8141132: JEP 254: Compact Strings
Summary: Adopt a more space-efficient internal representation for strings.
Reviewed-by: alanb, bdelsart, coleenp, iklam, jiangli, jrose, kevinw, naoto, pliden, roland, smarks, twisti
Contributed-by: Brent Christian <brent.christian@oracle.com>, Vivek Deshpande <vivek.r.deshpande@intel.com>, Tobias Hartmann <tobias.hartmann@oracle.com>, Charlie Hunt <charlie.hunt@oracle.com>, Vladimir Kozlov <vladimir.kozlov@oracle.com>, Roger Riggs <roger.riggs@oracle.com>, Xueming Shen <xueming.shen@oracle.com>, Aleksey Shipilev <aleksey.shipilev@oracle.com>, Sandhya Viswanathan <sandhya.viswanathan@intel.com>
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<h2 id="ValueBased">Value-based Classes</h2>
Some classes, such as <code>java.util.Optional</code> and
<code>java.time.LocalDateTime</code>, are <em>value-based</em>. Instances of a
value-based class:
<ul>
<li>are final and immutable (though may contain references to mutable
objects);</li>
<li>have implementations of <code>equals</code>,
<code>hashCode</code>, and <code>toString</code> which are computed
solely from the instance's state and not from its identity or the state
of any other object or variable;</li>
<li>make no use of identity-sensitive operations such as reference
equality (<code>==</code>) between instances, identity hash code of
instances, or synchronization on an instances's intrinsic lock;</li>
<li>are considered equal solely based on <code>equals()</code>, not
based on reference equality (<code>==</code>);</li>
<li>do not have accessible constructors, but are instead instantiated
through factory methods which make no committment as to the identity
of returned instances;</li>
<li>are <em>freely substitutable</em> when equal, meaning that interchanging
any two instances <code>x</code> and <code>y</code> that are equal
according to <code>equals()</code> in any computation or method
invocation should produce no visible change in behavior.
</li>
</ul>
<p>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.</p>
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