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Implementations of the singleton pattern ensure that only one instance of the singleton class ever exists and typically provide global access to that instance. Typically, this is accomplished by: Declaring all constructors of the class to be private, which prevents it from being instantiated by other objects
The difference between const and immutable is what they apply to: const is a property of the variable: there might legally exist mutable references to referred value, i.e. the value can actually change.
Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. Patterns that imply mutable state may be unsuited for functional programming languages.
The code above shows some of the conceptual differences between Java and Scala's handling of classes: Scala has no static variables or methods. Instead, it has singleton objects, which are essentially classes with only one instance. Singleton objects are declared using object instead of class.
The main difference between an arbitrary data structure and a purely functional one is that the latter is (strongly) immutable. This restriction ensures the data structure possesses the advantages of immutable objects: (full) persistency, quick copy of objects, and thread safety.
Data is immutable Type classes Garbage collection First appeared Common Lisp: No [1] Simulated with thunks [2] Dynamic [3] Yes [4] Extension [5] No [6]? Yes: 1984 ...
Primitive wrapper classes are not the same thing as primitive types. Whereas variables, for example, can be declared in Java as data types double, short, int, etc., the primitive wrapper classes create instantiated objects and methods that inherit but hide the primitive data types, not like variables that are assigned the data type values.
Concrete classes have to explicitly declare they implement the immutable interface. This may not be possible if the concrete class "belongs to" third-party code, for instance, if it is contained within a library. The object is not really immutable and hence not suitable for use in data structures relying on immutability like hash maps.