Lab Assignment 2

Due date : September 1, 12 PM EST

Total Marks : 30

Important Note :

1. The deadline will NOT be extended. Programs submitted after the due date and time will not be accepted.
2. You must work collaboratively with your group-mate.
3. One member of the group should upload all files including his/her peer evaluation form.
4. The other group member will only upload his/her peer evaluation form.
5. Any exception of rules 3 or 4 should be documented in comment section of drop box.

Today’s Exercise

Topics	Lab Exercises	Points
Syntax errors	Recognizing syntax errors Correcting syntax errors	10 + 5x5 10

Syntax:

Syntax rules of a programming language dictate exactly how the vocabulary elements should be combined to create a program statement. For example:

1. Each program statement should end with a semicolon.
2. Each method/function should open with an opening brace and end with an ending brace.

Appendix L in your textbook formally defines all syntax rules of java programming language.

You may use this link: http://lesliefranke.com/sandbox/ref/javacheatsheet.html as a guide to common syntax rules.

Semantics:

The semantics of a statement in a programming language define what will happen when that statement is executed. Semantics are always well defined. As a consequence, a programming language, unlike natural languages, is unambiguous i.e., there is one and only one interpretation of a statement. One specific English sentence has multiple meanings.

Errors:

• Compile-time error: The compiler checks to make sure that you are using correct syntax. If there is any sentence that does not conform to the syntactic rules of Java, the compiler issues an error. The compiler also tries to point out semantic mistakes such as incompatible data types. If a program contains compiler error, the executable file or the byte code is not produced.
• Run-time error: This type of error occurs during program execution. The program is syntactically and semantically correct. So there is no compiler error. The executable file is created. But a requested operation may be undefined. The system simply denies executing the rest of the program. Example: dividing by zero.
• Logical error: this is the error that is most difficult to discover. Here the program executes properly and produces a result but the result is not correct. The process of finding logical error is called debugging.

Recognizing Syntax Errors

When you make syntax errors in your program the compiler gives error messages and does not create the bytecode file. It

saves time and frustration to learn what some of these messages are and what they mean. Unfortunately, at this stage in the

game many of the messages will not be meaningful except to let you know where the first error occurred. Your only choice is

to carefully study your program to find the error. In the following you will introduce a few typical errors into a simple

program and examine the error messages.

1. Type the following program into a file called Hello.java. (This is the traditional first program a computer scientist writes

in a new language.)

// ********************************************

// Hello.java

//

// Print a Hello, World message.

// ********************************************

public class Hello

{

// -----------------------------------

// main method -- prints the greeting

// -----------------------------------

public static void main (String[] args)

{

System.out.println ("Hello, World!");

}

}

Compile and run the program to see what it does. Then make the changes below, answering the questions as you go.

2. Class name different from file name. Delete one l (el) from the name of the class (so the first non-comment line is

public class Helo), save the program, and recompile it. What was the error message?

3. Misspelling inside string. Correct the mistake above, then delete one l from the Hello in the message to be printed

(inside the quotation marks). Save the program and recompile it. There is no error message—why not? Now run the

program. What has changed?

4. No ending quotation mark in a string literal. Correct the spelling in the string, then delete the ending quotation mark

enclosing the string Hello, World!. Save the program and recompile it. What error message(s) do you get?

5. No beginning quotation mark in a string literal. Put the ending quotation mark back, then take out the beginning one.

Save and recompile. How many errors this time? Lots, even though there is really only one error. When you get lots of

errors always concentrate on finding the first one listed!! Often fixing that one will fix the rest. After we study

variables the error messages that came up this time will make more sense.

6. No semicolon after a statement. Fix the last error (put the quotation mark back). Now remove the semicolon at the end

of the line that prints the message. Save the program and recompile it. What error message(s) do you get?

Correcting Syntax Errors

File Problems.java contains a simple Java program that contains a number of syntax errors. Save the program to your

directory, study it and correct as many of the errors as you can find. Then compile the program; if there are still errors,

correct them. Some things to remember:

• Java is case sensitive, so, for example, the identifiers public, Public, and PUBLIC are all considered different. For

            reserved words such as public and void and previously defined identifiers such as String and System, you have to get the

            case right. You will learn the conventions about case soon, but for now you can look at a sample program in the text to

            see what case should be used where.

• When the compiler lists lots of errors, fix the first one (or few) and then recompile—often the later errors aren't really

      errors in the program, they just indicate that the compiler is confused from earlier errors.

• Read the error messages carefully, and note what line numbers they refer to. Often the messages are helpful, but even

            when they aren't, the line numbers usually are.

• When the program compiles cleanly, run it.

// ********************************************

// Problems.java

//

// Provide lots of syntax errors for the user to correct.

//

********************************************

public class problems

{

public Static main (string[] args)

{

System.out.println ("!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");

System.out.println (This program used to have lots of problems,");

System.0ut.println ("but if it prints this, you fixed them all.")

System.out.println (" *** Hurray! ***);

System.out.println ("!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");

}

}

Submission:

• Submit a word document with answers to questions 2, 3, 4, 5, 6.
• Submit corrected version of Problems.java.