Introduction to Java Computer Science Courseware

Paolo scientifico

Introduction to Java Computer Science Courseware

Paolo scientifico

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Abstract

These lecture notes are designed for use in the first year Computer Science modules at the University of KwaZulu-Natal. They provide an introduction to problem solving, programming, and the Java language. They are not intended to be complete in themselves but serve as a complement to the formal lectures, and students are urged to make use of the books referenced in addition to these notes.

Figures (125)

This description applies to most programming languages such as C and Pascal. The objec code which the compiler produces is, of course, specific to the actual system on which the compilation takes place - ie. your object code will only execute on the system it was compilec on since the machine code is only recognisable by that system. If you want to use the same program on another system you have to recompile your original source code using a compile for that system. The process is:

operating system it instead generates machine code for a hypothetical machine called the J ava Virtual Machine (J VM). J ava calls this machine code bytecodes. The bytecodes are not the native machine code of the computer, so they are executed by a bytecode interpreter running on your machine, which is a much simpler program than a compiler. A bytecode interpreter is supplied with the J ava language, and there is also an automatic bytecode interpreter built into most Internet browsers such as Netscape and Internet Explorer. Java programs come in two forms, applications and applets. J ava applications are just like any ordinary program, while applets are mini-applications that are embedded in WWW pages When a page containing an applet is displayed, the applet (in bytecodes) is downloaded to the users computer and executed locally. This is only possible because J ava is architecturally neutral and the bytecode programs can run on any machine with an interpreter. Because applets are downloaded from a remote site to run on a local computer they are subject to ¢ number of security restrictions, for example, they cannot access local files or print. Applications on the other hand, are not restricted in what they can do, and any kind of program can be written

For example, let us look forward to the year 2000. Our household robot, Robbie, helps us with some simple chores. Each morning we would like Robbie to serve us breakfast. Unfortunately Robbie is an early production model and to get him to perform even the simplest task we must provide him with a detailed list of instructions. In this case, the problem to be solved is getting Robbie to serve breakfast.

or in J SP notation (J ackson Structured Programming): Solving these 4 subproblems will give us the solution to the original problem.

Step 2 could stand as itis, although we could consider opening and closing the coffee tin.

Steps 1.2 and 1.3 are OK, but we probably need to expand step 1.1 a little

Problem: Which is the largest of the numbers 10, 15, 3, 2, 14, 18, 27, 19, 7, 23 ? The correct algorithm would therefore be: Remember that you are working with memory cells and there is no guarantee that any particular memory cell is set to zero initially. Sometimes this is so, but often a memory cell may be holding left-over data from earlier use. If the memory cell total happens to be holding 0 the algorithm will work correctly because 0+x=x, but if total is holding some other value the result of total+x is not equal to x. To ensure we always get the right answer irrespective of whether or not anything was stored in the memory cell beforehand, a memory cell that is going to be used in a calculation should always be initialised.

Will this algorithm give the correct answer?

The solution is to scan the list of numbers and compare each number to the "largest so far" until all the numbers have been considered at which stage the “largest so far" is the largest in the list. The algorithm includes the control structure comparison and conditional execution, and can once again be generalised to deal with any number of values along the lines of the averaging algorithm. Then we need 3 memory cells only, n, value and max.

The interesting part is how to divide the minutes into days, hours and minutes. We know there are 60 minutes in an hour, so if we divide the total minutes by 60 we should get the number of hours, and the remainder will give us the number of minutes left. Then we can divide the hours by 24 to get the number of days, and the reminder gives the number of hours left.

