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Java Programming Understanding a Java Program

Java IDEs

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!

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

Code listing 2.1: Distance.java publicclassDistance{privatejava.awt.Pointpoint0,point1;publicDistance(intx0,inty0,intx1,inty1){point0=newjava.awt.Point(x0,y0);point1=newjava.awt.Point(x1,y1);}publicvoidprintDistance(){System.out.println("Distance between "+point0+" and "+point1+" is "+point0.distance(point1));}publicstaticvoidmain(String[]args){Distancedist=newDistance(intValue(args[0]),intValue(args[1]),intValue(args[2]),intValue(args[3]));dist.printDistance();}privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}}

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 thejavac command fails, review theinstallation instructions.)

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

If you get ajava.lang.NumberFormatException exception, some arguments are not a number. If you get ajava.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 0Distance 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 19Distance 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.

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.

Code listing publicclassDistance{privatejava.awt.Pointpoint0,point1;publicDistance(intx0,inty0,intx1,inty1){point0=newjava.awt.Point(x0,y0);point1=newjava.awt.Point(x1,y1);}publicvoidprintDistance(){System.out.println("Distance between "+point0+" and "+point1+" is "+point0.distance(point1));}publicstaticvoidmain(String[]args){Distancedist=newDistance(intValue(args[0]),intValue(args[1]),intValue(args[2]),intValue(args[3]));dist.printDistance();}privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}}

Figure 2.1: Basic Java syntax.

For a further treatment of the syntax elements of Java, see alsoSyntax.

Thesyntax 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 asclass andpublic which representkeywords(in purpleabove) — special words with reserved meaning in Java. Other words such asDistance,point0,x1, andprintDistance are not keywords butidentifiers (in grey). Identifiers have many different uses in Java but primarily they are used as names. Java also has tokens to represent numbers, such as1 and3; these are known asliterals(in orange).String literals(in blue), such as"Distance between ", consist of zero or more characters embedded in double quotes, andoperators(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 encloseblocks. The body of a class is one such block. Some tokens are punctuation, such as periods. and commas, and semicolons;. You usewhitespace 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.

Figure 2.2: Declarations and Definitions.

Sequences of tokens are used to construct the next building blocks of Java classes as shownabove: 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 andclass followed by the identifierDistance.

This means that we are defining a class namedDistance. Other classes, or in our case, the command line, can refer to the class by this name. Thepublic keyword is anaccess modifier which declares that this class and its members may be accessed from other classes. Theclass keyword, obviously, identifies this declaration as a class. Java also allows declarations ofinterfaces andannotations.

The class declaration is then followed by a block (surrounded by curly braces) which provides the class's definition(in blue infigure 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, namedpoint0 andpoint1(in green)
2. A constructor declaration(in orange)
3. Three method declarations(in red)

The declaration

Code section 2.1: Declaration. privatejava.awt.Pointpoint0,point1;

...declares twoinstance fields. Instance fields represent named values that are allocated whenever an instance of the class is constructed. When a Java program creates aDistance instance, that instance will contain space forpoint0 andpoint1. When anotherDistance object is created, it will contain space for itsownpoint0 andpoint1 values. The value ofpoint0 in the firstDistance object can vary independently of the value ofpoint0 in the secondDistance object.

This declaration consists of:

1. Theprivate 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 isjava.awt.Point.
   This is the classPoint in thejava.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. privatejava.awt.Pointpoint0;privatejava.awt.Pointpoint1;

Since the type of these fields is a reference type (i.e. a field thatrefers to or can hold areference to an object value), Java will implicitly initialize the values ofpoint0 andpoint1 to null when aDistance instance is created. The null value means that a reference value does not refer to an object. The special Java literalnull 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. privatejava.awt.Pointpoint0=null;privatejava.awt.Pointpoint1=null;

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

Aconstructor 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 fieldspoint0 andpoint1 to null.

Code section 2.4: The constructor for the class publicDistance(intx0,inty0,intx1,inty1){point0=newjava.awt.Point(x0,y0);point1=newjava.awt.Point(x1,y1);}

The constructor above consists of five parts:

1. The optionalaccess modifier(s).
   In this case, the constructor is declaredpublic
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 optionalthrows clause which declares theexceptions 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, namedx0, y0, x1 andy1. Each parameter requires a parameter type declaration, which in this example isint for all four parameters. The parameters in the parameter list are separated by commas.

The two assignments in this constructor use Java'snew operator to allocate twojava.awt.Point objects. The first allocates an object representing the first point,(x0, y0), and assigns it to thepoint0 instance variable (replacing the null value that the instance variable was initialized to). The second statement allocates a secondjava.awt.Point instance with(x1, y1) and assigns it to thepoint1 instance variable.

