基本类型和原子类型有什么区别？

Question

Many programming language specifications talk about the language's primitive types and their atomic types. In some, these are mentioned separately, and some mix the definitions together. After reading some material I have come to realize that there is a difference between the two terms, however it seems to be either very subtle or too obscure.

What exactly is the difference between an atomic type and a primitive type ? Is one contained within the other?

Answer 1

The difference is that primitive types ( int etc) are not objects; although in Java there are classes associated to each primitive type ( Integer for int etc).

Atomic types are "just" regular Java objects; the difference with a "plain" object is that the methods to manipulate them are guaranteed to be atomic, therefore thread safe. For int , that would be AtomicInteger , which you can increment, .getAndSet() etc atomically.

It is partially incorrect to talk about "atomic types" vs "primitive types" however, since some classes which support atomic operations are not linked to primitive types; one such example is AtomicReference , but you also have AtomicIntegerArray and a few others.

See also the Wikipedia entry on compare and swap , or CAS for short.

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Also, while one would think that primitive types are fundamentally thread safe, this is not the case; each thread can have its own local storage (this is referred to, uninspiringly, as TLS, Thread Local Storage) for optimization, and can therefore have its own copy of primitive variables; this is why in Java you have volatile .

Answer 2

Primitive types are atomic. In other words, they cannot be de-constructed into simpler types.

Answer 3

In Java there are Primitive types and there are boxed types, Box types are actual objects and primitive types are just variables of a type.

For example int is a primitive type, while Integer is a boxed type.

int pi ; Integer bi;

pi is declared AND allocated and can be used immediately, eg: pi = 100 ;

bi is declared, but it is NOT allocated until: bi = new Integer(1);

Before allocation referencing bi will throw a java.lang.NullPointer error.