A goal of this programming course is for you to learn to write programs that are not only correct but also understandable. These guidelines should help you toward that goal. We ask that you follow these guidelines when writing programs in this course. They will give you a good basis for developing a style of your own as you become a more experienced programmer.

This handout includes guidelines for various Java constructs, including some that will be covered later in the semester, and/or later in your study of the Java language.   Skim these 'extra' sections now and read them more carefully later when the topics are discussed.

Table of contents

• Reasons for following a disciplined programming style
• Conventions for names: package names, class names, method names, parameter names, field names, local-variable names, accessor names, constant names
• Indentation
• Statement-comments
• Specification of methods (procedures and functions)
• Definitions of variables (field names, parameters, and local variables)
• Specifying classes
• The assignment statement
• Acknowledgements

Your computer program should be readable, because if it is not readable, the chances of it being correct are slim. Moreover, if your program is unreadable, it will be difficult and time-consuming to find and correct the errors in it. So,

The major reason for using a disciplined style of programming is that it will save you time whenever you have to read your program or debug it.

Outside this course, making programs readable by others becomes even more important. Most programs live a long time and require "maintenance" --changes to adapt to new and different requirements, upgrades in other software, new hardware, etc. And the author of the program is quite likely not going to be around when the maintenance is required. Someone else must read the program and understand it enough to update it successfully.

Even the programs you write for yourself should be readable; if not, four weeks after finishing it you will not remember it enough to make changes simply.

Thus, simply for your own sake and for the sake of others, it makes sense to develop programming habits that lend themselves to writing readable, understandable, and correct, programs.

Part of these habits concern simple, syntactical measures like indenting program parts properly and using a few conventions for names of variables, methods, etc. The more important part concerns recording enough information in comments for the reader to understand how a program is designed and why.

A computer program is the result of many design decisions. These decisions --why this variable was introduced, what that function does, etc.-- are often not reflected in the final Java code, which consists of low-level, detailed declarations and statements. However, the higher-level design must be understood if a programmer is to modify the program successfully. Trying to understand decisions that are not recorded in comments in the code is tedious, error-prone, and aggravating --but all too common.

So, it will be to your advantage to instill in yourself some disciplined programming habits, right from the beginning, like

• Write a precise definition (in a comment) of a variable as you write its declaration
• Update the definition of the variable (the comment) when you decide to change its meaning
• Write a precise specification for a method as you write the method's heading and before you write the body.
• Change the specification for a method whenever you make changes to the body of the method that make the body not fit the specification.

You will find that writing good comments as you write a program will help you clarify your ideas and write better, correct code sooner. If you can write down clearly what your program is doing you are more likely to have a good understanding of the problem and your program is more likely to be correct. Time spent on careful thinking and writing is more than repaid in time saved during testing and debugging.

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Conventions for names
Good naming conventions can help you and any reader of a program understand the program easily; bad naming conventions or no naming conventions can lead to confusion and misunderstanding. Some people advocate using very long names that define what the named entity (e.g. method, field) represents. However, in general, this is not feasible. A good definition for an entity may require three lines of explanation or more ---how can you have such a long name? On the other hand, extremely short names --one or two characters long-- don't help at all. Thus, some in-between measures have to be adopted. There are contexts where a short name is best and contexts where a longer name is best.

Remember that a name can rarely be used to give a complete, precise definition of the entity it names, and a complete definition should always be given where the entity is defined.

The people developing Java programs have developed some conventions for identifiers, which help one readily see what the identifier represents (to some extent). You can see these conventions at work by looking at the large set of classes that are provided with the language, including source code;  (for example ava.awt , the Abstract Window Toolkit).  Below, we state the conventions, with a few additions.

Class names: e.g. FilterInputStream, LivingMammals

Since a class represents a set of possible objects, each of which is an instance of the class, a class name should generally be a noun phrase that identifies the possible objects. A class names should begin with a capital letter, and all successive words in the class name should be capitalized.

Method names: e.g. drawOval, fillOval, toString, length, equals, isAtMost, binarySearch

A method that is a command to do something should be given by a verb phrase that gives some indication of what the method does. (But this name should never be used in place of a precise comment for the method!) A method that implements a function should be a phrase that describes the value. The method name can also be the name of an algorithm (e.g. binarySearch). For a method that both executes some task and returns a value, use common sense in creating its name.

A method name should begin with a small letter, but all successive words in the class name should be capitalized.

Names of parameters of methods: e.g. x, y

The precise meaning of a parameter, and any restrictions on it, should be given in the comment of the heading of the method. Therefore, particularly if the method is fairly short, it is wise to give parameters short names.

For example, in the two method headings given below, the first is preferable because it is shorter and easier to understand. Moreover, the body of the method of the first method will also be shorter and far easier to understand and manipulate.

