence of tokens. There are different kinds of tokens. For example, there are word tokens such as class and public which represent keywords (in purple above) — special words with reserved meaning in Java. Other words such as Distance, point0, x1, and printDistance are not keywords but identifiers (in grey). Identifiers have many different uses in Java but primarily they are used as names. Java also has tokens to represent numbers, such as 1 and 3; these are known as literals (in orange). String literals (in blue), such as "Distance between ", consist of zero or more characters embedded in double quotes, and operators (in red) such as + and = are used to express basic computation such as addition or String concatenation or assignment. There are also left and right braces ({ and }) which enclose blocks. The body of a class is one such block. Some tokens are punctuation, such as periods . and commas , and semicolons ;. You use whitespace such as spaces, tabs, and newlines, to separate tokens. For example, whitespace is required between keywords and identifiers: publicstatic is a single identifier with twelve characters, not two Java keywords.

Declarations and DefinitionsEdit

public class Distance {

  private java.awt.Point point0, point1;

  public Distance(int x0, int y0, int x1, int y1) {
    point0 = new java.awt.Point(x0, y0);
    point1 = new java.awt.Point(x1, y1);

  public void printDistance() {
    System.out.println("Distance between " + point0 + " and " + point1
      + " is " + point0.distance(point1));

  public static void main(String[] args) {
    Distance dist = new Distance(
      intValue(args[0]), intValue(args[1]),
      intValue(args[2]), intValue(args[3]));

  private static int intValue(String data) {
    return Integer.parseInt(data);


Figure 2.2: Declarations and Definitions.

Sequences of tokens are used to construct the next building blocks of Java classes as shown above: declarations and definitions. A class declaration provides the name and visibility of a class. In our example, public class Distance is the class declaration. It consists (in this case) of two keywords, public and class followed by the identifier Distance.

This means that we are defining a class named Distance. Other classes, or in our case, the command line, can refer to the class by this name. The public keyword is an access modifier which declares that this class and its members may be accessed from other classes. The class keyword, obviously, identifies this declaration as a class. Java also allows declarations of interfaces and annotations.

The class declaration is then followed by a block (surrounded by curly braces) which provides the class's definition (in blue in figure 2.2). The definition is the implementation of the class – the declaration and definitions of the class's members. This class contains exactly six members, which we will explain in turn.

  1. Two field declarations, named point0 and point1 (in green)
  2. A constructor declaration (in orange)
  3. Three method declarations (in red)

Example: Instance FieldsEdit

The declaration

  Code section 2.1: Declaration.
1 private java.awt.Point point0, point1;

...declares two instance fields. Instance fields represent named values that are allocated whenever an instance of the class is constructed. When a Java program creates a Distance instance, that instance will contain space for point0 and point1. When another Distance object is created, it will contain space for its own point0 and point1 values. The value of point0 in the first Distance object can vary independently of the value of point0 in the second Distance object.

This declaration consists of:

  1. The private access modifier,
    which means these instance fields are not visible to other classes.
  2. The type of the instance fields. In this case, the type is java.awt.Point.
    This is the class Point in the java.awt package.
  3. The names of the instance fields in a comma separated list.

These two fields could also have been declared with two separate but more verbose declarations,

  Code section 2.2: Verbose declarations.
1  private java.awt.Point point0;
2  private java.awt.Point point1;

Since the type of these fields is a reference type (i.e. a field that refers to or can hold a reference to an object value), Java will implicitly initialize the values of point0 and point1 to null when a Distance instance is created. The null value means that a reference value does not refer to an object. The special Java literal null is used to represent the null value in a program. While you can explicitly assign null values in a declaration, as in

  Code section 2.3: Declarations and assignments.
1  private java.awt.Point point0 = null;
2  private java.awt.Point point1 = null;

It is not necessary and most programmers omit such default assignments.

Example: ConstructorEdit

A constructor is a special method in a class which is used to construct an instance of the class. The constructor can perform initialization for the object, beyond that which the Java VM does automatically. For example, Java will automatically initialize the fields point0 and point1 to null.

  Code section 2.4: The constructor for the class
1 public Distance(int x0, int y0, int x1, int y1) {
2    point0 = new java.awt.Point(x0, y0);
3    point1 = new java.awt.Point(x1, y1);
4 }

The constructor above consists of five parts:

  1. The optional access modifier(s).
    In this case, the constructor is declared public
  2. The constructor name, which must match the class name exactly: Distance in this case.
  3. The constructor parameters.
    The parameter list is required. Even if a constructor does not have any parameters, you must specify the empty list (). The parameter list declares the type and name of each of the method's parameters.
  4. An optional throws clause which declares the exceptions that the constructor may throw. This constructor does not declare any exceptions.
  5. The constructor body, which is a Java block (enclosed in {}). This constructor's body contains two statements.

This constructor accepts four parameters, named x0, y0, x1 and y1. Each parameter requires a parameter type declaration, which in this example is int for all four parameters. The parameters in the parameter list are separated by commas.

