Simple Java Programming Constructs 4

Transcription

1 Simple Java Programming Constructs 4 Course Map In this module you will learn the basic Java programming constructs, the if and while statements. Introduction Computer Principles and Components Software Development Language Rules and Constructs The Java Language Rules and Tools Simple Java Programming Constructs Advanced Java Programming Constructs Objects, Arrays, and Methods Object Orientation Methods Arrays Advanced Object Orientation 4-1

2 Relevance Discussion How do you write code to have your program make decisions? What thought processes do you go through in making a decision? Try to get students to suggest the if X, then Y; if A, else if B, else if C, and so on. Also ask them about situations that allow something to occur until a condition is met. 4-2 Java Programming for Non-Programmers

3 Objectives Upon completion of this module, you should be able to: Use simple Java programming constructs to make your programs more generic and versatile Use the if statement to make decisions within your programs Use the while statement to repeat part of your program Simple Java Programming Constructs 4-3

4 Simple Programming Constructs The word construct suggests a building block that has been designed to incorporate a sound structure. Two basic constructs required to make our programs more than just a sequence of statements executing one after another. These are: Conditions The ability to decide at runtime whether to perform certain statements is essential. The basic conditional branching statement is if. Loops The ability to decide at runtime how many times to perform certain statements is also essential. The basic looping statement is while. There are other condition and loop constructs that will be explained in the next module. The others are alternatives to if and while, however, and if you master these two then you will be able to code any situation your programs require. 4-4 Java Programming for Non-Programmers

5 Simple Programming Constructs Note All the constructs in this course apply to a single statement only. For example, you can choose between only two statements using the if construct. But remember that a statement can be replaced with a code block under all circumstances, and would be used when you need to have several statements executed after a decision or in a loop. Simple Java Programming Constructs 4-5

6 The if Construct Overview The if construct allows your program to make simple decisions based on stored values. The simplest form is: if (boolean expression) statement; As it executes the above statement, the JVM checks whether the Boolean expression is true or false. If it is true then the statement is executed; otherwise, it is ignored. Note All the condition and loop constructs work on the basis of a Boolean expression, something that is true or false. Conditions happen if the statement is true and loops continue while the Boolean expression is true. 4-6 Java Programming for Non-Programmers

7 The if Construct Overview (Continued) The following example uses a code block to conditionally execute several statements: if (boolean expression) { statement; statement; statement; } In fact, many programmers choose to use code blocks all the time, even when there is only one statement. For example: if (boolean expression) { statement; } Simple Java Programming Constructs 4-7

8 The if Construct Choosing Between Two Statements Often you will need to decide between two different statements with one condition. Extend the if statement with the else clause. For example: if (boolean expression) statement; else statement; This can also be used with code blocks. For example: 1 if (boolean expression) 2 { 3 statement; 4 statement; 5 statement; 6 } 7 else 8 { 9 statement; 10 statement; 11 statement; 12} 4-8 Java Programming for Non-Programmers

9 The if Construct Choosing From More Than Two Statements You can chain if and else structures together to permit the selection of one item from a series of statements or code blocks. For example, with single statements: if (boolean expression) statement; else if (boolean expression) statement; else statement; and with code blocks: 1 if (boolean expression) 2 { 3 statement; 4 statement; 5 } 6 else if (boolean expression) 7 { 8 statement; 9 statement; 10} 11else 12{ 13 statement; 14 statement; 15} Simple Java Programming Constructs 4-9

10 Logical Operations Table 4-1 lists the different conditions you can test for in conditional structures and loops. Table 4-1 Logical Operators Operation Operator Example is equal to == if (i == 1) is not equal to!= if (i!= 1) is less than < if (i < 1) is less than or equal to <= if (i <= 1) is greater than > if (i > 1) is greater than or equal to >= if (i >= 1) Remind students that = is assignment. Ask students what would happen in an if statement should they use = instead of ==. You will also need the ability to make a single decision based upon more than one comparison Under such circumstances you apply boolean logic and operators. Table 4-1 lists the Boolean operators in the Java programming language. Table 4-2 Boolean Operators Operation Operator Example AND && if ((i < 1) && (j > 6)) OR if ((i < 1) (j > 6)) NOT! if (!(i < 1)) 4-10 Java Programming for Non-Programmers

