Today’s topics

1. Today’s topics Parsing Java Programming Notes from Tammy Bailey Reading Great Ideas, Chapter 3 & 4

2. Java programs • Java programs are created as text files using a text editor (like emacs) • Save to disk with .java file extension HelloWorld.java • The file contains characters (stored as bytes) • file can be printed, displayed on monitor, or edited • file cannot be directly executed (run) by the computer system • Java must first translate the program into bytecodes before it can be run

3. Bytecodes • Java bytecode • machine instruction for the Java processor • Java compiler javac translates the source program into bytecodes • Bytecode file has same name as the source program with a .class file extension: HelloWorld.class HelloWorld.java javac HelloWorld.class source program Java bytecodes Javacompiler

4. Java Virtual Machine (JVM) • Bytecode (class) file will contain exactly the same bytecodes no matter what computer system is used • Bytecode file is executed by a Java bytecode interpreter • processor specific executable program • Each type of computer system has its own Java interpreter that can run on that system • Any computer system can execute Java bytecode programs if it has a Java interpreter • Computers with Java interpreters are called Java Virtual Machines • a “computer” with a Java processor that can run Java bytecodes

5. Java applets • An applet is a Java bytecode program that runs on a Web browser • Most newer Web browsers have Java interpreters • Web pages on the Internet contain instructions that send Java bytecodes to your computer • Web browser runs the Java applet with its built-in interpreter

6. Data types • Computer memory stores arbitrary bit patterns • Meaning of a bit pattern depends on its use • Pattern used for a particular string of bits is a data type • values are any kind of data a computer can process • all values are represented using some data type • Example: What does the following pattern of 16 bits represent? 0000000001100111 • No way to know without more information • If data type is short (a Java type) it represents 103

7. Java data types • Primitive • types of data that are so fundamental ways to represent them are built into Java • Object • built-in or user-defined

8. Primitive data types • All primitive values belong to one of eight primitive types byte short int long floatdouble char boolean • Primitive data types use a fixed number of bytes • four of these types designate different sizes of bounded integers: byte, short, int, long • A programmer can not create new primitive data types • Any data type you invent will be a type of object • Most commonly used types in practice: int, boolean, and double

9. Java primitive data types

10. Basic operators

11. Variable declaration • Declarationtype <variable-name>; • Declaration + initialization: type <variable-name> = <value>; • Variable names • any combination of letters, numbers, and the underscore character • may not start with number • may not be reserved word • e.g. int, return, if, for, while • may not be same as a subroutine name • case-sensitive (num and Num are different)

12. Examples • int x, y, z; • int sum = 0; • float f; • double pi = 3.14; • char first = ‘T’, middle = ‘L’, last = ‘B’; • char first = ‘T’; char middle = ‘L’; char last = ‘B’;

13. Operator precedence • Evaluate a + b * c • multiplication first? a + (b * c) • addition first? (a + b) * c • Java solves this problem by assigning priorities to operators (operator precedence) • operators with high priority are evaluated beforeoperators with low priority • operators with equal priority are evaluated left to right Operator priority (highest to lowest) • ( ) • * / % • + - • =

14. When in doubt, use parentheses • a + b * c = a + (b * c) • because * has higher priority than + • To perform the + operation first we need to use parentheses • (a + b) * c • If in any doubt use extra parentheses to ensure the correct order of evaluation • parentheses are free! • cause no extra work for the computer • only make it easier for you to work out what is happening

15. Examples • Java adheres to traditional order of operations • * and / have higher priority than + and – int x = 3 + 5 * 6; (x = 33) int y = (3 + 5) * 6; (y = 48) • Parentheses are free, use them liberally int z = ((3 + 5) * (6)); (z = 48) • Equal priority operations are evaluated left-to-right in the absence of parentheses int w = 3 * 4 / 2 * 6; (w = 36) int x = 3 * 4 / (2 * 6); (x = 1) int y = 3 * 4 + 2 * 6; (y = 24) int z = 3 * (4 + 2) * 6; (z = 108)

16. Syntax and semantics • Addition, subtraction: + and –, int and double int x = 21+4; (x = 25) double y = 14.1-2; (y = 12.1) • Multiplication: *, int and double int x = 21*4; (x = 84) double y = 14.1*2.5; (y = 35.25) • Division: /, different for int and double int x = 21/4; (x = 5) double y = 21/4; (y = 5.0) double y = 21/4.0; (y = 5.25) • Modulus: %, only for int int x = 21%4; (x = 1)

17. Mixed type expressions are converted to higher compatible types If all operands are of type int then result is type int If any operand is of type double then result is of type double Cannot convert to a lower type Conversion may result in loss of precision Example: Convert Fahrenheit to Celsius double F=41.0; double C=(F-32.0)*(5/9); Question: What is the value of C? 5 0.0 9.0 5.0 9 Automatic type conversion

18. int g = 12 + 2.5; What is the value of g? 0 12 14 14.5 error int x = 8 * (7 – 6 + 5) % (4 + 3 / 2) – 1; What is the value of x? -1 0 2 3 none of the above More expressions int n = 1 – 2 * 3 – (4 + 5); What is the value of n?

