Modified version from the original at
http://ece.vuse.vanderbilt.edu/multimedia/CLASSWORK/java-intro.txt
1. Our first Java application example:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello World");
} // main
} // HelloWorld
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Notes: - naming conventions .java and .class
- JDK: compiler: javac - creates .class files from .java files
interpreter : java - runs .class files
- command line arguments - main's parameters
array of Strings
args.length tells how many strings
Note for C/C++ programmers: program name not included in args
2. Our first Java applet example:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
import java.applet.*;
import java.awt.*;
public class HelloWorldApplet extends Applet
{
public void paint(Graphics g)
{
g.drawString("Hello World",25,50);
} // paint
} // HelloWorldApplet
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Corresponding html file:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
<H1>Hello World Applet Test Page</H1>
Notes: - appletviewer (best for debugging) or Netscape
3. Name space
- No global variables
- every class is part of a package
- fully qualified name:
java.lang.Math.PI
- package statement
first line of source code
otherwise: default unnamed package
- import statement:
convenience
top of file (after package statement)
three forms: - import package;
- import package.class;
- import package.*;
examples: import java.util;
// access: util.Hashtable
import java.util.Hashtable;
// access: Hashtable
import java.util.*
// access: Hashtable
// plus all other class in package
default: java.lang.* (all .java files get this automatically)
access restrictions: public
private
protected
private protected
package (default)
4. Local variables (declared in methods)
- no global namespace - there are no global variables
- declare anywhere in the method code (MY STYLE - declare them first)
- inside for loop ( ) (this is the only exception)
5. Comments:
/* comment */
// comment to end of line
/** JavaDoc comment **/
6. No prepocessor
- constants:final
public final class Math {
...
public static final double PI = 3.14159;
...
}
no name collision
convention: capital letters
- no macros
- no include files
directory and file structure
no variable and function declaration
Java file: interface definition and implementation
for a class
7. Unicode characters: 16 bit, lower 8-bit is ASCII
8. Primitive data types
- fixed platform-independent length
- all numerical types are signed
- default values (0 or null)
- types: type size
boolean 1
char 16
byte 8
short 16
int 32
long 64
float 32
double 64
- boolean: true or false
conditional must be of boolean type:
int i =10;
while(i--) // WON'T WORK in Java, you C/C++ programmers
...;
9. Reference data types
- objects and arrays
- different variables may refer to the same object:
Button a,b;
a = new Button();
b = a;
a.setLabel("OK");
String s = b.getLabel(); // s contains "OK"
this is not true of simple types:
int i = 3;
int j = i;
i = 2; // i == 2 and j == 3
- equality: == tests whether two objects refer to the
same object, NOT whether they have the same
values
- pointers: object instances are similar to pointers, but:
- cannot be cast to integers
- no pointer arithmetic
- no sizeof operator
reasons: - simpler, easier
- security
- default value: null (reserved keyword)
10. Objects
- creating objects: new (class name) : call to contructor method
Button b;
b = new Button();
ComplexNumber c = new ComplexNumber(1.1,2.2);
- deleting objects: automatic: garbage collection
- accessing object's variables and methods: . operator
ComplexNumber c = new ComplexNumber();
c.x = 1.1;
c.y = 2.2;
double magnitude d = c.magnitude();
11. Arrays
- similar to objects : - by reference
- created by new
- garbage collected
- creating an array does not create the objects!
Button buttons[]; // buttons == null
buttons = new Button[12]; // buttons[0] == null
buttons[0] = new Button(); // buttons[0] refers to a valid Button
- static initializer:
int powers_of_two[] = { 1, 2, 4, 8, 16, 32 };
- multidimensional
byte two_dim[][] = new byte[256][16];
int test[][] = new int[12][];
Button bad[][] = new Button[][5]; // ILLEGAL!
int triangle[][] = { { 1 },
{ 2, 1 },
{ 3, 2, 1},
{ 4, 3, 2, 1}};
- accessing elements:
int square[] = new int[32];
for(int i = 0; i < square.length; i++)
square[i] = i * i;
- special syntax:
char[] buffer;
int[] twodim[];
12. Strings
- not null-terminated
- immutable: Sister class: StringBuffer
- initialization:
String s = "This is a String";
- concatenation:
String s2 = "Does this work?" + " Yepp";
- some methods: length(), charAt(), equals(), and many more! Check the API
13. Operators
- same as C, except
MINUS: - dereference *
- address of &
- ->
- sizeof
PLUS: - string concatenation +
- instanceof
- right shift with zero extension >>>
- logical and & , or | : always evaluate both operands
14. Statements
- SAME AS IN C:
- if/else
- while
- do/while
- for
- switch
- labeled break and continue - 10 POINTS OFF IF YOU USE THESE!!!!
(except in switch statements)
- no label: same as C
- label: (made-up example)
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
test: if(checkit == 0)
{
outer: for(int i = 0 ; i < 10; i++)
{
inner: for(j = 0; j < 5; j++)
{
if(j == i)
continue outer;
if(a[i][j] == null)
break test;
a[i][j].do_something();
}
}
}
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- labeled break: any enclosing statement
- labeled continue: enclosing loop
- package, import
- LATER:
- synchronized
- try, catch, finally, throw, throws
15. Miscellaneous
- forward references
except for variable initialization
- method overloading
signature: number, type and order of parameters
class exmp {
...
void print_me(int i); // OK
void print_me(int i, int j); // OK
void print_me(double d); //OK
int print_me(double oops); // CONFLICT!
...
}
- Modifiers:
- final
- static
- native
- synchronized
- volatile
- NO: (for those of you who know C or C++)
- structures (simulate with class with no methods)
- unions
- enumerated types (simulate with static final variables)
- bitfields
- typedef (built in!)
- variable length argument list (sumulate with array of Object parameter)