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The static class definition LazyHolder within it is not initialized until the JVM determines that LazyHolder must be executed. The static class LazyHolder is only executed when the static method getInstance is invoked on the class Something, and the first time this happens the JVM will load and initialize the LazyHolder class.
Initializers are blocks of code that are executed when a class or an instance of a class is created. There are two kinds of initializers, static initializers and instance initializers. Static initializers initialize static fields when the class is created. They are declared using the static keyword:
In a software design pattern view, lazy initialization is often used together with a factory method pattern. This combines three ideas: Using a factory method to create instances of a class (factory method pattern) Storing the instances in a map, and returning the same instance to each request for an instance with same parameters (multiton pattern)
A section of code that performs such initialization is generally known as "initialization code" and may include other, one-time-only, functions such as opening files; in object-oriented programming, initialization code may be part of a constructor (class method) or an initializer (instance method).
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 ...
C# has a static class syntax (not to be confused with static inner classes in Java), which restricts a class to only contain static methods. C# 3.0 introduces extension methods to allow users to statically add a method to a type (e.g., allowing foo.bar() where bar() can be an imported extension method working on the type of foo ).
Notably, a constructor is a class method as there is no object (instance) available until the object is created, but destructors, initializers, and finalizers are instance methods. Further, constructors and initializers often can accept arguments, while destructors and finalizers generally do not as they are often implicitly callable.
In C#, a static constructor is a static data initializer. [4]: 111–112 Static constructors are also called class constructors. Since the actual method generated has the name .cctor they are often also called "cctors". [5] [6] Static constructors allow complex static variable initialization. [7] Static constructors are called implicitly when ...