JAVA FEATURES

Java Virtual Machine & Runtime Environment >>

Web Design & Development CS506

Lesson 1

JAVA FEATURES

This handout is a traditional introduction to any language features. You might not be able to comprehend

some of the features fully at this stage but don't worry, you'll get to know about these as we move on with

the course

Design Goals of Java

The massive growth of the Internet and the World-Wide Web leads us to a completely new way of looking

at development of software that can run on different platforms like Windows, Linux and Solaris etc.

Right Language, Right Time

Java came on the scene in 1995 to immediate popularity.

Before that, C and C++ dominated the software development

1. compiled, no robust memory model, no garbage collector causes memory leakages, not great

support of built in libraries

Java brings together a great set of "programmer efficient" features

2. Putting more work on the CPU to make things easier for the programmer.

Java Buzzwords (Vocabulary)

From the original Sun Java whitepaper: "Java is a simple, object-oriented, distributed, interpreted,

robust, secure, architecture-neutral, portable, high- performance, multi-threaded, and dynamic language."

Here are some original java buzzwords...

Java -- Language + Libraries

Java has two parts...

1. The core language -- variables, arrays, objects

o The Java Virtual Machine (JVM) runs the core language

o The core language is simple enough to run on small devices -- phones, smart cards, PDAs.

2. The libraries

o Java includes a large collection of standard library classes to provide "off the shelf" code.

(Useful built-in classes that comes with the language to perform basic tasks)

o Example of these classes is String, ArrayList, HashMap, StringTokenizer (to break

string into substrings), Date ...

o Java programmers are more productive in part because they have access to a large set of

standard, well documented library classes.

Simple

Very similar C/C++ syntax, operators, etc.

The core language is simpler than C++ -- no operator overloading, no pointers, and no multiple inheritance

The way a java program deals with memory is much simpler than C or C++.

Object-Oriented

Java is fundamentally based on the OOP notions of classes and objects.

Java uses a formal OOP type system that must be obeyed at compile-time and run-time.

This is helpful for larger projects, where the structure helps keep the various parts consistent. Contrast to

Perl, which has a more anything-goes feel.

Web Design & Development CS506

Distributed / Network Oriented

Java is network friendly -- both in its portable, threaded nature, and because common networking

operations are built-in to the Java libraries.

Robust / Secure / Safe

Java is very robust

Both, vs. unintentional errors and vs. malicious code such as viruses.

Java has slightly worse performance since it does all this checking. (Or put the other way, C

can be faster since it doesn't check anything.)

The JVM "verifier" checks the code when it is loaded to verify that it has the correct structure --

that it does not use an uninitialized pointer, or mix int and pointer types. This is one-time "static"

analysis -- checking that the code has the correct structure without running it.

The JVM also does "dynamic" checking at runtime for certain operations, such as pointer and array

access, to make sure they are touching only the memory they should. You will write code that runs

into

As a result, many common bugs and security problems (e.g. "buffer overflow") are not possible in

java. The checks also make it easier to find many common bugs easy, since they are caught by the

runtime checker.

You will generally never write code that fails the verifier, since your compiler is smart enough to

only generate correct code. You will write code that runs into the runtime checks all the time as

you debug -- array out of bounds, null pointer.

Java also has a runtime Security Manager can check which operations a particular piece of code is

allowed to do. As a result, java can run un-trusted code in a "sandbox" where, for example, it can

draw to the screen but cannot access the local file system.

Portable

"Write Once Run Anywhere", and for the most part this works.

Not even a recompile is required -- a Java executable can work, without change, on any Java enabled

platform.

Support for Web and Enterprise Web Applications

Java provides an extensive support for the development of web and enterprise applications

Servlets, JSP, Applets, JDBC, RMI, EJBs and JSF etc. are some of the Java technologies that

can be used for the above mentioned purposes.

High-performance

The first versions of java were pretty slow.

Java performance has gotten a lot better with aggressive just-in-time-compiler (JIT) techniques.

Web Design & Development CS506

Java performance is now similar to C -- a little slower in some cases, faster in a few cases. However

memory use and startup time are both worse than C.

Java performance gets better each year as the JVM gets smarter. This works, because making the

JVM smarter does not require any great change to the java language, source code, etc.

Multi-Threaded

Java has a notion of concurrency wired right in to the language itself.

This works out more cleanly than languages where concurrency is bolted on after the fact.

Dynamic

Class and type information is kept around at runtime. This enables runtime loading and inspection

of code in a very flexible way.

Java Compiler Structure

The source code for each class is in a .java file. Compile each class to produce ".class" file.

Sometimes, multiple .class files are packaged together into a .zip or .jar "archive" file.

On unix or windows, the java compiler is called "javac". To compile all the .java files in a directory use

"javac *.java".

Java: Programmer Efficiency

Faster Development

Building an application in Java takes about 50% less time than in C or C++. So, faster time to market

Java is said to be "Programmer Efficient".

OOP

Java is thoroughly OOP language with robust memory system

Memory errors largely disappear because of the safe pointers and garbage collector. The lack of memory

errors accounts for much of the increased programmer productivity.

Libraries

Code re-use at last -- String, ArrayList, Date, ... available and documented in a standard way

Microsoft vs. Java

Microsoft hates Java, since a Java program (portable) is not tied to any particular operating system.

If Java is popular, then programs written in Java might promote non-Microsoft operating systems. For

basically the same reason, all the non- Microsoft vendors think Java is a great idea.

Microsoft's C# is very similar to Java, but with some improvements, and some questionable

features added in, and it is not portable in the way Java is. Generally it is considered that C# will be

successful in the way that Visual Basic is: a nice tool to build Microsoft only software.

Microsoft has used its power to try to derail Java somewhat, but Java remains very popular on its

merits.

Web Design & Development CS506

Java Is For Real

Java has a lot of hype, but much of it is deserved. Java is very well matched for many modern

problems

Using more memory and CPU time but less programmer time is an increasingly appealing tradeoff.

Robustness and portability can be very useful features

A general belief is that Java is going to stay here for the next 10-20 years

References

Majority of the material in this handout is taken from the first handout of course cs193j at Stanford.

The JavaTM Language Environment, White Paper, by James Gosling & Henry McGilton

Java's Sun site: http://java.sun.com

Java World: www.javaworld.com

Table of Contents: