Java version historyThe Java language has undergone several changes since JDK 1.0 as well as numerous additions of classes and packages to the standard library. Since J2SE 1.4, the evolution of the Java language has been governed by the Java Community Process (JCP), which uses Java Specification Requests (JSRs) to propose and specify additions and changes to the Java platform. The language is specified by the Java Language Specification (JLS); changes to the JLS are managed under JSR 901.
Java bytecodeIn computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming languages, see List of JVM languages. A Java programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the IBM developerWorks journal, "Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembly helps the C or C++ programmer.
Virtual machineIn computing, a virtual machine (VM) is the virtualization or emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve specialized hardware, software, or a combination of the two. Virtual machines differ and are organized by their function, shown here: System virtual machines (also called full virtualization VMs) provide a substitute for a real machine. They provide the functionality needed to execute entire operating systems.
Java compilerA Java compiler is a compiler for the programming language Java. The most common form of output from a Java compiler is s containing platform-neutral Java bytecode, but there are also compilers that output optimized native machine code for a particular hardware/operating system combination, most notably the now discontinued GNU Compiler for Java. Most Java-to-bytecode compilers do virtually no optimization, leaving this until run time to be done by the Java virtual machine (JVM).
Garbage (computer science)In computer science, garbage includes data, objects, or other regions of the memory of a computer system (or other system resources), which will not be used in any future computation by the system, or by a program running on it. Because every computer system has a finite amount of memory, and most software produces garbage, it is frequently necessary to deallocate memory that is occupied by garbage and return it to the heap, or memory pool, for reuse.
Java appletJava applets were small applications written in the Java programming language, or another programming language that compiles to Java bytecode, and delivered to users in the form of Java bytecode. The user launched the Java applet from a web page, and the applet was then executed within a Java virtual machine (JVM) in a process separate from the web browser itself. A Java applet could appear in a frame of the web page, a new application window, Sun's AppletViewer, or a stand-alone tool for testing applets.
Memory managementMemory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed. This is critical to any advanced computer system where more than a single process might be underway at any time. Several methods have been devised that increase the effectiveness of memory management.
Free Java implementationsFree Java implementations are software projects that implement Oracle's Java technologies and are distributed under free software licences, making them free software. Sun released most of its Java source code as free software in May 2007, so it can now almost be considered a free Java implementation. Java implementations include compilers, runtimes, class libraries, etc. Advocates of free and open source software refer to free or open source Java virtual machine software as free runtimes or free Java runtimes.
Object lifetimeIn object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction. Rules for object lifetime vary significantly between languages, in some cases between implementations of a given language, and lifetime of a particular object may vary from one run of the program to another. In some cases, object lifetime coincides with variable lifetime of a variable with that object as value (both for static variables and automatic variables), but in general, object lifetime is not tied to the lifetime of any one variable.
GNU Compiler for JavaThe GNU Compiler for Java (GCJ) is a discontinued free compiler for the Java programming language. It was part of the GNU Compiler Collection. GCJ compiles Java source code to Java virtual machine (JVM) bytecode or to machine code for a number of CPU architectures. It could also compile and whole that contain bytecode into machine code. The GCJ runtime-libraries original source is from GNU Classpath project, but there is a code difference between the libgcj libraries. GCJ 4.3 uses the Eclipse Compiler for Java as a front-end.