11 Exercise: Using the if Construct Exercise objective The objective of this exercise is to write an application that will generate a random number between 1 and 10 and print a message ( Bananas ) if the number is greater than 5. Tasks 1. Change directory to the MyApps directory. 2. Create a new subdirectory called If and change directory to it. 3. In a text editor, write a program called RandomOne that satisfies the above objective. 4. Run your application several times to make sure that you receive the message Bananas approximately half the time. 5. Change your program to print out the random number as well as the message to ensure your application is working properly. Note Remember to use the application structure outlined in the previous module. The following expression can be used to create a random integer number in the range of 1 and a variable called max. (int)((math.random() * max) + 1) This can be used (for example) in an assignment statement, to store the value in an integer variable: int i = (int)((math.random() * max) + 1); Random numbers were chosen as the core of these exercises because the above is a single expression that can be explained or taken on faith, but either way provides a way of creating a program that is self-contained and does something different each time it runs. At this stage students need to run things several times to properly learn the commandline stuff, so it s good from that perspective. Simple Java Programming Constructs 4-11

12 Exercise: Using the if Construct Tasks (Continued) 6. In the If directory, create an application called RandomTwo that generates two different random numbers in the range 1 to 10 called rand1 and rand2, and prints out the following messages based on their values: If rand1 is less than or equal to 3 print the message Bananas. If rand1 is greater than 3 and rand2 is less than or equal to 5 print the message Oranges. If rand1 is greater than 3 and rand2 is greater than 5 print the message Pears. 7. Compile the application and run it several times to see different messages appear. 8. Change your program so that it prints out the numbers rand1 and rand2 to ensure it is working correctly Java Programming for Non-Programmers

13 Exercise: Using the if Construct Exercise Summary Discussion Take a few minutes to discuss what experiences, issues, or discoveries you had during the lab exercises. Manage the discussion here based on the time allowed for this module, which was given in the About This Course module. If you find you do not have time to spend on discussion, then just highlight the key concepts students should have learned from the lab exercise. Experiences Ask students what their overall experiences with this exercise have been. You may want to go over any trouble spots or especially confusing areas at this time. Interpretations Ask students to interpret what they observed during any aspects of this exercise. Conclusions Have students articulate any conclusions they reached as a result of this exercise experience. Applications Explore with students how they might apply what they learned in this exercise to situations at their work place. Simple Java Programming Constructs 4-13

14 Arithmetic Concepts Arithmetic Operators The Java programming language incorporates the five basic math operations listed in Table 4-3. Table 4-3 Simple Math Operators Purpose Operator Example Addition + sum = num1 + num2 Subtraction - diff = num1 - num2 Multiplication * prod = num1 * num2 Division / quot = num1 / num2 Modulus % mod = num1 % num2 Operator Precedence All complex arithmetic calculations follow the rules of operator precedence: 1. Operators within pairs of parentheses are evaluated first, then 2. Multiplication and division operators 3. Addition and subtraction operators 4. If the same operator appears successively in a statement they are evaluated from left to right Java Programming for Non-Programmers

15 Arithmetic Concepts Operator Precedence (Continued) The following example demonstrates the need for operator precedence: c = 25-5 * 4 / ; It is not clear what the author intended, either from the statements or any supporting comments, but the subsequent value of c is 9: c = 25 - (5 * (4 / 2)) ; It is important to use parentheses to provide the intended structure: c = (((25-5) * 4) / (2-10)) + 4; c = ((20 * 4) / (2-10)) + 4; c = (80 / (2-10)) + 4; c = (80 / -8) + 4; c = ; c = -6; Many programmers insert parentheses to emphasize their intention even when the precedence is not in doubt. Simple Java Programming Constructs 4-15

16 Arithmetic Concepts Increment and Decrement Operators A common requirement of a program is to add or subtract a value from a variable In the next example, 9 is subtracted from the variable count. count = count - 9; In the next example, 5 is added to the variable score. testscore = testscore + 5; A more common requirement is to add or subtract 1 from a variable. Table 4-4 lists the increment and decrement operators and their syntax. Table 4-4 Increment and Decrement Operators Purpose Operator Example Pre-Increment ++ j = ++i; Post-Increment ++ j = i++; Pre-Decrement -- j = --i; Post-Decrement -- j = i--; 4-16 Java Programming for Non-Programmers

