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[1] [2] An instance variable has similarities with a class variable, [3] but is non-static. An instance variable is a variable which is declared in a class but outside of constructors, methods, or blocks. Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class.
An example would be a static method to sum the values of all the variables of every instance of a class. For example, if there were a Product class it might have a static method to compute the average price of all products. A static method can be invoked even if no instances of the class exist yet.
In some languages, for example C++, Java, and Raku this or self is a keyword, and the variable automatically exists in instance methods. In others, for example, Python, Rust, and Perl 5, the first parameter of an instance method is such a reference. It needs to be specified explicitly.
Used in the declaration of a method or code block to acquire the mutex lock for an object while the current thread executes the code. [8] For static methods, the object locked is the class's Class. Guarantees that at most one thread at a time operating on the same object executes that code.
Getting the total number of televisions manufactured could be a static method of the television class. This method is associated with the class, yet is outside the domain of each instance of the class. A static method that finds a particular instance out of the set of all television objects is another example.
A singleton implementation may use lazy initialization in which the instance is created when the static method is first invoked. In multithreaded programs, this can cause race conditions that result in the creation of multiple instances. The following Java 5+ example [6] is a thread-safe implementation, using lazy initialization with double ...
Actual implementation happens outside Java code, and such methods have no body. strictfp - Declares strict conformance to IEEE 754 in carrying out floating-point operations. synchronized - Declares that a thread executing this method must acquire monitor. For synchronized methods the monitor is the class instance or java.lang.Class if the ...
A common example is the iostream library in C++, which uses the << or >> operators for the message passing, sending multiple data to the same object and allowing "manipulators" for other method calls. Other early examples include the Garnet system (from 1988 in Lisp) and the Amulet system (from 1994 in C++) which used this style for object ...