الجوري محمد
05-03-2011, 04:09 AM
السلام عليكم ورحمه الله وبركاته
اخواني واخوتي انا عندي بحث عن ماده الجافا طلعت البحث بعدين قالنت لي التيتشر سوي له برافريز يعني ارجع واكتب الكلام واخذ الزبده منه وانا صراحه عجزت عنه فاليت احديسوي لي الله يوفقكم اجمعين
edit] Java Platform
Main articles: Java (software platform) and Java Virtual Machine
One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.
Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time compilers were introduced from an early stage that compile bytecodes to machine code during runtime.
[edit] Implementations
Sun Microsystems officially licenses the Java Standard Edition platform for Linux,[21] Mac OS X,[22] and Solaris. Although in the past Sun has licensed Java to Microsoft, the license has expired and has not been renewed.[23] Through a network of third-party vendors and licensees,[24] alternative Java environments are available for these and other platforms.
Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.[25] As a result, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows.
Platform-independent Java is essential to the Java EE strategy, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications, such as Web services, Java Servlets, and Enterprise JavaBeans, as well as with embedded systems based on OSGi, using Embedded Java environments. Through the new GlassFish project, Sun is working to create a fully functional, unified open source implementation of the Java EE technologies.
Sun also distributes a superset of the JRE called the Java Development Kit (commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, Jar, and debugger.
[edit] Performance
Main article: Java performance
Programs written in Java have a reputation for being slower and requiring more memory than those written in C.[26] However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,[27] the addition of language features supporting better code analysis (such as inner classes, StringBuffer class, optional assertions, etc.), and optimizations in the Java Virtual Machine itself, such as HotSpot becoming the default for Sun's JVM in 2000. Currently, Java code has approximately half the performance of C code.[28]
Some platforms offer direct hardware support for Java; there are microcontrollers that can run java in hardware instead of a software JVM, and ARM based processors can have hardware support for executing Java bytecode through its Jazelle option.
[edit] Automatic memory management
Java uses an automatic garbage collector to manage memory in the object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.[29][30]
One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used.
Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.
Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.
As in C++ and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as commonly true for objects (but see Escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0, autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class.
Java contains multiple types of garbage collectors. By default, HotSpot uses the Concurrent Mark Sweep collector, also known as the CMS Garbage Collector. However, there are also several other garbage collectors that can be used to manage the Heap. For 90% of applications in Java, the CMS Garbage Collector is good enough.[31]
[edit] Syntax
Main article: Java syntax
The syntax of Java is largely derived from C++. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language. All code is written inside a class, and everything is an object, with the exception of the primitive data types (integers, floating-point numbers, boolean values, and characters), which are not classes for performance reasons.
Java suppresses several features (such as operator overloading and multiple inheritance) for classes in order to simplify the language and to prevent possible errors and anti-pattern design.
Java uses similar commenting methods to C++. There are three different styles of comment: a single line style marked with two slashes (//), a multiple line style opened with a slash asterisk (/*) and closed with an asterisk slash (*/), and the Javadoc commenting style opened with a slash and two asterisks (/**) and closed with an asterisk slash (*/). The Javadoc style of commenting allows the user to run the Javadoc executable to compile documentation for the program.
هذا البراقراف بس ابي منه الشي المفيد ياليت احد يساعدني
اخواني واخوتي انا عندي بحث عن ماده الجافا طلعت البحث بعدين قالنت لي التيتشر سوي له برافريز يعني ارجع واكتب الكلام واخذ الزبده منه وانا صراحه عجزت عنه فاليت احديسوي لي الله يوفقكم اجمعين
edit] Java Platform
Main articles: Java (software platform) and Java Virtual Machine
One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.
Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time compilers were introduced from an early stage that compile bytecodes to machine code during runtime.
[edit] Implementations
Sun Microsystems officially licenses the Java Standard Edition platform for Linux,[21] Mac OS X,[22] and Solaris. Although in the past Sun has licensed Java to Microsoft, the license has expired and has not been renewed.[23] Through a network of third-party vendors and licensees,[24] alternative Java environments are available for these and other platforms.
Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.[25] As a result, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows.
Platform-independent Java is essential to the Java EE strategy, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications, such as Web services, Java Servlets, and Enterprise JavaBeans, as well as with embedded systems based on OSGi, using Embedded Java environments. Through the new GlassFish project, Sun is working to create a fully functional, unified open source implementation of the Java EE technologies.
Sun also distributes a superset of the JRE called the Java Development Kit (commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, Jar, and debugger.
[edit] Performance
Main article: Java performance
Programs written in Java have a reputation for being slower and requiring more memory than those written in C.[26] However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,[27] the addition of language features supporting better code analysis (such as inner classes, StringBuffer class, optional assertions, etc.), and optimizations in the Java Virtual Machine itself, such as HotSpot becoming the default for Sun's JVM in 2000. Currently, Java code has approximately half the performance of C code.[28]
Some platforms offer direct hardware support for Java; there are microcontrollers that can run java in hardware instead of a software JVM, and ARM based processors can have hardware support for executing Java bytecode through its Jazelle option.
[edit] Automatic memory management
Java uses an automatic garbage collector to manage memory in the object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.[29][30]
One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used.
Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.
Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.
As in C++ and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as commonly true for objects (but see Escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0, autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class.
Java contains multiple types of garbage collectors. By default, HotSpot uses the Concurrent Mark Sweep collector, also known as the CMS Garbage Collector. However, there are also several other garbage collectors that can be used to manage the Heap. For 90% of applications in Java, the CMS Garbage Collector is good enough.[31]
[edit] Syntax
Main article: Java syntax
The syntax of Java is largely derived from C++. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language. All code is written inside a class, and everything is an object, with the exception of the primitive data types (integers, floating-point numbers, boolean values, and characters), which are not classes for performance reasons.
Java suppresses several features (such as operator overloading and multiple inheritance) for classes in order to simplify the language and to prevent possible errors and anti-pattern design.
Java uses similar commenting methods to C++. There are three different styles of comment: a single line style marked with two slashes (//), a multiple line style opened with a slash asterisk (/*) and closed with an asterisk slash (*/), and the Javadoc commenting style opened with a slash and two asterisks (/**) and closed with an asterisk slash (*/). The Javadoc style of commenting allows the user to run the Javadoc executable to compile documentation for the program.
هذا البراقراف بس ابي منه الشي المفيد ياليت احد يساعدني