This document provides information about the CS3101-3 Programming Language - JAVA course for Fall 2004. It introduces the instructor, Ke Wang, and his contact information. It states the class will meet on Wednesdays from 11am-1pm for 6 weeks ending on October 20th. There will be 5-6 homework assignments due on Tuesdays at 11:59:59pm. Late submissions are allowed once with a 24-hour extension. The document outlines topics that will be covered in the course like Java basics, objects, classes, inheritance, and GUI programming. It provides references to online textbooks and tutorials that can be used. Finally, it encourages students to practice programming to learn the language.
4. Homework
5 or 6 homework
One homework per week
All programming
Goes out every Wed night
Due next Tuesday 11:59:59pm
Submission and HW return eletronically
Grade percentage to be determined
Final?
5. Late policy
You have one 24-hour extension
Can be used only once
Otherwise, no late homework will be
accepted
6. Academia Integrity
The work you submit should be
implemented BY YOURSELF
Can get help from me, or friends
Must acknowledge all help given.
8. Textbook
No required textbook
Most of the information you need can be found online,
especially at http://java.sun.com
Tutorials and code camp:
http://java.sun.com/learning/tutorial/index.html
Java API specification:
http://java.sun.com/j2se/1.4.2/docs/api/index.html
Important one! Should visit often when coding
9. Reference books
Core Java 2, Volume I: Fundamentals
Core Java 2, Volume II: Advanced
Features
Thinking in Java, 3rd Edition
Electronic version available:
http://64.78.49.204/
JAVA in a Nutshell (fourth Edition)
10. A word about learning a programming
language
PRACTICE
PRACTICE
…
PRACTICE
…
11. road map today
Brief intro to Java
Develop and compile environment
A simple example
Intro to object/class
Java basics
Differences from C
12. Intro to Java
Java programming language
The one we use to write our program
Compiled to byte code of JVM
Java virtual machine (JVM)
Java interpreter – interpret the compiled byte code
Software simulated CPU architecture
Cross-platform: support Linux, Windows, PalmOS…etc.
Java runtime environment (JRE)
Predefined set of java classes available to use
Core Java APIs – basic utilities, I/O, graphics,
network…
13. Java is portable,
As long as there is a JVM compiled for
that particular processor and OS
Typical program, like C or C++, is
compiled for a particular processor
architecture and OS.
“Write once, run everywhere!”
Sun’s motto for Java
14. Bottom line: slow but safe
Not suitable for high-performance computation
Scientific computation, games, OS kernel
Compiled to byte codes of JVM, not native machine
language instructions
New release of Java is improving the speed a lot
Just-in-time (JIT) compiler: convert byte codes to native
machine language on the fly
Very safe
No pointer
Automatic garbage collection
Check array access bound
15. Java
Java is an object-oriented language, with
a syntax similar to C
Structured around objects and methods
A method is an action or something you do with
the object
Avoid those overly complicated features of
C++:
Operator overloading, pointer, templates, friend
class, etc.
16. Getting and using java
J2SDK freely download from http://java.sun.com
“Your first cup of Java”:
detailed instructions to help you run your first program
http://java.sun.com/docs/books/tutorial/getStarted/cupojava/index.h
All text editors support java
Vi/vim, emacs, notepad, wordpad
Just save to .java file
Have IDEs that comparable to Visual Studio
JCreator (simple)
Eclipse (more complicated)
17. Compile and run an application
Write java class Foo containing a main()
method and save in file “Foo.java”
The file name MUST be the same as class
name
Compile with: javac Foo.java
Creates compiled .class file: Foo.class
Run the program: java Foo
Notice: use the class name directly, no .class!
18. Hello World!
/* Our first Java program – Hello.java */
public class Hello {
//main()
public static void main ( String[] args ) {
System.out.println( "hello world!" );
}
}
File name: Hello.java
Command line
arguments
Standard output, print with new line
19. About class
Fundamental unit of Java program
All java programs are classes
Each class define a unique kind of object (
a new data type)
Each class defines a set of fields, methods
or other classes
public: modifier. This class is publicly
available and anyone can use it.
