Draft specification for java.lang.Record
Stuart Marks
stuart.marks at oracle.com
Tue Aug 20 23:46:48 UTC 2019
It might be worth also saying something about finalization. I think the choices
are to do nothing (be finalizer-agnostic) or to prohibit it. Generating a
finalize() method for every record class seems like a non-starter.
I'm leaning toward prohibiting finalization, for a couple reasons. First,
Object.finalize() is deprecated, and finalization will eventually be removed, so
we'd like to discourage new code from using finalizers. Second, if a
user-provided record constructor throws an exception, the partially-initialized
record instance will be visible to the finalizer and it potentially can be
resurrected. This seems like a really bad idea.
Suggestion:
/**
* Finalization of record class instances is disallowed.
*
* @implSpec
* This method implementation does nothing.
*/
protected final void finalize() { }
s'marks
On 8/15/19 10:34 AM, Brian Goetz wrote:
>
>
> Re-sending as plain text, since the formatting got mangled by mailers.
>
>
> /*
> * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
> * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
> *
> * This code is free software; you can redistribute it and/or modify it
> * under the terms of the GNU General Public License version 2 only, as
> * published by the Free Software Foundation. Oracle designates this
> * particular file as subject to the "Classpath" exception as provided
> * by Oracle in the LICENSE file that accompanied this code.
> *
> * This code is distributed in the hope that it will be useful, but WITHOUT
> * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
> * version 2 for more details (a copy is included in the LICENSE file that
> * accompanied this code).
> *
> * You should have received a copy of the GNU General Public License version
> * 2 along with this work; if not, write to the Free Software Foundation,
> * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
> *
> * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
> * or visit www.oracle.com if you need additional information or have any
> * questions.
> */
> package java.lang;
>
> /**
> * This is the common base class of all Java language record classes.
> *
> * <p>More information about records, including descriptions of the
> * implicitly declared methods synthesized by the compiler, can be
> * found in section 8.10 of
> * <cite>The Java™ Language Specification</cite>.
> *
> * <p>A <em>record class</em> is a shallowly immutable, transparent carrier for
> * a fixed set of values, called the <em>record components</em>. The Java™
> * language provides concise syntax for declaring record classes, whereby the
> * record components are declared in the record header. The list of record
> * components declared in the record header form the <em>record descriptor</em>.
> *
> * <p>A record class has the following mandated members: a public <em>canonical
> * constructor</em>, whose descriptor is the same as the record descriptor;
> * a private static field corresponding to each component, whose name and
> * type are the same as that of the component; a public accessor method
> * corresponding to each component, whose name and return type are the same as
> * that of the component. If not explicitly declared in the body of the record,
> * implicit implementations for these members are provided.
> *
> * <p>The implicit declaration of the canonical constructor initializes the
> * component fields from the corresponding constructor arguments. The implicit
> * declaration of the accessor methods returns the value of the corresponding
> * component field. The implicit declaration of the {@link
> Object#equals(Object)},
> * {@link Object#hashCode()}, and {@link Object#toString()} methods are derived
> * from all of the component fields.
> *
> * <p>The primary reasons to provide an explicit declaration for the
> * canonical constructor or accessor methods are to validate constructor
> * arguments, perform defensive copies on mutable components, or normalize groups
> * of components (such as reducing a rational number to lowest terms.) If any
> * of these are provided explicitly.
> *
> * <p>For all record classes, the following invariant must hold: if a record R's
> * components are {@code c1, c2, ... cn}, then if a record instance is copied
> * as follows:
> * <pre>
> * R copy = new R(r.c1(), r.c2(), ..., r.cn());
> * </pre>
> * then it must be the case that {@code r.equals(copy)}.
> *
> * @jls 8.10
> * @since 14
> */
> public abstract class Record {
> /**
> * Indicates whether some other object is "equal to" this one. In addition
> * to the general contract of {@link Object#equals(Object)},
> * record classes must further participate in the invariant that when
> * a record instance is "copied" by passing the result of the record component
> * accessor methods to the canonical constructor, the resulting copy is
> * equal to the original instance.
> *
> * @implNote
> * The implicitly provided implementation returns {@code true} if and
> * only if the argument is an instance of the same record type as this object,
> * and each component of this record is equal to the corresponding component
> * of the argument, according to {@link Object#equals(Object)} for components
> * whose types are reference types, and {@code ==} for components whose
> * types are primitive types.
> *
> * @see Object#equals(Object)
> *
> * @param obj the reference object with which to compare.
> * @return {@code true} if this object is the same as the obj
> * argument; {@code false} otherwise.
> */
> @Override
> public abstract boolean equals(Object obj);
>
> /**
> * {@inheritDoc}
> *
> * @implNote
> * The implicitly provided implementation returns a hash code value derived
> * by combining the hash code value for all the components, according to
> * {@link Object#hashCode()} for components whose types are reference types,
> * or the primitive wrapper hash code for components whose types are primitive
> * types.
> *
> * @see Object#hashCode()
> *
> * @return a hash code value for this object.
> */
> @Override
> public abstract int hashCode();
>
> /**
> * {@inheritDoc}
> *
> * @implNote
> * The implicitly provided implementation returns a string that is derived
> * from the name of the record class and the names and string representations
> * of all the components, according to {@link Object#toString()} for
> components
> * whose types are reference types, and the primitive wrapper {@code toString}
> * method for components whose types are primitive types.
> *
> * @see Object#toString() ()
> *
> * @return a string representation of the object.
> */
> @Override
> public abstract String toString();
> }
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