17 Arithmetic Concepts Increment and Decrement Operators (Continued) Using these operators can lead to confusion. It is possible to increment and decrement variables inside other expressions. For example: int i = 6; int j = ++i; The first line declares an int variable called i and assigns it the value 6. The second line declares an int variable called j and assigns it the value ++i. The pre-increment operator means increment the variable before using it inside the expression, so i becomes 7 before it is assigned to j, which also becomes 7. The following example is the same as above but uses the postincrement operator instead. int i = 6; int j = i++; This time the variable i is incremented after it is used in the assignment expression, so j has the value 6 and i becomes 7. Many programmers do not use these. It is more readable to explicitly increment i before or after assigning the value to j. For example int i = 6; int j = i; i = i + 1; The increment and decrement operators are inherited from the C language. Many C and C++ programmers use them in preference to other ways of adding 1 to a value. The name C++ was invented because of the increment operator. It is meant to suggest that the C++ language is an increment or improvement on C. Simple Java Programming Constructs 4-17

18 Control Flow Statements The while Loop There will be times when you want to perform a repetitive action until a certain condition is met. A while statement can be used for this purpose. The general while statement looks like: while (condition) statement; You can perform more than one statement using a code block: while (condition) { statement; statement; } 4-18 Java Programming for Non-Programmers

19 Control Flow Statements The while Loop (Continued) The Java code to use for a while statement that loops through the numbers 10, 20, 30 up to 100 may look like: int count = 10; while (count <= 100) { // statement or statements count = count + 1; } Because the condition is at the top of the loop structure it is tested first. If it is false immediately then the statements inside the loop will not be executed at all. This is called a zero or many iterative loop. Simple Java Programming Constructs 4-19

20 Exercise: Using the while Construct Exercise objective The objective of this exercise is to write some programs using the while construct. Tasks 1. Change directory to the MyApps directory. 2. Create a new subdirectory called While and change directory to it. 3. Write a program called WhileOne that uses a while loop to print the values from 1 to Write a program called WhileSquares that uses a while loop to print the first 10 square numbers (a square number is a number which is the square of another, so 9 is a square number because it is 3 * 3). 5. Write a program called WhileTriangles that uses a while loop to print the first 10 triangle numbers. (Imagine stacking circular tins on their sides. If you start with four along the bottom you could place three in the row above, then two above that, then one at the top. The number of tins you have used to do this ( )is the fourth triangle number: 1 is the first triangle number, and so on.) 4-20 Java Programming for Non-Programmers

21 Exercise: Using the while Construct 6. Write a program called WhileTimes that uses while loops to print the first 10 times tables Note The statement System.out.println("...") prints the string in the quotes and leaves the cursor on the next line (the ln in the method name means linefeed ). To print something out and leave the cursor at the end of that line so that your next text to print appears beside the first, use System.out.print("...") instead. Simple Java Programming Constructs 4-21

22 Exercise: Using the while Construct Exercise Summary Discussion Take a few minutes to discuss what experiences, issues, or discoveries you had during the lab exercises. Manage the discussion here based on the time allowed for this module, which was given in the About This Course module. If you find you do not have time to spend on discussion, then just highlight the key concepts students should have learned from the lab exercise. Experiences Ask students what their overall experiences with this exercise have been. You may want to go over any trouble spots or especially confusing areas at this time. Interpretations Ask students to interpret what they observed during any aspects of this exercise. Conclusions Have students articulate any conclusions they reached as a result of this exercise experience. Applications Explore with students how they might apply what they learned in this exercise to situations at their work place Java Programming for Non-Programmers

23 Check Your Progress Before continuing on to the next module, check that you are able to accomplish or answer the following: Use simple Java programming constructs to make your programs more generic and versatile Use the if statement to make decisions within your programs Use the while statement to repeat part of your program Simple Java Programming Constructs 4-23

24 Think Beyond Are there other decision constructs and other types of loops available for programming? If so, what differences exist between the different constructs? 4-24 Java Programming for Non-Programmers