20. Things to notice
Java is case sensitive
whitespace doesn’t matter for compilation
File name must be the same as one of the
class names, including capitalization!
At most one public class per file
If there is one public class in the file, the
filename must be the same as it
Generally one class per file
21. What is an object?
Object is a thing
An object has state, behavior and identity
Internal variable: store state
Method: produce behavior
Unique address in memory: identity
An object is a manifestation of a class
22. What is class?
Class introduces a new data type
A class describes a set of objects that
have identical characteristics (data
elements) and behaviors (methods).
Existing classes provided by JRE
User defined classes
Once a class is established, you can make
as many objects of it as you like, or none.
23. Simple example: class Person
A Person has some attributes
The class defines these properties for all
people
Each person gets his own copy of the
fields
Attributes = properties = fields
24. Class Person: definition
class Person {
String name;
int height; //in inches
int weight; //in pounds
public void printInfo(){
System.out.println(name+" with height="+height+", weight="+weight);
}
}
class ClassName{ /* class body goes here */ }
class: keyword
25. Class Person: usage
Person ke; //declaration
ke = new Person(); //create an object of Person
ke.name= “Ke Wang”; //access its field
Person sal = new Person();
sal.name=“Salvatore J. Stolfo”;
ke.printInfo();
Sal.printInfo(); // error here??
26. Class Person
Name: Ke Wang
height: 0
weight: 0
Name: Salvatore J. Stolfo
height: 0
weight: 0
ke
sal
27. Class Person: variables
Person x;
x=ke;
x.printInfo();
x=sal;
x.printInfo();
This gives the same output as previous code !
28. Class Person: variables
Name: Ke Wang
height: 0
weight: 0
Name: Salvatore J. Stolfo
height: 0
weight: 0
ke
sal
x
references objects
29. Reference
We call x, as well as ke and sal, “reference” to
the object
Handles to access an object
Reference itself is not accessible/manipulable
Different from C/C++, cannot increment/decrement it
Implemented as pointer+
Java runtime is watching all assignment to references
Why? – garbage collection (later)
30. Reference
Person ke; //only created the reference, not an object.
It points to nothing now (null).
ke = new Person(); //create the object (allocate storage
in memory), and ke is initialized.
ke.name=“Ke Wang”; //access the object through
the reference
31. More on reference
Have distinguished value null, meaning
pointing to nothing
if( x==null) { … }
Multiple references can point to one object
When no reference point to an object, that
object is never accessible again.
32. Class Person: problem
ke.weight = 150; // too bad, but possible
ke.weight = -20; // Houston, we have a problem!!
Need to ensure the validity of value.
Solution: ask the class to do it!
ke.setWeight(150); // OK, now ke’s weight is 150
ke.setWeight(-10); ******** Error, weight must be positive number
33. Class Person: add method
class Person{
...
void setWeight(int w){
if(w<=0)
System.err.println("***** error, weight must be positive
number! ");
else
weight = w;
}
}
34. Class Person: new problem
ke.setWeight(-10);
******** Error, weight must be positive number
ke.weight = -20; //haha, I’m the boss!
How about we forgot to use the set function? Or
we just don’t want to?
Solution: just make the variable inaccessible
from outside!