Write a program to determine how many days, hours and minutes there are in a giver number of minutes. 4.6 Complete examples

Consider the following demonstration program:

The effect of formatting to tabulate data is clearly seen in the output

When this program is run, it pauses at the Keyboard.getChar statement and waits for the user to enter a character. The user types the character (say 0) and the program completes its execution: As an example of character input, consider a program that inputs a character and then displays a heart made up of that character:

As another example of using methods to structure a program, consider a program to display a Mother's Day message on the screen, such as We can plan a program to do this using a style of programming known as top-down design where we start at the top and consider what needs to be done to achieve the goal, and break the overall goal into sub-goals each of which can be solved independently. In this case in order to achieve the goal of displaying the message we need first to display the word "TO", then after a few blank lines the word "MOM". Each of these are sub-goals, so we can consider them independently

When a program is executed control is passed to the main method. The first statement is the call to DisplayTo, so control is transferred to the first statement of DisplayTo which is a call to DisplayT, so control is transferred there, and the sequence of println statements are executed. When the end of DisplayT is reached, control is transferred back to DisplayTo and the next statement is the call to Display0, so control is transferred there, and its println statements are executed. Then control is transferred back to DisplayTo, but the end has now been reached so control is transferred back to the main method and its next statement is executed. Execution continues in this manner until the end of the entire program is reached.

Then | checked using a time with non-zero departure time so | could see the total minutes calculation was still OK, then different old and new depart times. If possible, just change 1 value ata time so that if you detect an error it is easier to isolate what caused it. Use simple values to start with, but once you are satisfied that it seems to be working, use more complex values and check using a selection of inputs with different characteristics.

The blocks and the scope of variables in the NewCar program are: For example, the duration of the original journey is called oldJourney in the main method and time in CalcOldjJourneyTime, while the identifiers departMins and arriveMins are used in both CalcOlLdJourneyTime (to refer to the original departure and arrival times expressed as minutes after midnight) and in CalcNewArrivalTime (to refer to the new departure and arrival times expressed as minutes after midnight). The identifiers hr and min are used in a number of places to refer to hours and minutes in whatever context applies in the method (block) in which they are used.

The first if checks for the one possibility and if true the then-part is executed. If false one possibility has been eliminated and the next if is contained in the else-part and checks a second possibility, and so on. The last condition is the default because if a number is not smaller or larger than another then it must be equal, so the last condition does not need a test although the comment clarifies what state exists at this point and is helpful for the human reader. Layout is important here, and helps the user to understand that a series of successive, related conditions are being tested. The following examples appear at first glance to be similar, but are incorrect:

Sometimes conditions contain sub-conditions, in which case a more "classic" type of nested if can be used. For example, we want to count the number of passes and fails, and within each category to classify them further:

‘his gives us the final program to input the size and draw a triangle of stars of that size:

So in the program itself the first thing to do is to output the column headings which involves a loop to display each column number. Then we can write the row loop which displays the row number as a label, then loops through all the columns in the row and displays the product, then goes to the next line. Formatting must be used so that the columns line up nicely and the whole effect is pleasing.

The reasons are that not all decimal fractions can be represented exactly in a finite number of bits, 0.1 being one of these values. So when 0.1 is added to num each iteration it is actually a value slightly larger than 0.1 that is being used which starts to have an effect after a few iterations. We can do away with the extra decimal places by using formatting to display only 1 decimal place, but that will not solve the second problem. The loop terminates early as a result of the additional value being added each iteration. The terminating condition is num<=2.5;

The first mark input is 60 which is valid so the test is false and the loop body is not executed. The next mark is -10 which is out of range so the test returns true and the loop body is entered to display the error and allow the mark to be re-entered. This time a valid mark is input so execution continues with the next statements. The third mark entered is 110 which is out of range so the user is prompted to re-enter, again an invalid mark (-1) is input so the user is prompted to re-enter, and only once a valid mark is input does the program continue. Finally two more invalid marks are entered and rejected before the program accepts the sentinel value and the program ends.

Check that the requirements have been satisfied:

The third while loop example - adding the digits of a number - can be successfully rewritten as a do-while loop, since in most cases a non-zero value is entered so we need to execute the loop body at least once, and even if zero is entered the digit split off is zero so the correct answer (0) is still obtained:

Methods of a class can reference and manipulate the fields of that class, and can also declare their own fields which are then local to that method and are not accessible to other methods, even in the same class. For example, NumRoots has a local field, a double variable called disc. Methods may have their own inner classes, declared for their own Specific purpose, but may not have their own local methods.

