Immutable objects cannot be changed after creation, and can be created either by defining an immutable class or by explicitly declaring a mutable object as immutable. Various programming languages allow for creating immutable objects, and using a keyword to designate an object as immutable is easier but requires caution. Immutable objects should always be treated as such, and if mutable objects require immutable data to be created, the immutable data must be initialized first. Immutable objects cannot be made mutable after creation, but can sometimes be copied as mutable objects.
In object-oriented programming, an immutable object is one whose properties cannot be changed after it has been created. Most objects, by contrast, are mutable objects, meaning that some or all of their properties can be changed freely after creation. A variety of computer languages have the ability to create immutable objects, including Java, C++, Erlang, Tcl, and Scala.
An immutable object can be created in two ways: by creating a class that is by definition immutable or by explicitly declaring an otherwise mutable object immutable. The methodology for creating an immutable class varies depending on the computer language used. Regardless of language, an immutable class must have no methods that could change its internal data, and no way to change methods—that is, override or overload them—in a way that would change its internal data. This is the more complicated of the two approaches. To create an immutable object from an otherwise mutable object, on the other hand, one can often apply a keyword to designate the object as immutable.
For example, creating immutable objects in Java relies heavily on the “final” keyword. A quick way to make a class immutable in Java is to declare the class, as well as all of its methods and data members, final. A similar strategy can also be used in C++ with the “const” keyword. However, these approaches, depending on the content of the class, may not be the best. In general, objects can usually be made immutable by careful maintenance of their internal data.
Creating an immutable object using a keyword, like in the example above, is often much easier than writing an entire class as immutable. Using this approach, however, has its dangers. When explicitly designating an object immutable in this way, it is important for the programmer to make sure that all the appropriate properties and data have been incorporated into the object first. Relying on an incomplete object could be dangerous, especially if essential information is missing.
If an object is mutable but requires immutable data to be successfully created, the immutable data must be initialized before the mutable object can be created. Immutable objects are always immutable, whether they are data values or large objects, and should always be treated as such. In general, an immutable object cannot be made mutable after it has been created, although they can sometimes be copied as mutable objects. An immutable object and all copies of it can usually be assumed to be immutable, which can prevent data or memory problems.
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