from c++ to java snu idb lab. hyoung-woo park march 26, 2007

From C++ to Java

SNU iDB Lab.Hyoung-woo Park

March 26, 2007

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Contents History Primary Goals of Java Comparing Java to C++

Features Omitted from C++ Features Added in Java

Improving the Performance Summary Reference

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History 1991

Java started as a project called Oak by James Gosling of Sun Microsystems

1993 The name is changed to Java

1994 Major web browsers like Netscape Navigator

incorporated the ability to run secure Java applets within web pages

1995 Java 1.0 was released

… 2006

J2SE (Java 2 Standard Edition) 6 was released

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Primary Goals of Java (1/10) Java is simple (1/2)

int *a;a = (int *)malloc(sizeof(int) * 50);

C++

#include <iostream>#include <algorithm>

Manual management of memory allocation Major source of bugs

Same file can be included multiple timesMajor source of code bloat

int [ ]a;a = new int[50];

import java.io.*;import java.util.*;

Java

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Primary Goals of Java (2/10) Java is simple (2/2)

There is neither pointers nor macros in Java

Pointer Macro

MethodMemberVariable

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Primary Goals of Java (3/10) Java is familiar

Syntax and general principles of Java are similar to C++ But there are so many differences, of course

Syntaxof

C++

Syntaxof

Java

Similar

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Primary Goals of Java (4/10) Java is object-oriented

As with C++, Java has object-orientation facilities Encapsulation Inheritance Polymorphism

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Primary Goals of Java (5/10) Java is independent to architecture (1/2)

Win32 UNIX MacOS

Compilerfor Win32

C++ Java

Compilerfor UNIX

Compilerfor MacOS

Source codefor Win32



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Primary Goals of Java (6/10) Java is independent to architecture (2/2)

Java uses virtual machine which reads platform-independent code and execute corresponding commands The same code can be run on any machine if

appropriate JVM is installed on it Java can be run on

LINUX Mac OS Windows NT Windows XP AIX…

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Primary Goals of Java (7/10) Java supports threads (1/2)

Modern, networked applications are often required to perform multiple tasks simultaneously

Although concurrent execution can be handled at OS level, it is more convenient for programmers to handle it using language constructs

P: thread: processP

P P P

job1 job2 job3

job1 job2 job3

Concurrencycontrolled by OS

Concurrencycontrolled by language

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Primary Goals of Java (8/10) Java supports threads (2/2)

Java has built-in support for threads via Thread class and Runnable interface

Thread

Runnacle

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Primary Goals of Java (9/10) Java is secure (1/2)

Security is getting more important as people are getting more and more online

Program

SecurityResources

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Primary Goals of Java (10/10) Java is secure (2/2)

Java has built-in security features Security manager Digital Signature Cryptographic interfaces

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Features Omitted from C++ (1/9) Destructors (1/2)

Object destruction is often associated with the explicit freeing of dynamic memory

Java performs garbage collection instead of explicitly deallocating memory

Java uses finalize( ) function to release / close / clean up resources before garbage collection

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Features Omitted from C++ (2/9) Destructors (2/2)

class A{public: void f( ){ Foo *x = new Foo( ); delete x; }};

C++ Java

public class A{ public void f( ){ Foo x = new Foo( ); }}

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Features Omitted from C++ (3/9) Operator overloading (1/2)

C++ allows the primitive operators to be overloaded to work with classes This can lead to elegant code if carefully used But operator overloading is one of the most

confusing and consistently misused facilities of C++

Matrix Matrix+ : Matrix addition(Good and intuitive)

LinkedList

>> : ???(Unnatural and misused) HashTable

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Features Omitted from C++ (4/9) Operator overloading (2/2)

Java uses methods instead of overloaded operators

map<string, string>

foo;

foo["DB"] = "2006";

Map foo;

foo.put("DB",

"2006");C++ Java

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Features Omitted from C++ (5/9) Multiple inheritance

The concept of multiple inheritance is sound but the implementation proves difficult and adds complexity to the language

Studentid

Staffid

Workstudyid

Ambiguous!!

In this example we can resolve conflict by substituting id with Staff::id.But in reality, the class hierarchy may much more complicated than this example.

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Features Omitted from C++ (6/9) Pointers and references (1/2)

Java does not support pointer arithmetic because it is unnecessary to deal with memory allocation and

deallocation with the existence of garbage collector type safety and security can no longer be

guaranteed if arbitrary manipulation of pointers is allowed1 2 3

pointer1 pointer3pointer2

Pointer2 and pointer3 refer to illegal addresses

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Features Omitted from C++ (7/9) Pointers and references (2/2)

int *a;

a = (int *)malloc(sizeof(int) *

50);

*(a + 100) = 1; // pointer

error

free(a); C++ Java

int[ ] a;

a = new int[50];

a[100] = 1; // exception

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Features Omitted from C++ (8/9) Friend classes (1/2)

class Complex {public: friend class Pal;private: double r, i;};

