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. The syntax of Java is similar to C and C++, but 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] and it is ranked fourth on TIOBE index as of October 2023[update].[20] Although still widely popular, there has been a gradual decline in use of Java in recent years with other languages using JVM gaining popularity.[21]
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James Gosling, Mike Sheridan, and Patrick Naughton initiated the Java language project in June 1991.[22] Java was originally designed for interactive television, but it was too advanced for the digital cable television industry at the time.[23] 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.[24] Gosling designed Java with a C/C++-style syntax that system and application programmers would find familiar.[25]
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.
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.[54] If methods for a non-existent object are called, a null pointer exception is thrown.[55][56]
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.[54] 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.[54]
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.
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.
Having solved the memory management problem does not relieve the programmer of the burden of handling properly other kinds of resources, like network or database connections, file handles, etc., especially in the presence of 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.
All source files must be named after the public class they contain, appending the suffix .java, for example, HelloWorldApp.java. It must first be compiled into bytecode, using a Java compiler, producing a file with the .class suffix (Main.class, in this case). Only then can it be executed or launched. The Java source file may only contain one public class, but it can contain multiple classes with a non-public access modifier and any number of public inner classes. When the source file contains multiple classes, it is necessary to make one class (introduced by the class keyword) public (preceded by the public keyword) and name the source file with that public class name.
A class that is not declared public may be stored in any .java file. The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with .class appended. For class file generation, anonymous classes are treated as if their name were the concatenation of the name of their enclosing class, a $, and an integer.
The keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy.[61] The class hierarchy is related to the name of the directory in which the .java file is located. This is called an access level modifier. Other access level modifiers include the keywords private (a method that can only be accessed in the same class) and protected (which allows code from the same package to access).[61] If a piece of code attempts to access private methods or protected methods, the JVM will throw a SecurityException.
The method name main is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise JavaBeans do not use or need a main() method. A Java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from.
The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a JAR) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a command line.
In 2004, generics were added to the Java language, as part of J2SE 5.0. Prior to the introduction of generics, each variable declaration had to be of a specific type. For container classes, for example, this is a problem because there is no easy way to create a container that accepts only specific types of objects. Either the container operates on all subtypes of a class or interface, usually Object, or a different container class has to be created for each contained class. Generics allow compile-time type checking without having to create many container classes, each containing almost identical code. In addition to enabling more efficient code, certain runtime exceptions are prevented from occurring, by issuing compile-time errors. If Java prevented all runtime type errors (ClassCastExceptions) from occurring, it would be type safe. 2351a5e196
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