| // |
| // Licensed to the Apache Software Foundation (ASF) under one |
| // or more contributor license agreements. See the NOTICE file |
| // distributed with this work for additional information |
| // regarding copyright ownership. The ASF licenses this file |
| // to you under the Apache License, Version 2.0 (the |
| // "License"); you may not use this file except in compliance |
| // with the License. You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, |
| // software distributed under the License is distributed on an |
| // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
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| // specific language governing permissions and limitations |
| // under the License. |
| // |
| |
| = DevFaqLookupVsHashmap |
| :jbake-type: wiki |
| :jbake-tags: wiki, devfaq, needsreview |
| :jbake-status: published |
| :keywords: Apache NetBeans wiki DevFaqLookupVsHashmap |
| :description: Apache NetBeans wiki DevFaqLookupVsHashmap |
| :toc: left |
| :toc-title: |
| :syntax: true |
| |
| === Why use this strange convention - wouldn't a Map be good enough? |
| |
| Other platforms do use string-keyed maps for this sort of thing, but there are some weaknesses with that approach: |
| |
| 1. it is impossible to enforce link:DevFaqModuleDependencies.asciidoc[dependencies]. With `link:DevFaqLookup.asciidoc[Lookup]`, a module's code cannot request an object of type X unless it can load/access the `X.class`. The module trying to look up an X will not be able to see `X.class` unless it link:DevFaqModuleDependencies.asciidoc[declares a dependency] on the module that defines X _and_ the module which defines X says that it allows access to the Java package X lives in. (A `Map<Class<T>,T>` would do the same job as `Lookup`.) |
| 2. The class of values in a map can change without notice - so if you have `(SomeIface) foo = (SomeIface) globalMap.get("foo")`, some new version of the module that provides "foo" can change the return type, causing `ClassCastException` s; with `Lookup`, you cannot ever get an object that is not of the type you passed in the request - so Lookup's approach is more robust. |
| 3. `Lookup` supports listening to changes in the result. |
| 4. `Lookup` supports multiple instances for one key - if you call `lookup(X.class)` you get one instance. If you call `lookupAll(X.class)` you get a `Collection<? extends X>` (so with `lookupAll()` it is more like a `Map<Class<T>,List<T>>`) |
| |
| There are some other capabilities of `Lookup` (such as getting the specific type or number of results without actually creating objects, and providing named result items) but these are rarely used in practice. |
| |
| `Lookup` is very powerful, yet simple and generic; people quickly learn to love it, once they realize what it can do. |
| |
| See also the javadoc: [link:http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/Lookup.html[http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/Lookup.html] `org.openide.util.Lookup`] |
| |
| === Apache Migration Information |
| |
| The content in this page was kindly donated by Oracle Corp. to the |
| Apache Software Foundation. |
| |
| This page was exported from link:http://wiki.netbeans.org/DevFaqLookupVsHashmap[http://wiki.netbeans.org/DevFaqLookupVsHashmap] , |
| that was last modified by NetBeans user Tboudreau |
| on 2010-02-18T23:58:49Z. |
| |
| |
| *NOTE:* This document was automatically converted to the AsciiDoc format on 2018-02-07, and needs to be reviewed. |