C++

package kr.ac.snu.idb.oasis;public class SLGuide { void func2(GOGuide g) { int i = g.a; g.func1( ); } Java

class Pal { void negate(Complex &c) { c.r = –c.r; c.i = –c.i; }};

package kr.ac.snu.idb.oasis;public class GOGuide { int a; void func1( ) { … }};

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Features Omitted from C++ (9/9) Friend classes (2/2)

Allows one class to have access to protected or private data members and methods of another class in C++

Although Java doesn't support friend classes, Java supports packages Package members are accessible to classes in the

same package

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Features Added in Java (1/16) Java virtual machine (1/3)

An abstract computer architecture Software on top of a real hardware Can run the same application on different

machines JVM has two primary jobs

Execute code Manage memory

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Java system overview

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Features Added in Java (4/16) Garbage collection (1/2)

Unlike C++, in Java objects cannot be reclaimed or freed by explicit language directives

Objects become garbage when there are no more references to the object

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Features Added in Java (5/16) Garbage collection (2/2)

root

Deallocatedby GC

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Features Added in Java (6/16) Packages (1/3)

In Java, a package is a container for related classes and interfaces

A package can have packages, i.e. package structure is similar to directory structure we are familiar with

Java does allow a default friendliness among all classes inside a package

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java

lang util io net applet

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package kr.ac.snu.idb.oasis;public class GOGuide { int a; void func1( ) { …}

package kr.ac.snu.idb.oasis;public class SLGuide { void func2(GOGuide g) { int i = g.a; g.func1( ); }

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Features Added in Java (9/16) Interfaces (1/3)

Interfaces are a named collection of method definitions without implementations and constants

Interfaces are used to resolve ambiguity problem without losing the usefulness of multiple inheritance

Interfaces are so different from multiple inheritance No interfaces or classes inherit variables No interfaces or classes inherit method implementations The interface hierarchy is independent of the class

hierarchy

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public interface Sleeper { public static final ONE_SECOND = 1000; public void wakeup( );}public class SleeperImpl implements Sleeper { public void wakeup( ){ System.out.println("This is an example."); }}An example interface and a class implementing it

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A

C

B A

C

B

: inheritance: implementation

: interface: class

C++ Java

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Features Added in Java (12/16) Multi-threading (1/3)

Modern, networked applications are often required to perform multiple tasks simultaneously

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The Java platform is designed from the ground up to support concurrent programming

Multi-threading is the ability of a single process to spawn multiple, simultaneous execution paths

Threads exist within a process and provide an execution environment

Threads share the process's resources Threads require fewer resources than processes

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P

: thread: processP

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Features Added in Java (15/16) Anonymous classes (1/2)

In Java, you can declare a class within the body of a method without naming it

Anonymous classes enhance developer efficiency in many cases, but they can reduce code readability

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Features Added in Java (16/16) Anonymous classes (2/2)

public class Spot extends Applet { public void init( ) { addMouseListener(new MyMouseAdapter( )); } class MyMouseAdapter extends MouseAdapter { // implementation of custom adapter }}

Using named class

Using anonymous class

public class Spot extends Applet { public void init( ) { addMouseListener(new MouseAdapter( ) { // implementation of custom adapter } ); }}

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Improving the Performance (1/5) Just-in-time compilation (1/2)

JIT compilation (dynamic translation) is a technique for improving the runtime performance of program

JIT compiler caches the result of translation (execution code) to improve performance

Programmers had to decide whether or not to use JIT compilers in the past, but now it's automatic in most JVM

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Improving the Performance (2/5) Just-in-time compilation (2/2)

Java source code

Java bytecode

output

compiler

interpreter

JIT compiler

native code

native exec

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Improving the Performance (3/5) Java native interface (1/3)

JNI is a programming framework that allows Java code to call and be called by native applications Native applications are programs which are

dependent to platforms and possibly written in other languages

JNI is used to implement platform-specific features or to write performance-critical modules

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An example program using JNI

public class HelloJNI { private native void print( ); public static void main(String[ ] args) { new HelloJNI( ).print( ); } static { System.loadLibrary("HelloJNI"); }}#include <jni.h>#include "HelloJNI.h"JNIEXPORT void JNICALL Java_HelloJNI_print(JNIEnv *env, jobject obj) { printf("Hello JNI\n" ); return;}

Compilerfor Win32

HelloJNI.class

HelloJNI.dll

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Summary Although C++ and Java has commonalities, some

features of C++ are omitted in Java, and some features are added in Java

Java is fit to current web environment because Java is platform-independent code reuse & sharing Java is object-oriented modular programming Java is multi-threaded distributed computing Java is secure secure web programming

Through JNI programmers can merge native codes into Java program

JIT compiler improves the performance of Java

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Reference Setrag, K., Razmik A. (1995). Object

Orientation. John Wiley & Sons, Inc., 267-319. Richard C. (1998). Java Tutorial 2nd Ed.

Addison-Wesley, 105-158. Michael D. (1996). Java for C/C++

Programmers, Wiley Computer Publishing, 245-287.

Barry B. (1996). Java Essentials for C and C++ Programmers, Wesley Developers Press, 3-11.

from c++ to java snu idb lab. hyoung-woo park march 26, 2007

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