Transcript Java Goals

“Perfect Quote”
You know you've achieved
perfection in design,
Not when you have nothing
more to add,
But when you have nothing
more to take away.
- Antoine de Saint Exupery.
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Language Design
• One thing the language designer should not do is
to include untried ideas of ones own. The task is
one of consolidation, not innovation.
- C. A. R. Hoare
• In language design, the central problem is to
organize a variety of concepts in a way which
exhibits uniformity and generality.
- J. C. Reynolds
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• Conflicting goals lead to difficult trade-offs.
– Safety checks “burden” the compiler and/or the
run-time.
– Safety and machine independence may exclude
efficient low-level operations.
– Type systems restrict programming style in
exchange for strong guarantees.
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A language addition should whenever
possible solve new problems beyond its
original purpose --- not create new
problems through its interaction with other
mechanisms. A prudent language designer,
having toyed with a possible new facility,
and encountering incompatibilities with
more important properties of the design,
quickly retreats.
-- Bertrand Meyer
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Language Specification
Programs
– (dynamic) Software Machinery
– (static) Naming Environments
• What entities can be named?
• How are the names organized into groups?
• How are the names made known where ever they
are needed?
Conventions are no substitute for enforced
constraints.
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Java : Design Goals
A general-purpose concurrent
object-oriented language
for the Internet.
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Top Ten Design Goals
Simple
ObjectOriented
Secure
Familiar
(~ C/C++ )
Robust
Interpreted
Portable
Distributed
Efficient
Concurrent
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Simplicity
• Programmer Perspective
– Automatic garbage collection.
– “Unconstrained” array type.
• Implementer Perspective
– Only “single” class inheritance.
– Restricted overloading.
• E.g., Pascal > Modula-2 > Modula-3.
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Portability
– Hardware Independence
• Java source => “Architecture Neutral” Byte code
– Operating System Independence
• Java APIs for I/O, networking, GUI, concurrent
programming, etc.
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Portability
• Architecture-neutral.
– Size of integer, char, etc and meaning of
floating point operations fixed by the language
(not by an implementation).
• All “behavioral” aspects of a Java program
defined by the Java language spec.
– Order of evaluation of operands fixed.
– Error and Exception handling.
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Object-Oriented Programming
• Programming with abstract data types
– Data Abstraction
• Separation of Spec. and Impl.
– Modularity
• Logically related entities are textually close.
– Encapsulation
• Controlling visibility of names.
• Code and Interface Reuse
– Inheritance ; Dynamic binding
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Java
• Class-based language.
• Has object types (based on classes).
• Subclassing implies subtyping
– but there is subtyping independent of
subclassing.
• Static typing
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Reliability and Robustness
• Strong Typing
– every variable and every expression has a type
known at compile-time.
– type limits the values a variable can hold and
the values an expression can produce at runtime.
• Exceptions
– signaling violation of semantic constraints.
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Java : Compiler and Interpreter
source code
javac
byte code
java
mips
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native code
pentium
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sparc
alpha
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Interpretation and Security
• Java programs are compiled into byte codes
for a (hardware-independent) Java Virtual
Machine, and is executed by an interpreter.
• Just-In-Time compiler converts byte codes
into native codes, for efficiency.
• An interpreter for JVM verifies and then
executes Java byte codes. This step
incorporates Security Checks.
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Evolution of Sun’s JDK
• Java 1.0: Interpreter
• Java 1.1: Interpreter + JIT Compiler
• Java 2 : Hotspot
• Profiling and Adaptive Dynamic Compilation of “hot” code
• Method in-lining and other aggressive optimizations, and
Decompilation
• Improved Memory Management for long-running (server)
programs
• Fast Thread Synchronization
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Concurrent Programming
• Threads can share address space. In Java,
threads are modeled after Hoare’s monitors.
• Java is a multi-threaded system, which is
very convenient for coding interactive or
multi-media applications.
– Doing I/O concurrently with reading a
document. (downloading via browser)
– Periodic updating of the screen with data
acquired in real-time. (sports/stocks ticker)
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Distributed Processing
• HTML and WWW contributed immensely
to the sharing of (distributed) static data.
• CGI scripts enabled customization of
responses based on user input (client-server).
• A Java applet can further react to user input
and change behavior dynamically. It can
provide an alternative to client-server model
in that it runs on the client machine.
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(cont)
• Java supports network programming.
• Java supports mobile computing.
– Compiled to compact, platform
independent byte codes
– Dynamic linking.
– KVM
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