The class diagram gives a pictorial view of the classes and their relationships, and is ven helpful in seeing the overall picture. The notation used here for depicting classes is a modification of the UML notation. It uses rectangles to denote classes, with connecting lines and annotations to denote class relationships. Within a class rectangle, the first section gives the class name, the nex section lists the attributes and the last section lists the methods. Private fields and methods are enclosed in vertical lines (eg | costs |), and objects are underlined and their classes are given.

For the field class, we need to decide what data types our attributes need to be, write é constructor that will store the values supplied when the object is instantiated in the object’ attributes, and design and write a method that will use the attributes to determine the area o the field. In this case this is trivial - multiply length by width. For the crop class, we need to decide what data types our attributes need to be, write a constructor that will store the values supplied when the object is instantiated in the object's attributes, and design and write a method that will use the attributes to determine crop's profi per square metre. We do this by calculating the income per square metre by multiplying yield by price per sq metre, and then subtracting the costs. Note this method uses a local variable, income.

The co-ordinating class instantiates the objects it needs and uses them to calculate the total profit. Its design is

The coordinating class will merely request a test (or more) and supply the number of questions and the largest value to use. The question class has 3 attributes, the two values and the answer. The answer does not have to be stored, it could be calculated from the values, but since it is used at least twice (checking the answer and returning the value) it makes sense to calculate it once and store it so that we are certain that the answer used in checking is the same as that made available outside the class. Also, a possible extension is to allow more than one guess.

The methods available in FileIO are: The following program demonstrates use of class FileIO by reading a file name from the keyboard and then reading a name (String), a student number (Long) and 3 marks (int, float and double) from the file and calculates the average mark. All output is to a results file.

However, this does not copy the entire array, but merely copies the reference so that both arrays will refer to the same storage. Any changes made to one array will affect the other.

Contrast this method with another, seemingly similar method:

The following code is not as efficient as the first because all the values are checked even when it is already known that the arrays contain unequal values, but at least it gives the right result.

The method to perform a simple Bubble Sort to sort an array of integers into ascending order is shown below. It forms part of class mySort which contains a number of sorting algorithms.

Note that the number of elements declared for a column of a 2-dimensional array can be determined by ascertaining the Length property for a row of the table matrix[0].length

With an unordered array the data is in random order, so we need to check every element in the array to see whether or not it is the one we are looking for, and we also need to be able to tell if the value we’re looking for does not appear in the array. The processing required is:

With a linear search, if there are n elements in the array then on average n/2 elements need to be examined to either locate the key value or to ascertain that it is not in the array - ie. the work done is O(n/2). With the unordered search, if the key is in the array the work done is O(n/2), but is O(n) if the key is not in the array. In both cases, the execution time increases linearly with the number of elements in the array, which is a problem for large n.

(vue CS1A does not equal CS1B CS1A does not equal csla CS1A equals (no case) cslé

The fibonacci method is called with N=5, and before it completes it calls fibonacci again with N=4, ... and so on until a base case (N=2) and this instance of the method completes it execution and returns a value (1), which is passed back to the previous instance of the method (N=3), which still needs fibo; to complete so it calls fibonacci again with N=1, a base case and returns a result to the previous instance (N=3) which can now complete and return a result to the previous call (N=4) which still needs fibo2 so calls the method again ... and so on until finally all the method calls are complete and a result is returned to the main method which displays the result.

When N=1, the solution is trivial: simply move the 1 disc to the other rod. Legend has it that in a temple in the Far East (Hanoi?), is a gold rod with 64 golden discs stacked on it, arranged in order of size so that each disc rests on a larger disc. Monks are attempting to move all the discs from this rod to another one (using a third in the process), by only moving 1 disc at a time and ensuring that each disc always rests on a larger disc. When they complete their task, the world would end!