The two assignments in this constructor use Java's new operator to allocate two java.awt.Point objects. The first allocates an object representing the first point, (x0, y0), and assigns it to the point0 instance variable (replacing the null value that the instance variable was initialized to). The second statement allocates a second java.awt.Point instance with (x1, y1) and assigns it to the point1 instance variable.

This is the constructor for the Distance class. Distance implicitly extends from java.lang.Object. Java inserts a call to the super constructor as the first executable statement of the constructor if there is not one explicitly coded. The above constructor body is equivalent to the following body with the explicit super constructor call:

  Code section 2.5: Super constructor.
1 {
2    super();
3    point0 = new java.awt.Point(x0, y0);
4    point1 = new java.awt.Point(x1, y1);
5 }

While it is true that this class could be implemented in other ways, such as simply storing the coordinates of the two points and computing the distance as  , this class instead uses the existing java.awt.Point class. This choice matches the abstract definition of this class: to print the distance between two points on the plane. We take advantage of existing behavior already implemented in the Java platform rather than implementing it again. We will see later how to make the program more flexible without adding much complexity, because we choose to use object abstractions here. However, the key point is that this class uses information hiding. That is, how the class stores its state or how it computes the distance is hidden. We can change this implementation without altering how clients use and invoke the class.

Example: MethodsEdit

Methods are the third and most important type of class member. This class contains three methods in which the behavior of the Distance class is defined: printDistance(), main(), and intValue()

The printDistance() methodEdit

The printDistance() method prints the distance between the two points to the standard output (normally the console).

  Code section 2.6: printDistance() method.
1 public void printDistance() {
2   System.out.println("Distance between " + point0
3     + " and " + point1
4     + " is " + point0.distance(point1));
5 }

This instance method executes within the context of an implicit Distance object. The instance field references, point0 and point1, refer to instance fields of that implicit object. You can also use the special variable this to explicitly reference the current object. Within an instance method, Java binds the name this to the object on which the method is executing, and the type of this is that of the current class. The body of the printDistance method could also be coded as

  Code section 2.7: Explicit instance of the current class.
1   System.out.println("Distance between " + this.point0
2     + " and " + this.point1
3     + " is " + this.point0.distance(this.point1));

to make the instance field references more explicit.

This method both computes the distance and prints it in one statement. The distance is computed with point0.distance(point1); distance() is an instance method of the java.awt.Point class (of which point0 and point1 are instances). The method operates on point0 (binding this to the object that point0 refers to during the execution of the method) and accepting another Point as a parameter. Actually, it is slightly more complicated than that, but we'll explain later. The result of the distance() method is a double precision floating point number.

This method uses the syntax

  Code section 2.8: String concatenation.
1  "Distance between " + this.point0
2  + " and " + this.point1
3  + " is " + this.point0.distance(this.point1)

to construct a String to pass to the System.out.println(). This expression is a series of String concatenation methods which concatenates Strings or the String representation of primitive types (such as doubles) or objects, and returns a long string. For example, the result of this expression for the points (0,3) and (4,0) is the String

"Distance between java.awt.Point[x=0,y=3] and java.awt.Point[x=4,y=0] is 5.0"

which the method then prints to System.out.

In order to print, we invoke the println(). This is an instance method from java.io.PrintStream, which is the type of the static field out in the class java.lang.System. The Java VM binds System.out to the standard output stream when it starts a program.

The main() methodEdit

The main() method is the main entry point which Java invokes when you start a Java program from the command line. The command

java Distance 0 3 4 0

instructs Java to locate the Distance class, put the four command line arguments into an array of String values, then pass those arguments to the public static main(String[]) method of the class. We will introduce arrays shortly. Any Java class that you want to invoke from the command line or desktop shortcut must have a main method with this signature or the following signature: public static main(String...).

  Code section 2.9: main() method.
1 public static void main(String[] args) {
2    Distance dist = new Distance(
3          intValue(args[0]), intValue(args[1]),
4          intValue(args[2]), intValue(args[3]));
5    dist.printDistance();
6 }

The main() method invokes the final method, intValue(), four times. The intValue() takes a single string parameter and returns the integer value represented in the string. For example, intValue("3") will return the integer 3.

People who do test-first programming or perform regression testing write a main() method in every Java class, and a main() function in every Python module, to run automated tests. When a person executes the file directly, the main() method executes and runs the automated tests for that file. When a person executes some other Java file that in turn imports many other Java classes, only one main() method is executed -- the main() method of the directly-executed file.

The intValue() methodEdit

The intValue() method delegates its job to the Integer.parseInt() method. The main method could have called Integer.parseInt() directly; the intValue() method simply makes the main() method slightly more readable.

  Code section 2.10: intValue() method.
1 private static int intValue(String data) {
2    return Integer.parseInt(data);
3 }

This method is private since, like the fields point0 and point1, it is part of the internal implementation of the class and is not part of the external programming interface of the Distance class.

