compsci 001 8.1 today’s topics parsing java programming notes from tammy bailey reading great...
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CompSci 001 8.1
Today’s topics
ParsingJava ProgrammingNotes from Tammy Bailey
ReadingGreat Ideas, Chapter 3 & 4
CompSci 001 8.2
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
CompSci 001 8.3
Java Details
Java tutorial http://java.sun.com/docs/books/tutorial1. Do “Your First Cup of Java” and create your First
Applet2. 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
CompSci 001 8.4
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 Javacompiler
Java bytecodes
CompSci 001 8.5
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
CompSci 001 8.6
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
CompSci 001 8.7
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
CompSci 001 8.8
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
CompSci 001 8.9
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
CompSci 001 8.10
Java primitive data types
Primitive Type Description Range
byte 8-bit integer -128 to 127
short 16-bit integer -32768 to 32767
int 32-bit integer-2147483648 to
2147483647
long 64-bit integer -263 to 263-1
float 32-bit floating point 10-46 to 1038
double 64-bit floating point 10-324 to 10308
char Unicode character
boolean Boolean variable false and true
CompSci 001 8.11
Basic operators
Operator Java Description
Assignment = assigns rhs to lhs
Arithmetic +,-,*,/,%addition, subtraction, multiplication, division, remainder
Unary -,++,--negative, auto increment, auto decrement
Equality ==, != equals to, not equals to
Relational <,<=,>,>=less than, less than or equals to, greater than, greater than or equals to
Logical &&,||,! AND, OR, NOT
CompSci 001 8.12
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)
CompSci 001 8.13
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’;
CompSci 001 8.14
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 before operators with low priority
operators with equal priority are evaluated left to right
Operator priority(highest to lowest)
1. ( ) 2. * / % 3. + - 4. =
CompSci 001 8.15
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
CompSci 001 8.16
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 liberallyint z = ((3 + 5) * (6)); (z = 48)
Equal priority operations are evaluated left-to-right in the absence of parenthesesint 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)
CompSci 001 8.17
Syntax and semantics
Addition, subtraction: + and –, int and doubleint x = 21+4; (x = 25) double y = 14.1-2; (y = 12.1)
Multiplication: *, int and doubleint x = 21*4; (x = 84)double y = 14.1*2.5; (y = 35.25)
Division: /, different for int and doubleint x = 21/4; (x = 5)double y = 21/4; (y = 5.0)double y = 21/4.0; (y = 5.25)
Modulus: %, only for intint x = 21%4; (x = 1)
CompSci 001 8.18
Automatic type conversion 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?a) 5b) 0.0c) 9.0d) 5.0e) 9
CompSci 001 8.19
More expressions
int g = 12 + 2.5;What is the value of g?a. 0b. 12c. 14d. 14.5
e. error
int x = 8 * (7 – 6 + 5) % (4 + 3 / 2) – 1;What is the value of x?a. -1b. 0c. 2d. 3
e. none of the above
int n = 1 – 2 * 3 – (4 + 5); What is the value of n?
CompSci 001 8.20
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++
}
}
CompSci 001 8.21
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++;
}
CompSci 001 8.22
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
CompSci 001 8.23
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)
CompSci 001 8.24
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
CompSci 001 8.25
Java String class
The String class represents character stringsString 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 */
CompSci 001 8.26
Instantiation
Creating an object is called instantiation the new operator is used with class name
Example: Create a TextField objectTextField t = new TextField();
Can create multiple instances of the same classTextField t1 = new TextField();TextField t2 = new TextField();
Exception the new operator is not required when creating a
String
CompSci 001 8.27
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
CompSci 001 8.28
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
CompSci 001 8.29
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
CompSci 001 8.30
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
CompSci 001 8.31
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);
CompSci 001 8.32
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?