19. Syntax errors • The following Java subroutine computes the inclusive sum between two integers. Find all the syntax errors. int sumBetween( x, y ) { int z = x; Int sum = 1; while( z <= y ){ sum = sum*z; z++ } }

20. Logic errors • The computer will do precisely what you say even though it may not be what you want • What is wrong with this code? int sumBetween( int x, int y ) { int z = x; int sum = 1; while( z <= y ) sum = sum*z; z++; }

21. Java objects • Java is an object-oriented programming language • use objects to define both the data type and the operations that can be applied to the data • Objects have attributes and functionality • attributes describe the state of the object • the functionality of an object is the set of actions the object can perform • In Java, we define an object’s attributes using variables and its functionality using methods

22. Real-world objects • Suppose we want to describe a car in terms of its attributes and functionality • Attributes: • int year; int mileage; • String make; String model; • boolean manual_transmission; • Methods: • void brake() • int getMileage() • boolean needsGas() • void shift(int gear)

23. Java classes • Java objects are created using classes • Encapsulation • combining elements to create a new entity • A class encapsulates the variables and methods that define an object • Instantiation • the act of creating an object • objects are called class instances • Java provides many predefined classes • You can also define your own classes

24. Java String class • The String class represents character strings String first = “Tammy”; String last = “Bailey”; • Strings can be concatenated (added together) using the concatenation operator + String fullname = first + “ ” + last; • Testing for equality: first.equals(“Tammy”); /* returns true */ first.equals(“Amy”); /* returns false */

25. Instantiation • Creating an object is called instantiation • the new operator is used with class name • Example: Create a TextField object TextField t = new TextField(); • Can create multiple instances of the same class TextField t1 = new TextField(); TextField t2 = new TextField(); • Exception • the new operator is not required when creating a String

26. Java TextField class • The TextField class allows the editing and display of a single line of text TextField t = new TextField(); • Methods • setText(String s) • set the text of the field to the string s • String getText() • get the text of the field and assign it to a variable of type string

27. Invoking an object’s methods • Once we create a text field, we can perform actions on it using its methods • The variables and methods of an object are accessed using the dot operator TextField t = new TextField(); t.setText(“Hello”); • Syntax • object.verb(data); • Perform verb on object using data

28. Interactive objects • User interaction determines the behavior of the program • Program receives user input through mouse and keyboard and performs associated method or action • Text fields • edit and display single line of text • Buttons • can specify action to occur when button is clicked

29. Action listeners • If we want a button to know when it is clicked, we have to enable it to “listen” for user input • Use the button method addActionListener Button b = new Button(“click!”); b.addActionListener(this); • If we don’t invoke the addActionListener method on a button, nothing will happen when the button is clicked

30. Example • We would like our applet to do the following: • get text from text field t1 and display it in text field t2 when button b is clicked TextField t1 = new TextField(); TextField t2 = new TextField(); Button b = new Button(“copy text”); b.addActionListener(this);

31. Numeric input • Suppose we want an applet that allows the user to enter two integers and display the minimum • a text field contains a character string • If we want to perform numeric operations on the input from a text field, we have to convert the string to a numeric data type • numbers are primitive data types, not objects • Can convert using Java type wrappers

32. If-Then statements if (logical expression) { “true” actions } If-Then-Else statements if (logical expression) { “true” actions } else (logical expression 2) { “false” actions } Logical expressions analogous to yes or no questions true or false Statements that are true (5 < 7) (100 == 100) (100 != 10) (10 <= 10) Statements that are false (-2 > -1) (10 != 10) Decision trees

33. A decision tree 3 He received the Physics Price in 1921. 1 Would you like to read about Einstein? 4 Try the Medicine Prize in 1962. 0 Would you like to read about a scientist? 5 Look up the Peace Prize in 1991. 2 Would you prefer a humanitarian? 6 Try A. Solzhenitsyn, Literature 1970.

34. Assignment statement variable = expression; Method invocation Also called function or procedure Invoking also called “calling” a function Methods can take arguments button.setText(“This text is an argument”); init() Variable declaration VariableType variableName; Button choice; More Java Syntax

35. Java Details • Java tutorial http://java.sun.com/docs/books/tutorial • Do “Your First Cup of Java” and create your First Applet • Go to “Learning the Java Language” and read “Language Basics” • Variable: an item of data named by an identifier • Operators • Arithmetic • Relational and conditional • Assignment • Other • Expression: a series of variables, operators, and method calls that evaluates to a single value

36. Dealing with numbers • Primitive data type: int • Does not require a new statement to create • Primitive types not classes • Must declare • Should initialize (Java sets to 0) • Other primitive types include: boolean, char, double • Operations using integers • +, -, *, /, % • Operator Precedence

37. Some arithmetic details • Java adheres to traditional order of operations • * and / have higher precedence than + and – int x = 3 + 5 * 6; int y = (3 + 5) * 6; • Parentheses are free, use them liberally • Arithmetic expressions are evaluated left-to-right in the absence of parentheses int x = 3 * 4 / 6 * 2; int y = (3*4)/(6*2); • There are limits on int and double value, be aware of them.