35. Class Person: private variable
class Person{
private String name;
private int weight;
private int height;
public void setWeight(int w){
if(w<=0)
System.err.println("***** error, weight must be positive
number! ");
else
weight = w;
}
}
Keyword private: no one can access the element except itself
Keyword public: everyone can access the element
36. Class Person
class Hello{
public static void main ( String[] args ) {
Person ke = new Person();
ke.weight = -20;
}
}
>javac Hello.java
Hello.java:5: weight has private access in Person
ke.weight = -20;
^
1 error
37. Access functions
Generally make fields private and provide
public getField() and setField() access
functions
O-O term for this is Encapsulation
C# does this by default
38. Java Basics: primitive types
One group of types get special treatment
in Java
Variable is not created by “new”, not a
reference
Variable holds the value directly
39. Primitive types
Primitive type Size Minimum Maximum Wrapper type
boolean 1-bit — — Boolean
char 16-bit Unicode 0 Unicode 216
- 1 Character
byte 8-bit -128 +127 Byte
short 16-bit -215
+215
-1 Short
int 32-bit -231
+231
-1 Integer
long 64-bit -263
+263
-1 Long
float 32-bit IEEE754 IEEE754 Float
double 64-bit IEEE754 IEEE754 Double
40. Primitive types
All numerical types are signed!
No unsigned keyword in Java
The “wrapper” class allow you to make a
non-primitive object to represent the
primitive one
char c =‘a’;
Character C = new Character(c);
Character C = new Character(‘a’);
41. Primitive types - boolean
boolean can never convert to or from other
data type, not like C or C++
boolean is not a integer
if(0) doesn’t work in java
Have to explicitly state the comparison
if( x ==0) {
42. Primitive types - char
Char is unsigned type
The Character wrapper class has several
static methods to work with char, like
isDigit(), toUpperCase() etc.
43. Default values for primitive members
When a primitive type
data is a member of a
class, it’s guaranteed
to get a default value
even if you don’t
initialize it.
Not true for those
local variables!!
There will be compile
error if you use it
without initialization
Primitive type Default
boolean false
char ‘u0000’ (null)
byte (byte)0
short (short)0
int 0
long 0L
float 0.0f
double 0.0d
44. Example
class Hello{
public static void main ( String[] args ) {
int x;
System.out.println(x);
}
}
>javac Hello.java
Hello.java:5: variable x might not have been initialized
System.out.println(x);
^
1 error
45. Arrays in Java
An ordered collection of something, addressed
by integer index
Something can be primitive values, objects, or even
other arrays. But all the values in an array must be of
the same type.
Only int or char as index
long values not allowed as array index
0 based
Value indexes for array “a” with length 10
a[0] – a[9];
a.length==10
Note: length is an attribute, not method
46. Arrays in Java: declaration
Declaration
int[] arr;
Person[] persons;
Also support: int arr[]; Person persons[];
(confusing, should be avoided)
Creation
int[] arr = new int[1024];
int [][] arr = { {1,2,3}, {4,5,6} };
Person[] persons = new Person[50];
47. Arrays in Java: safety
Cannot be accessed outside of its range
ArrayIndexOutOfBoundsException
Guaranteed to be initialized
Array of primitive type will be initialized to their
default value
Zeroes the memory for the array
Array of objects: actually it’s creating an array
of references, and each of them is initialized to
null.
48. Arrays in Java:
second kind of reference types in Java
int[] arr = new int [5];
arr
int[][] arr = new int [2][5];
arr[0]
arr[1]
arr
49. More on reference
Java doesn’t support & address of , or *, ->
dereference operators.
reference cannot be converted to or from
integer, cannot be incremented or decremented.
When you assign an object or array to a
variable, you are actually setting the variable to
hold a reference to that object or array.
Similarly, you are just passing a reference when
you pass object or array to a method
50. Reference vs. primitive
Java handle objects and arrays always by
reference.
classes and arrays are known as reference types.
Class and array are composite type, don’t have
standard size
Java always handle values of the primitive types
directly
Primitive types have standard size, can be stored in a
fixed amount of memory
Because of how the primitive types and objects
are handles, they behave different in two areas:
copy value and compare for equality
51. copy
Primitive types get copied directly by =
int x= 10; int y=x;
Objects and arrays just copy the
reference, still only one copy of the object
existing.
Name: Ke Wang
height: 0
weight: 0
ke
x
Person ke =new Person();
ke.name="Ke Wang";
Person x=ke;
x.name="Sal";
System.out.println(ke.name); // print Sal!
