META-INF.modules.java.base.classes.java.lang.doc-files.ValueBased.html Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of java.base Show documentation

Bytecoder java.base Module

There is a newer version: 2024-05-10




  Value-based Classes
  


Value-based Classes

Some classes, such as java.util.Optional and
java.time.LocalDateTime, are value-based.  Instances of a
value-based class:

    are final and immutable (though may contain references to mutable
        objects);
    have implementations of equals,
        hashCode, and toString which are computed
        solely from the instance's state and not from its identity or the state
        of any other object or variable;
    make no use of identity-sensitive operations such as reference
        equality (==) between instances, identity hash code of
        instances, or synchronization on an instances's intrinsic lock;
    are considered equal solely based on equals(), not
        based on reference equality (==);
    do not have accessible constructors, but are instead instantiated
        through factory methods which make no committment as to the identity
        of returned instances;
    are freely substitutable when equal, meaning that interchanging
        any two instances x and y that are equal
        according to equals() in any computation or method
        invocation should produce no visible change in behavior.
    


A program may produce unpredictable results if it attempts to distinguish two
    references to equal values of a value-based class, whether directly via reference
    equality or indirectly via an appeal to synchronization, identity hashing,
    serialization, or any other identity-sensitive mechanism.  Use of such
    identity-sensitive operations on instances of value-based classes may have
    unpredictable effects and should be avoided.