38. Types for Numbers • The type String is not a built-in type, technically it’s a class • There are many numerical types in Java We’ll use two • int, represents integers: {…-3,-2,-1,0,1,2,3,…} • Conceptually there are an infinite number of integers, but the range is limited to [-231, 231-1] or [Integer.MIN_VALUE,Integer.MAX_VALUE] • Alternatives? Why is range limited? • double, represents real numbers like , 2 • Not represented exactly, so expressions like 100*0.1 may yield unexpected results • Double precision floating point numbers, another type float exists, but it’s a terrible choice (generates poor results)

39. GIGO: program as good as its data? • In calculations involving floating point numbers it’s easy to generate errors because of accumulated approximations: • What is 1023 + 1? • When is (x + y) + z different from x + (y + z) ? • The type int is severely constrained on 16-bit computers, e.g., running DOS, largest value is 32,767 (215-1) • Even on 32-bit machines, how many seconds in a millennium? 60*60*24*365*1000, problems? • On UNIX machines time is measure in seconds since 1970, problems? • What was Y2K all about?

40. What arithmetic operations exist? • Syntax and semantics for arithmetic operations • Addition, subtraction: + and –, int and double 23 + 4x + yd – 14.0 + 23 • Multiplication: *, int and double 23 * 4 y * 3.0 d * 23.1 * 4 • Division: /, different for int and double 21 / 4 21 / 4.0 x / y • Modulus: %, only for int 21 % 4 17 % 2 x % y • Mixed type expressions are converted to “higher” type • Associativity of operators determines left-to-right behavior • Use parentheses liberally • Without () use operator precedence, *,/, % before +,-

41. Dealing with text • Strings are a class and not a primitive datatype • Declaration: String message; • String Constants “Good Morning World!” • String Assignment message = "It's Friday";

42. Manipulating Strings • Methods for manipulation int length() int indexOf(String st) String substring(int start, int end) • Getting String Data from user • The TextField class has getText() method • Use: message = mg.getText(); • where mg is a TextField and message is a String

43. Order of precedence Automatic type conversion Values of one type are promoted to another compatible type as part of the computation process You can convert Tf degrees Fahrenheit to Tc degrees Celsius using the formula: Tc = (5/9)*(Tf-32) Given the following expression: double Tc = (Tf – 40.0) * (5/9) If Tf is –40.0 what is Tc? -40.0 0.0 40.0 error unknown Evaluating expressions

44. int n = 1 - 2 * 3 - 4 + 5; What is n? -4 -2 0 2 4 error int n = 12 + “hello” 0 12 17 unknown errror int x = 8 * (7 – 6 + 5) % (54 + 3 / 2) – 1; What is x? -1 0 2 3 error something else More expressions

45. Repeating code • Repeating code is bad • Writing repetitive code is tedious • Debugging repetitive code is hard • Avoid repeating code through: • Subroutines/methods • Loops

46. Loops • If statements need to repeat, then you probably need a loop • Describe portion of program as: • Repeat • Continue until • For each value from 1 to n • For every object of a set, do something • We have already used iteration by using the buttons • How?

47. Problems • We want to: • Print out all numbers from 0up to 100 incrementing by 0.5 each time • Sum up the numbers from 1 to 100 • … • New Java syntax • New object type TextArea which is basically a big scrolling textbox • tArea is 80 character wide and 20 rows high text box with 20 rows TextArea tArea = new TextArea(20,80); • Add characters to the end of the TextArea using append tArea.append(“Hello\n”); • ‘\n’ is called a newline character which moves the next character to the next line

48. Anatomy of a while loop • While loops are one way to get rid of repetitive code • Print out numbers up to 100 by increments of 0.5 x = 0.0; while (x < 100) { x = x + 0.5; tArea.append(“x = “ + x); tArea.append(“\n”); } x  0 x < 100 x  x + 0.5 print x true false

49. Another loop • Summing the numbers 1 … 100 int sum = 0; int k = 0; while (k < 100) { k = k + 1; sum = sum + 1; } • Other Loop designs • Count down • Stopping and starting at computed values • Data dependent loop

50. Functions/Methods • Function example: distance from point (x,y) to origin • Function declaration • Name of the function • Type of each argument to the function with a descriptive name for each argument • The type of value a function returns