CS451 - Lecture 41

CS451Lecture 4: Introduction to Java

Yugi Lee

STB #555(816) 235-5932

leeyu@umkc.edu

www.sice.umkc.edu/~leeyu

*Acknowledgement: This lecture is based on material courtesy of Monash University

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Why Java? Application?

• Java can support what we are interested in ... Systems characterized as those which are typically

commercially useful; Systems which allow interaction with the user; Dynamical web pages which allow interaction only

between user and page, but not page source. interaction which involves the exchange of data

between remote user and local data source;

Distributed Web-based applications.

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Why Java? What’s Java?

simple object-oriented distributed interpreted robust secure architecture-neutral

portable high performance multithreaded dynamic buzzword-compliant

language!

(The Java Language: A White Paper)

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What’s Java?

• Green Project (1990)– Consumer device operating

software

– Requirements: small size, robust, real-time performance

• Oak– Originally used C++, then

realized a new language was needed

– Original requirements same as for current language

• Java (1993)– Intended market never

eventuated

– WWW starting to takeoff

– Language design "based on" many current OO languages (e.g., C++, Eiffel, Smalltalk, Cedar/Mesa, Objective C)

• JDK 1.0 (1996), JDK 1.1, JDK1.2, JDK1.3

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What distinguishes Java from other OO languages?

The Java Virtual Machine (JVM). Code is compiled into machine independent byte code.

Like some other languages it has a garbage collector

Prevents pointer arithmetic Prevents operator

overloading

Inbuilt security Support for threads and

exceptions as part of base language.

Provides a large collection of classes that support and simplify many common programming activities

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Java - Simple

Intentionally created to be syntactically similar to C/C++ Eliminates traditionally

troublesome features of C/C++ Pointer arithmetic Multiple inheritence Implicit type coercions Explicit memory management Preprocessor

Eliminates features of C/C++

struct typedef union enum operator overloading

Features included as part of base language:

Threads Exception handling

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Java - Object-Oriented

Systems are built from sets of classes Classes are instantiated at runtime to give objects Objects communicate via messages Everything* is part of a class OO concepts supported:

Encapsulation Inheritance Polymorphism Dynamic Binding

Logical cluster of classes == package

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Java - Distributed

Network programming support built into JDK class library: TCP sockets UDP packets IP addresses URLs Security features designed into language

• Java's network programming facilities are one of the language's best features.

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Java - Interpreted

Source code is initially compiled (javac) into architecture-neutral byte-codes

Byte-codes are interpreted by the java virtual machine (java or Netscape)

Seamless linking stage JIT compilers lead to a large performance increase in

compilation and runtime execution

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Java - Robust

Strongly-typed language Compile and runtime checking No pointer arithmetic Exception handling Automatic memory management

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Java - Secure

Generally very difficult to trick Java's security mechanisms

Depends very much on the program loader (e.g., Netscape)

The SecurityManager class defines the protocol used by the program loader

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Java - Architecture-neutral/Portable

Byte-codes are architecture neutral Performance suffers by using bytecodes Primitive type sizes are explicit - not architecture

dependent Strings and characters are (16-bit) Unicode compliant GUI libraries give a native graphic library-independent

mechanism for creating quality graphical interfaces

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Java - High performance

• Interpreting leads to quicker development cycle

Depends what you compare it to "Slightly faster than VB" JITC help greatly in this respect Can use native code for mission-critical

performance sections of code

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Java - Multithreaded

Based on well-known 20 year old Hoare monitor synchronization

Thread support built into language Thread synchronization primitives supplied

• Garbage collector runs permanently as a low priority background thread

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Java - Dynamic

Class linking, layout, name resolution and object references not resolved until run-time

Runtime Type Information (RTTI) available Class class for dynamic instantiation

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The Top 10 Worst Things About Java!

1. Performance (particularly for Swing graphics)

2. No templates/generic container classes

3. Poor debugger

4. Some poor or inconsistent design decisions (naming, IO Libraries)

5. No multiple inheritance?

6. No operator overloading?

7. Doesn't run on Windows 3.1

8. Requires JVM for each machine

9. Even though claimed as stable, new releases of JDK are issued regularly.

10. Many systems slow to support new versions of JDK