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. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Java primitive data types

11. Basic operators

12. 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)

13. 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’;

14. 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) • ( ) • * / % • + - • =

15. 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

16. 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)

17. 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)

18. 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

19. 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?

20. 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++ } }

21. 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++; }

22. 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

23. 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)

24. 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

25. 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 */

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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);

32. Think about it Puzzle: Toggling Frogs • You have 100 light switches, numbered 1-100, and 100 frogs, also numbered 1-100. • Whenever a frog jumps on a light switch, it toggles a light between on and off. All lights are initially off. • frog #1 jumps on every light switch (ie turning them all on). • frog #2 jumps on every 2nd light switch, toggling some of them back off. ... • frog #k jumps on every kth light switch. • After 100 frogs, which lights are on? Game: Don’t be last • You and a friend have a stack of 10 coins. • On each person's turn, they remove either 1 or 2 coins from the stack. • The person who removes the last coin wins. • What is a winning strategy? Should you go first or second?