Method calcClMark is also routine - averages the assignment and test marks and combines them in equal proportions. The number of tests and assignments is specified as a prameter so that as long as the overall departmental guidelines on how to calculate the class mark are met, individual module differences in the numbers of assignments and tests can be accommodated.

Each Student object in the array has to be instantiated, and this is done in the method setupStudents, which sets up all the names and student numbers for each student in the module from a specified file, and then sorts the attribute array into ascending order of student number. Because each array element is (a reference to) an object, the sort needs merely to exchange the references and not actually copy the values. A bubble sortis used.

In both methods the mark code is checked to ensure a valid assignment or test or the exam has been specified, and if not the method terminates returning false. The keyboard entry version displays each students number and name in turn and inputs that students mark, while the file entry version inputs marks one by one from a file, and for each checks that the student number for the mark matches the next student in the array. If the file of marks and the array are not in exactly the same order the method returns false. The method to calculate the student and module averages is a private method as it is automatically invoked each time a mark is entered or changed, and so the averages are always correct and there is never a need to specifically invoke the method to calculate the averages, they can merely be retrieved or displayed using the appropriate methods.

A number of the module methods require that a student number be specified so that a particular mark for that student can be retrieved or changed, or the student's results can be displayed. All these methods use a private method, findStudent to locate the array element in which the student’s data is stored using the student number as the key, and to return the array index or -1 if the student is not found in the array. It uses a linear search since the data is arranged in student number order (the key field).

The diagram below illustrates the processing required:

When using AWT, the size of an object on the screen is measured in pixels (picture elements). The actual size will depend on the screen resolution. In other words, a graphics screen is made up of a grid of tiny rectangles (pixels), each of which can have a colour. The more pixels on your screen, the greater the resolution. For example, a screen resolution of 800 x 600 means there are 800 pixels across the screen and 600 down the screen. The screen coordinate system has its origin in the top left corner of the screen, and the x- coordinate is measured from the left across the screen and the y-coordinate is measured from the top down the screen. The easiest way to get going with graphics is to take a simple program and adapt it. Consider the following program that produces this window:

The hierarchy of objects is shown in this diagram. A line linking two objects means the lowe Class is derived from the higher class.

As before, the method actionPerformed defines the actions to take place when a button is clicked The constructor first defines the basic window requirements, including the layout manager to be used when adding components to the window. It then creates a panel (textPanel) and defines its background colour and layout manager. Next various components are defined and added to this panel: first a label (pwLabel) is constructed with some text and then a Specialised text field JPasswordField of a given size (30) is constructed. The J] PasswordField does not display the actual characters entered by the user. The password variable is an instance variable and is declared outside the constructor. This is because it needs to be referenced in the actionPerformed method and this could not be done if it were local to the constructor. Another general text field JTextField, called status, is constructed and set to not editable (textfields are editable by default). Note that status is an instance variable as well, and is also modified in the actionPerformed method. The textPanel is added to the window and another panel (buttonPanel) is defined to hold some buttons in a manner similar to before.

Both text areas and text fields have getText and setText methods that return the text entered as a String, or display a string in the text area/field. If the submit button is clicked the contents of the text field are obtained and a confirmation message is sent to the status text field. If the clear button is clicked an empty string is stored in the password text field.

Key takeaways

Everything in a Java program must be part of a class, and the statement public class Hello defines a class called Hello.
A simple program that uses this class is shown here.
12.5 Write a class, Marks, which has as data member an array of student numbers and of marks (%) and write methods to input the arrays; output only those that are above average (3.2); determine the pass mark for a 75% pass rate (3.3), and assign letter grade to the marks (3.4) 12.6 Write a method that will sort the student number array into ascending numeric order, and order the mark array in the same way (so that the first student number corresponds to the first mark etc) 12.7 Write a method that will determine the median mark and output the median mark and the student who achieved it.
In Java, String is a class, so strings are objects and not variables.
An inner class can be defined to represent a student with suitable attributes, and the course class can have an array of such student objects as attribute.

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