// Draw an ellipse that fits exactly within the rectangle whose
// upper left corner is at position (x,y), whose width is w, and
// whose height is h. Use the current color to draw the ellipse.
void drawOval(int x, int y, int w, int h) {

// Draw an ellipse that fits exactly within the rectangle whose
// upper left corner is at position (xCoordinate,yCoordinate)
// whose width is width, and whose height is height. Use the
// current color to draw the ellipse.
void drawOval(int xCoordinate,int yCoordinate, int width, int height)

A name like theLoopCounter or firstNumber instead of k or x is useless and causes clutter.

A parameter used as a "flag" should be named for what the flag represents, like noMorePizza, rather than simply flag. Avoid generic names like count and value; instead, describe the items being counted or the value stored in the variable.

A field name, or instance variable as they are sometimes called, contains information that helps describe the "state" of the object in which it occurs. A field name should be a noun phrase that describes the information the field contains. (However, the field still needs a (more) precise comment that describes it.) All words in a field name, except the first, should be capitalized.

A local variable of a method contains information that helps describe the "state" of the method during its execution.. A local variable name should be a noun phrase that describes the information the field contains. However, the field still needs a (more) precise comment that describes it. All words in a field name, except the first, should be capitalized.

If the body of a method is short, or the places in which a local variable is used is fairly short, then a short, one-or-two letter name can be used for the local variable (see also the conventions for parameter names). A name like theLoopCounter or firstNumber instead of k or x causes clutter.

A variable used as a "flag" should be named for what the flag represents, like noMorePizza, rather than simply flag. Avoid generic names like count and value; instead, describe the items being counted or the value stored in the variable.

A getter method for a field returns the value in the field; a setter method stores a value in a field. Two examples will illustrate a convention for naming such getter methods. To reference a field named textLength, use a method textLength() or method getTextLength(). To store a value in the field, use a method setTextLength().

In Java, a constant is a field of a class that has been declared final. The Java convention is to use only capital letters in the names of constants and to separate adjacent words in a constant name with an underscore.

A package is a set of classes that have been grouped together. The name of a package is the name of the folder in which the source files for the classes appear. Windows and Macintosh systems don't make a distinction between upper- and lower-case in file and folder names, but Unix does. For portability purposes, be sure the name of a class (e.g. LivingMammal) and the file in which it appears (e.g. LivingMammal.java) have the same capitalization.

Indentation
Indentation is used to make the structure of a program clear. The basic rule is:

Substatements of a statement or declaration should be indented.

There are several methods for placing curly braces that delimit statements, none of which is liked by all and all of which are disliked by some. Below, we illustrate three methods, with pros and cons explained to the right.

(0)  if (x < y)        Pro: It is easy to match up braces
     {                 Con: Too many lines are used, and the
         x=  y;             number of lines is a scarce re-                      
         y=  0;             source on the monitor.
     }                 Con: The substatement {x= y; y= 0;} has
    else                    not been indented, because the 
    {                       braces are part of the substatement.
         x=  0;             Therefore, the basic rule concerning
         y=  y/2;           indentation has not been followed.

    }

(1)  if (x < y) {      Pro: There are no wasted lines.  
         x=  y;        Pro: The opening brace is out of the way
         y=  0;             and the closing brace indicates
     } else {               nicely the end of a substatement
         x=  0;        Con: It is difficult to match up be-
         y=  y/2;           ginning and end braces.
         }

(2)  if (x < y)        Pro: There are no wasted lines.
         {x=  y;       Pro: The basic indentation rule is followed.
         y=  0;        Pro: It is easy to match up beginning and
         }                  end braces and easy to see where a
     else                   substatement ends.
         {x=  0;       Con: The first statement of a substatement    
         y=  y/2;           indented differently from the rest.
         }

Below, we illustrate two ways to place the curly braces surrounding the body of a method. There are others. Choose one and use it consistently within a program.

(0) // Specification of method
    public void drawLine(int x1, int y1, int x2, int y2) {
        statement 1;
        statement 2;
    }

(1) // Specification of method
    public void drawLine(int x1, int y1, int x2, int y2)
        {statement 1;
        statement 2;
        }

Statement-comments

Just as the sentences of an essay are grouped in paragraphs, so the sequence of statements of the body of a method should be grouped into logical units. Often, the clarity of the program is enhanced by preceding a logical unit by a comment that explains what it does. This comment serves as the specification for the logical unit; it should say precisely what the logical unit does. .

The comment for such a logical unit is called a "statement-comment". It should be written as a command to do something. Here is an example.

    // Truthify x >= y by swapping x and y if needed.
         if (x < y) {            
            int tmp = x;
            x = y;
            y = tmp;
         }

Statement comments must be complete. The comment

     // Test for valid input

Placement of statement-comments. In the example above, and in the following example, the statements specified by a statement comment are indented. In a program with several levels of statement-comments, this indentation is useful in clarifying the structure of a program. In fact, with several levels of statement-comments, even judicious use of blank lines cannot eliminate all ambiguity concerning what statement belongs to what statement-comment. The program fragment given below illustrates this.