Static vs. Instance MethodsEdit

Both the main() and intValue() methods are static methods. The static keyword tells the compiler to create a single memory space associated with the class. Each individual object instantiated has its own private state variables and methods but use the same static methods and members common to the single class object created by the compiler when the first class object is instantiated or created. This means that the method executes in a static or non-object context — there is no implicit separate instance available when the static methods run from various objects, and the special variable this is not available. As such, static methods cannot access instance methods or instance fields (such as printDistance()) or point0) directly. The main() method can only invoke the instance method printDistance() method via an instance reference such as dist.

Data TypesEdit

Most declarations have a data type. Java has several categories of data types: reference types, primitive types, array types, and a special type, void.

Primitive TypesEdit

The primitive types are used to represent boolean, character, and numeric values. This program uses only one primitive type explicitly, int, which represents 32 bit signed integer values. The program also implicitly uses double, which is the return type of the distance() method of java.awt.Point. double values are 64 bit IEEE floating point values. The main() method uses integer values 0, 1, 2, and 3 to access elements of the command line arguments. The Distance() constructor's four parameters also have the type int. Also, the intValue() method has a return type of int. This means a call to that method, such as intValue(args[0]), is an expression of type int. This helps explain why the main method cannot call:

  Code section 2.11: Wrong type.
1 new Distance(args[0], args[1], args[2], args[3]) // This is an error

Since the type of the args array element is String, and our constructor's parameters must be int, such a call would result in an error because Java will not automatically convert values of type String into int values.

Java's primitive types are boolean, byte, char, short, int, long, float and double. Each of which are also Java language keywords.

Reference TypesEdit

In addition to primitive types, Java supports reference type. A reference type is a Java data type which is defined by a Java class or interface. Reference types derive this name because such values refer to an object or contain a reference to an object. The idea is similar to pointers in other languages like C.

Java represents sequences of character data, or String, with the reference type java.lang.String which is most commonly referred to as String. String literals, such as "Distance between " are constants whose type is String.

This program uses three separate reference types:

  1. java.lang.String (or simply String)
  2. Distance
  3. java.awt.Point
For more information see chapter: Java Programming/Classes, Objects and Types.

Array TypesEdit

Java supports arrays, which are aggregate types which have a fixed element type (which can be any Java type) and an integral size. This program uses only one array, String[] args. This indicates that args has an array type and that the element type is String. The Java VM constructs and initializes the array that is passed to the main method. See arrays for more details on how to create arrays and access their size.

The elements of arrays are accessed with integer indices. The first element of an array is always element 0. This program accesses the first four elements of the args array explicitly with the indices 0, 1, 2, and 3. This program does not perform any input validation, such as verifying that the user passed at least four arguments to the program. We will fix that later.


void is not a type in Java; it represents the absence of a type. Methods which do not return values are declared as void methods.

This class defines two void methods:

  Code section 2.12: Void methods
1 public static void main(String[] args) { ... }
2 public void printDistance()  { ... }


Whitespace in Java is used to separate the tokens in a Java source file. Whitespace is required in some places, such as between access modifiers, type names and Identifiers, and is used to improve readability elsewhere.

Wherever whitespace is required in Java, one or more whitespace characters may be used. Wherever whitespace is optional in Java, zero or more whitespace characters may be used.

Java whitespace consists of the

  • space character ' ' (0x20),
  • the tab character (hex 0x09),
  • the form feed character (hex 0x0c),
  • the line separators characters newline (hex 0x0a) or carriage return (hex 0x0d) characters.

Line separators are special whitespace characters in that they also terminate line comments, whereas normal whitespace does not.

Other Unicode space characters, including vertical tab, are not allowed as whitespace in Java.

Required WhitespaceEdit

Look at the static method intValue:

  Code section 2.13: Method declaration
1 private static int intValue(String data) {
2   return Integer.parseInt(data);
3 }

Whitespace is required between private and static, between static and int, between int and intValue, and between String and data.

If the code is written like this:

  Code section 2.14: Collapsed code
1 privatestaticint intValue(String data) {
2   return Integer.parseInt(data);
3 }

...it means something completely different: it declares a method which has the return type privatestaticint It is unlikely that this type exists and the method is no longer static, so the above would result in a semantic error.


Java ignores all whitespace in front of a statement. As this, these two code snippets are identical for the compiler:

  Code section 2.15: Indented code
 1 public static void main(String[] args) {
 2   Distance dist = new Distance(
 3            intValue(args[0]), intValue(args[1]),
 4            intValue(args[2]), intValue(args[3]));
 5   dist.printDistance();
 6 }
 8 private static int intValue(String data) {
 9   return Integer.parseInt(data);
10 }
  Code section 2.16: Not indented code
 1 public static void main(String[] args) {
 2 Distance dist = new Distance(
 3 intValue(args[0]), intValue(args[1]),
 4 intValue(args[2]), intValue(args[3]));
 5 dist.printDistance();
 6 }
 8 private static int intValue(String data) {
 9 return Integer.parseInt(data);
10 }

However, the first one's style (with whitespace) is preferred, as the readability is higher. The method body is easier to distinguish from the head, even at a higher reading speed.

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