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Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let programmers write once, run anywhere (WORA),[16] meaning that compiled Java code can run on all platforms that support Java without the need to recompile.[17] Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. Although its syntax is similar to that of C and C++, the Java language has fewer low-level facilities than either of them. The Java runtime provides dynamic capabilities (such as reflection and runtime code modification) that are typically not available in traditional compiled languages.
Java gained popularity shortly after its release, and has been a very popular programming language since then.[18] Java was the third most popular programming language in 2022[update] according to GitHub.[19] Although still widely popular, there has been a gradual decline in use of Java in recent years with other languages using JVM gaining popularity.[20]
Java was originally developed by James Gosling at Sun Microsystems. It was released in May 1995 as a core component of Sun's Java platform. The original and reference implementation Java compilers, virtual machines, and class libraries were originally released by Sun under proprietary licenses. As of May 2007, in compliance with the specifications of the Java Community Process, Sun had relicensed most of its Java technologies under the GPL-2.0-only license. Oracle offers its own HotSpot Java Virtual Machine, however the official reference implementation is the OpenJDK JVM which is free open-source software and used by most developers and is the default JVM for almost all Linux distributions.
James Gosling, Mike Sheridan, and Patrick Naughton initiated the Java language project in June 1991.[21] Java was originally designed for interactive television, but it was too advanced for the digital cable television industry at the time.[22] The language was initially called Oak after an oak tree that stood outside Gosling's office. Later the project went by the name Green and was finally renamed Java, from Java coffee, a type of coffee from Indonesia.[23] Gosling designed Java with a C/C++-style syntax that system and application programmers would find familiar.[24]
In 1997, Sun Microsystems approached the ISO/IEC JTC 1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process.[27][28][29] Java remains a de facto standard, controlled through the Java Community Process.[30] At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System.
On November 13, 2006, Sun released much of its Java virtual machine (JVM) as free and open-source software (FOSS), under the terms of the GPL-2.0-only license. On May 8, 2007, Sun finished the process, making all of its JVM's core code available under free software/open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright.[31]
Oracle released the last zero-cost public update for the legacy version Java 8 LTS in January 2019 for commercial use, although it will otherwise still support Java 8 with public updates for personal use indefinitely. Other vendors such as Adoptium continue to offer free builds of OpenJDK's Long-Term Support (LTS) versions. These builds may include additional security patches and bug fixes.[39]
One design goal of Java is portability, which means that programs written for the Java platform must run similarly on any combination of hardware and operating system with adequate run time support. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to architecture-specific machine code. Java bytecode instructions are analogous to machine code, but they are intended to be executed by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their device for standalone Java applications or a web browser for Java applets.
The use of universal bytecode makes porting simple. However, the overhead of interpreting bytecode into machine instructions made interpreted programs almost always run more slowly than native executables. Just-in-time (JIT) compilers that compile byte-codes to machine code during runtime were introduced from an early stage. Java's Hotspot compiler is actually two compilers in one; and with GraalVM (included in e.g. Java 11, but removed as of Java 16) allowing tiered compilation.[48] Java itself is platform-independent and is adapted to the particular platform it is to run on by a Java virtual machine (JVM), which translates the Java bytecode into the platform's machine language.[49]
Programs written in Java have a reputation for being slower and requiring more memory than those written in C++.[50][51] However, Java programs' execution speed improved significantly with the introduction of just-in-time compilation in 1997/1998 for Java 1.1,[52] the addition of language features supporting better code analysis (such as inner classes, the StringBuilder class, optional assertions, etc.), and optimizations in the Java virtual machine, such as HotSpot becoming Sun's default JVM in 2000. With Java 1.5, the performance was improved with the addition of the java.util.concurrent package, including lock-free implementations of the ConcurrentMaps and other multi-core collections, and it was improved further with Java 1.6.
Some platforms provide direct hardware support for Java. Certain microcontrollers can execute Java bytecode directly in hardware, bypassing the need for a software-based Java Virtual Machine (JVM). Additionally, some ARM-based processors were equipped with a feature called Jazelle, which enabled them to run Java bytecode natively. However, most modern ARM implementations have phased out support for Jazelle.
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.[53] If methods for a non-existent object are called, a null pointer exception is thrown.[54][55]
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.[53] 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 or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Garbage collection does not prevent logical memory leaks, i.e. those where the memory is still referenced but never used.[53]
In Java, garbage collection can happen at any time, but it usually runs when the program is idle. If there isn't enough free memory in the heap for a new object, garbage collection will definitely occur, which might cause a brief pause in the program. It's also important to know that Java doesn't let programmers manage memory explicitly.
Java does not support C/C++ style pointer arithmetic, where object addresses can be arithmetically manipulated (e.g. by adding or subtracting an offset). 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 either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as is commonly true for non-primitive data types (but see escape analysis). This was a conscious decision by Java's designers for performance reasons.
Java contains multiple types of garbage collectors. Since Java 9, HotSpot uses the Garbage First Garbage Collector (G1GC) as the default.[56] However, there are also several other garbage collectors that can be used to manage the heap. For most applications in Java, G1GC is sufficient. Previously, the Parallel Garbage Collector was used in Java 8.
Even though memory management issues are resolved, programmers still need to properly handle other resources like network or database connections, file handles, and so on, especially when dealing with exceptions.
The syntax of Java is largely influenced by C++ and C. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language.[17] All code is written inside classes, and every data item is an object, with the exception of the primitive data types, (i.e. integers, floating-point numbers, boolean values, and characters), which are not objects for performance reasons. Java reuses some popular aspects of C++ (such as the printf method).
Java uses comments similar to those of C++. There are three different styles of comments: a single line style marked with two slashes (//), a multiple line style opened with /* and closed with */, and the Javadoc commenting style opened with /** and closed with */. The Javadoc style of commenting allows the user to run the Javadoc executable to create documentation for the program and can be read by some integrated development environments (IDEs) such as Eclipse to allow developers to access documentation within the IDE.
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