    // Truthify the definition of t --return false if not possible
        // Eliminate whitespace from the beginning and end of t

            while (t.length() != 0 && isWhitespace(t.charAt(0)))

                 t= t.substring(1);
        // If t is empty, print an error message and return         
            if (t.length() == 0)
                {...
                return false;
                }
        if (containsCapitals(t))
           {...
           }
    // Store the French translation of t in tFrench        
       ...

In this example, note how reliance on the definition of t in the statement-comment "Truthify the definition of t" allows the statement-comment to be short but precise. Of course, a suitable definition for t must appear at its declaration.)

Some people prefer not to indent substatements of a statement-comment, but rather rely on blank lines to separate the end of the substatements of a statement-comments. Below, we show the above program fragment in this style. Note the extra comment to help the reader see the end of the substatements for "truthify the definition of t". This style has its problems. Don't use it.

     // Truthify the definition of t --return false if not possible

     // Eliminate whitespace from the beginning and end of t
     while (t.length() != 0 && isWhitespace(t.charAt(0)))
         t= t.substring(1);

     // If t is empty, print an error message and return
     if (t.length() == 0)
         {...
        return false;
         }

     if (containsCapitals(t))
         {...
         }

     // (End of Truthify the definition of t)

     // Store the French translation of t in tFrench
      ...
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Specification of methods (procedures and functions)
Every method should be preceded by a comment that gives its specification. This specification, together with the heading of the method, which gives the number and types (classes) of the parameters and the type (class) of the result, should provide all the information needed to use the method --and no more. It should describe any restrictions on the parameters and what the method does, not how it does it. One should never have to look at the body of a method to understand how to use it. The specification comment should describe the parameters of the method. The specifications can usually be written in few sentences. Here is an example.

     // Print on System.in the most frequently occurring temperature
     // in t[0..c-1]. If there is more than one possibility, print
     // the least temperature.
     void findCommon (Temperature[] t, int c)

     // Find most frequent temperature
     void findCommon (Temperature [] t, int c)

For a function --i.e. a method that returns a value-- it is often easiest to simply describe the value returned, using the word "yield":

    // = the distance between points (x1,y1) and (x2,y2)
    double dist (int x1, int y1, int x2, int y2)

Omit needless words.

Use the active voice.

     // Yield "y is in list x"
     boolean isIn(int y, List x)

Every significant variable and data structure needs a precise and complete definition, which provides whatever information is needed to understand the variable The most useful information is often the invariant properties of the data: facts that are always true except, perhaps, momentarily, when several related variables are being updated.

Important hint: Write the definition of a set of variables when you first conceive of using them, and type the definitions as comments into the program when you type in the declarations of the variables. If you later decide to change the meanings of the variables, change the definitions before you change the statement that use the variables.

     // 0 <= i <= currentItem < numberItems and i is the smallest value
     // such that item i's price is at most item currentItem's price.

    boolean b; // = " user selected menu item File | Quit"



    boolean b; // true if the user selected menu item File | Quit;
               // false otherwise

Related variables should be declared and described together. For example, the definition of a table should describe not only the array that holds the data but also the integer variable that contains the number of items currently in the table. In the example below, for utmost clarity tabs are used to line up identifier names and to line up the comments.

    final int maxTemp= 150; // max number of temperature readings.
    int nTemp= 0;            // Temperature readings are in temp[0..nTemp-1], where
    double temps=  new double{maxTemp];// 0 <= nTemp < maxTemp

   // Temperature readings are in temp[0..nTemp-1], where
   // 0 <= nTemp <= maxTemp and maxTemp is the maximum number of
   // temperature readings
         final int maxTemp= 150;
         int        nTemp= 0;
         double    temps= new double[maxTemp]

Specifying classes
In Java, typically, each class is given in a separate file. The beginning of the file should contain a comment that explains what the class is for. Often, one also puts information concerning the author, the date of last modification, and so on. Here is an example.

 // An object of class Auto represents a car
 // Author: John Doe.
 // Date of last modification: 25 August 1998
 public static class Car;

The assignment statement
The programming language C (and then C++ and Java) introduced the use of == for equality and = for assignment --only because programmers wrote more assignments than equality tests. This went against a long-standing tradition --of more than 300 years-- of the use of = for equality, a tradition that is worldwide. No other notation device has caused more confusion, expense, and loss of time than the use of == for equality and = for assignment. It causes confusion and misunderstanding to beginners, and it causes bugs in programs (in C and C++), some of which take a long time to find.

Equality is a symmetric operation: a = b and b = c (using = in the mathematical sense) yield the same value. Assignment is not symmetric. To get across the idea that the assignment in Java is not symmetric, we place NO blank to the left of = and one blank to its right. For example, here is th assignment of x to y:

    y= x;

Use this rule in your Java nad C programming! It will take a while to get into this habit, but once you do, you will happier.
 

Acknowledgements
The ideas in this missive originated in the structured-programming movement of the 1970's. They are every bit as applicable today. Many examples and ideas were taken from old Cornell CS100 and and CS211 handouts, originating in the work of Richard Conway and David Gries (see their text on An Introduction to Programming, using PL/C). Hal Perkins had a hand in writing this also.

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