This is the constructor for the Distance class. Distance implicitly extends fromjava.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. {super();point0=newjava.awt.Point(x0,y0);point1=newjava.awt.Point(x1,y1);}

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${\displaystyle \sqrt{(x_1-x_0{)}^2+(y_1-y_0{)}^2}}$, this class instead uses the existingjava.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.

Methods are the third and most important type of class member. This class contains threemethods in which the behavior of theDistance class is defined:printDistance(),main(), andintValue()

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

Code section 2.6:printDistance() method. publicvoidprintDistance(){System.out.println("Distance between "+point0+" and "+point1+" is "+point0.distance(point1));}

Thisinstance method executes within the context of an implicitDistance object. The instance field references,point0 andpoint1, refer to instance fields of that implicit object. You can also use the special variablethis to explicitly reference the current object. Within an instance method, Java binds the namethis to the object on which the method is executing, and the type ofthis is that of the current class. The body of theprintDistance method could also be coded as

Code section 2.7: Explicit instance of the current class. System.out.println("Distance between "+this.point0+" and "+this.point1+" 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 withpoint0.distance(point1);distance() is an instance method of thejava.awt.Point class (of whichpoint0 andpoint1 are instances). The method operates onpoint0 (bindingthis to the object thatpoint0 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 thedistance() method is a double precision floating point number.

This method uses the syntax

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

to construct a String to pass to theSystem.out.println(). This expression is a series ofString 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

Output "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 toSystem.out.

In order to print, we invoke theprintln(). This is an instance method fromjava.io.PrintStream, which is the type of the static fieldout in the classjava.lang.System. The Java VM bindsSystem.out to the standard output stream when it starts a program.

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

Output 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 thepublic 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. publicstaticvoidmain(String[]args){Distancedist=newDistance(intValue(args[0]),intValue(args[1]),intValue(args[2]),intValue(args[3]));dist.printDistance();}

Themain() method invokes the final method,intValue(), four times. TheintValue() 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 testingwritea main() method in every Java class, anda 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 filethat in turn imports many other Java classes,only one main() method is executed --the main() method of the directly-executed file.

TheintValue() method delegates its job to theInteger.parseInt() method. The main method could have calledInteger.parseInt() directly; theintValue() method simply makes themain() method slightly more readable.

Code section 2.10:intValue() method. privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}

This method isprivate since, like the fieldspoint0 andpoint1, it is part of the internal implementation of the class and is not part of the external programming interface of theDistance class.

Both themain() andintValue() methods arestatic methods. Thestatic 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 samestatic 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 variablethis is not available. As such, static methods cannot access instance methods or instance fields (such asprintDistance()) orpoint0) directly. Themain() method can only invoke the instance methodprintDistance() method via an instance reference such asdist.

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

Theprimitive 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 usesdouble, which is the return type of thedistance() method ofjava.awt.Point.double values are 64 bit IEEE floating point values. Themain() method uses integer values 0, 1, 2, and 3 to access elements of the command line arguments. TheDistance() constructor's four parameters also have the typeint. Also, theintValue() method has a return type ofint. This means a call to that method, such asintValue(args[0]), is an expression of typeint. This helps explain why the main method cannot call:

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

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

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

In addition to primitive types, Java supportsreference 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 valuesrefer to an object or contain areference to an object. The idea is similar to pointers in other languages like C.

Java represents sequences of character data, orString, with the reference typejava.lang.String which is most commonly referred to asString.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.

Java supportsarrays, 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 thatargs has an array type and that the element type isString. The Java VM constructs and initializes the array that is passed to themain method. Seearrays 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 theargs array explicitly with the indices 0, 1, 2, and 3. This program doesnot 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 asvoid methods.

This class defines two void methods:

Code section 2.12: Void methods publicstaticvoidmain(String[]args){...}publicvoidprintDistance(){...}

Whitespace in Java is used to separate the tokens in a Java source file. Whitespace is required in some places, such as betweenaccess 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.

Look at thestatic methodintValue:

Code section 2.13: Method declaration privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}

Whitespace is required betweenprivate andstatic, betweenstatic andint, betweenint andintValue, and betweenString anddata.

If the code is written like this:

Code section 2.14: Collapsed code privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}

...it means something completely different: it declares a method which has the return typeprivatestaticint 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 publicstaticvoidmain(String[]args){Distancedist=newDistance(intValue(args[0]),intValue(args[1]),intValue(args[2]),intValue(args[3]));dist.printDistance();}privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}

Code section 2.16: Not indented code publicstaticvoidmain(String[]args){Distancedist=newDistance(intValue(args[0]),intValue(args[1]),intValue(args[2]),intValue(args[3]));dist.printDistance();}privatestaticintintValue(Stringdata){returnInteger.parseInt(data);}

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.

Execution

Java Programming Understanding a Java Program

Java IDEs

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