52. Compare equality
Primitive use ==, compare their value directly
int x = 10; int y=10;
if(x==y) { // true !
Object or array compare their reference, not
content
Person ke =new Person();
ke.name="Ke Wang";
Person ke2 =new Person();
ke2.name="Ke Wang";
if(ke==ke2) //false!!
Person x = ke;
if(ke==x) //true
53. Copy objects and arrays
Create new object, then copy all the fields
individually and specifically
Or write your own copy method in your class
Or use the special clone() method (inherited by
all objects from java.lang.Object)
int[] data = {1,2,3}; //an array
int[] copy = (int[]) data.clone(); //a copy of the array
Notice: clone() is shallow copy only! The copied object or
array contains all the primitive values and references in the
original one, but won’t clone those references, i.e., not
recursive.
54. Compare objects and arrays
Write you own comparison method
Or use default equals() method
All objects inherit equals() from Object, but
default implementation simply uses == for
equality of reference
Check each class for their definition of equals()
String s = "cs3101";
int num=3101;
String t ="cs"+num;
if(s.equals(t)) { //true!
Notice: + operator also
concatenate string. If either of the
operand to + is a string, the
operator converts the other
operand to a string
55. Scoping
Scope determines both the visibility and lifetime
of the names defined within the scope
Scope is determined by the placement of {},
which is called block.
{
int x = 10;
//only x available
{
int y = 20;
//both x and y available
}
//only x available, y out of scope!
}
56. Scoping
Notice, you cannot do the following,
although it’s legal in C/C++.
{
int x = 10;
{
int x = 20;
}
}
Compile error
Hello.java:6: x is already defined in
main(java.lang.String[])
int x =20;
^
1 error
57. Scope of objects
When you create an object using new, the
object hangs around past the end of the
scope, although the reference vanishes.
{
String s = new String("abc");
}
Reference s vanishes, but the String
object still in memory
Solution: Garbage Collector!
58. Garbage collector
In C++, you have to make sure that you
destroy the objects when you are done
with them.
Otherwise, memory leak.
In Java, garbage collector do it for you.
It looks at all the objects created by new and
figure out which ones are no longer being
referenced. Then it release the memory for
those objects.
59. Importing library
If you need any routines that defined by
java package
import java.util.*;
import java.io.*;
Put at the very beginning of the java file
java.lang.* already been imported.
Check javadoc for the classes
60. Static keyword
Want to have only one piece of storage for a
data, regardless how many objects are created,
or even no objects created
Need a method that isn’t associated with any
particular object of this class
static keyword apply to both fields and methods
Can be called directly by class name
Example: java.lang.Math
Non-static fields/methods must be called
through an instance
61. main()
class Hello{
int num;
public static void main(String[] args) {
num = 10;
}
}
>javac Hello.java
Hello.java:4: non-static variable num cannot be referenced
from a static context
num = 10;
^
1 error
62. Main() doesn’t belong in a class
Always static
Because program need a place to start, before any
object been created.
Poor design decision
If you need access non-static variable of class
Hello, you need to create object Hello, even if
main() is in class Hello!
class Hello{
int num;
public static void main(String[] args){
Hello h = new Hello();
h.num = 10;
}
}
63. Difference between C and Java
No pointers
No global variable across classes
Variable declaration anywhere
Forward reference
Method can be invoked before defined
Method overloading
As long as the methods have different parameter lists
No struct, union, enum type
No variable-length argument list
65. Input: importing library
Need routines from java.io package
import java.io.*;
System.in is not ready to use, need to build a
fully functional input object on top of it
InputStreamReader(System.in)
Basic byte-to-char translation
BufferedReader(InputStreamReader isr)
Allows us to read in a complete line and return it as a
String
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
//BufferedReader in = new BufferedReader(new FileReader(filename));
String line = in.readLine();
67. Integer.parseInt()
Static method
Can take the String returned by readLine()
and spit out an int
Throws NumberFormatException if String
cannot be interpreted as an integer