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CS 335 Lecture 02
Java Programming
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Programming in Java
Define data
z Calculate using data
z Output result
Java is object-oriented:
z
• Merge data and functions into object
• Invoke functions to operate on data
Java program must:
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• Define data and functions (in a class)
• Invoke functions to compute things
Object-Oriented Programming: Classes
A class is an object definition, and includes data and
functions on that data:
public class MyCourseGrade
{
int pset1;
int pset2;
:
int final_exam;
computeAverage()
{
:
}
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}
object
data
object
method
Classes
Class: code which defines an object
Object: a variable (data + methods) which is an instance of a class
Java program: a bunch of class definitions, variables, etc.
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Classes
One special class (the "mother of all classes") contains main(),
and this is where flow of control begins:
class Test
main
Kernel of execution is here!
class AnotherClass (like MyCourseGrade)
memberFunc1()
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memberFunc2()
Notes
z
z
z
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Java flow of control starts in main(), in
whichever class main() is defined
There can only be one class per file (unless
you are defining subclasses)
The filename must match the class name in a
Java source file!
Example 1
A Java program with one class and one member called main():
import java.io.*;
public class Test
{
public static void main( String[] )
throws IOException
{
System.out.println("That it, folks!");
}
}
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Example 1: Scoping
class Test
member functions
main()
data (none defined)
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Example 2
Add another member function:
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import java.io.*;
public class Test
{
public static void main( String[] )
throws IOException
{
System.out.println(“That it, folks!");
}
public void doStuff()
{
System.out.println(“doing stuff." );
}
}
Example 2: Scoping
class Test
member functions
doStuff()
data (none defined)
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main()
Example 3
Define an object of class Test and make a function call
import java.io.*;
public class Test
{
public static void main( String[] )
throws IOException
{
Test t; // t is of type "Test"
t = new Test(); // allocate object
t.doStuff(); // call member function
System.out.println( "That it, folks!");
}
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Example 3 (continued)
public void doStuff()
{
System.out.println( "I'm doing stuff." );
}
}
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Notes:
•Static methods cannot access nonstatic class members
directly
•main() must always be static
Example 4
Test.java:
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import java.io.*;
public class Test
{
public static void main( String[] ) throws
IOException
{
Stuff t;
t = new Stuff();
t.doStuff();
System.out.println( "That it, folks!" );
}
}
Put main class and a different class in separate files:
Example 4
Stuff.java:
public class Stuff
{
public void doStuff()
{
System.out.println( "I'm doing stuff." );
}
}
Notes
• One class per file
• To compile: javac Test.java
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Java I/O
The System object provides a way to manage I/O from a
more traditional "stream" (terminal window).
GUI-based I/O requires the action() method to deal with
GUI mouse events.
The System object requires no action() method
But terminal I/O is inadequate in a browser-based (GUI)
environment.
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Summary of Some Basic Java
Constructs
Everything is related to objects:
Data declaration:
int i;
Test t;
// declare i to be an int
// declare t to be
// an object of type Test
i = 0;
// set the int i equal to 0
t = new Test();
// initialize t and allocate space
// using a constructor
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Java Constructs
Flow of Control:
Traditional, but with object-oriented syntax for function calls
and member functions
Where control starts in the Applet class is important
Executable statements
Similar to C/C++: while, for, if/else,
switch, etc.
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Java Control Structures
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Selection:
–
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Repetition (looping):
–
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While, For, Do/While
Assignment:
–
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If, If/Else, Switch
Expressions, increment/decrement
Java Reserved Words
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Example: Average
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Assignments and Expressions
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Increment/Decrement
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Java Operators
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Java Operators
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Order is important in post/pre increment
operators
Increment operators (and combined
operators like *=) implicitly are assignments;
they must always have an LVALUE as a
target
One can still confuse “=“ with “==“
Primitive Data Types
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Java Data Types
z
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Java is a strongly-typed language
Primitive data types are meant to be portable
across all platforms
Note that char is 16 bits in Java, not 8!
CS 335 Lecture 03
Java Programming – Methods
Fall 2003
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Java Methods
z
z
z
z
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Methods are functions/procedures attached
to data via classes
Methods are always invoked on an object,
either implicitly or explicitly
As with procedures, methods do not need to
return a value (void is an option)
As with functions, methods can return a value
The Math Class
z
The Math class is an object library, and one
must use a Math object to invoke methods:
System.out.println (Math.sqrt (16.0));
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Example
class
class SquareRoot
SquareRoot {{
public
public static
static void
void main(String[]
main(String[] args)
args) {{
String
String outputString;
outputString;
double
double result;
result;
result
result == Math.sqrt(64.0);
Math.sqrt(64.0);
//
// result
result == mysquare(result);
mysquare(result);
outputString
outputString == Double.toString(result);
Double.toString(result);
//
// outputString
outputString == "The
"The result
result is
is "" ++ outputString;
outputString;
System.out.println(outputString);
System.out.println(outputString);
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}}
Example (Continued)
public
public static
static double
double mysquare
mysquare (double
(double s)
s) {{
return
return s*s;
s*s;
}}
}}
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Method Definition
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Note user-defined method square()
Parameter passing: reference and value
Type definitions and “coercion of arguments”
Explicit cast necessary when there are no
promotion rules
Defining Methods
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Note scope and purpose of method:
–
–
–
–
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constructor
accessor
mutator
…
Parameter passing vs. object-based data
access
Public/private method access and data hiding
Java’s Allowed Promotions
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The Java API Packages
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Java.awt: abstract windowing toolkit
javax.swing: “Swing” GUI components
java.util: date, time, random numbers, etc.
java.sql: database connectivity
java.io: streams, etc
java.rmi: remote method invocation
Random Numbers
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Random number generation is important in
most gaming/simulation situations
java.util contains the random number
package
Typical call (returns value between 0 and 1)
Math.random();
value = 1+(int)(Math.random()*12)
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