What is Stateless Object in Java?

Question

Currently I'm reading "Java concurrency in practice", which contains this sentence:

Since the action of a thread accessing a stateless object can't affect the correctness of operations on other threads, stateless objects are thread-safe.

So, what is stateless object?

Answer 1

Stateless object is an instance of a class without instance fields (instance variables). The class may have fields, but they are compile-time constants (static final).

A very much related term is immutable. Immutable objects may have state, but it does not change when a method is invoked (method invocations do not assign new values to fields). These objects are also thread-safe.

Answer 2

If the object doesn't have any instance fields, it it stateless. Also it can be stateless if it has some fields, but their values are known and don't change.

This is a stateless object:

class Stateless {     void test() {         System.out.println("Test!");     } } 

This is also a stateless object:

class Stateless {     //No static modifier because we're talking about the object itself     final String TEST = "Test!";      void test() {         System.out.println(TEST);     } } 

This object has state, so it is not stateless. However, it has its state set only once, and it doesn't change later, this type of objects is called immutable:

class Immutable {     final String testString;      Immutable(String testString) {         this.testString = testString;     }      void test() {         System.out.println(testString);     } }