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Introduction to Java
CSIS 3701: Advanced Object Oriented Programming
Java Background
Original purpose: web page applets
– Executable/dynamic applications running on web page
– No longer main use, but affected language design
Server
Client
requested
by browser
applet
browser
applet
copy
downloaded
to browser
Java code
executed on
client computer
Platform Independence
• Java applet must run on any client
– Different OS, architecture, etc.  different machine code
– Cannot compile applet to single executable used by all
• Stage 1:
Java source code compiled to “byte code”
– Code for an abstract “Java virtual machine” (JVM)
Hello.java
Source code
(must end in .java)
Hello.class
Byte code stored
on server
Platform Independence
• Stage 2: JVM on client runs “byte code”
– Converted to native machine code line-by line and
executed on the fly
Client
browser
– JVM can be:
• Part of browser
• Built into NetBeans
• Run separately from
command line
(java Hello.class)
• Built directly into chip
(mobile devices)
JVM
applet
Line 1
Line 2
Line 3
Line 4
…
convert and
execute
processor
Security and Safety
• Applet = unknown code running on your computer!
– How to prevent malicious applets?
• Applets vs. Applications
– Applets not allowed access to local files, network, etc.
– Application: separate standalone process not run in
browser
Security and the Sandbox
• All Java programs execute in restricted area of
memory (the “sandbox”)
• No explicit pointers
– int *ptr = 100; // outside sandbox
– *ptr = 0;
// overwrite that memory
• Array bounds checking
– int A[100];
– A[1000000] = 0; // outside sandbox
Safety and Exception Handling
• Java programs cannot “crash” to OS
• Exceptions caught and handled within JVM
– Browser/NetBeans/etc. notified
JVM
applet
int x = 0/0;
ArithmeticException thrown;
– Can handle as needed (error message displayed, etc.)
Tradeoffs
• Many tradeoffs in language design
– No “best” choices
– Different languages make different choices
• Portability vs. Speed
– “On the fly” interpretation slower than direct execution
of machine code
• Safety vs. Speed
– Array bounds checking
– Exception handling
Require extra time
Basic Java Syntax
• Java syntax mostly same as C++
– Java developed by C++ programmers
• Examples
– Lines/blocks: ; {}
– Control structures: if else for while switch…
– Operators:
= + - * / ++ -- +=…
== != > < >= <= && ||…
– Comments: /*
*/
//