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Guava: Google Core Libraries for Java

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Guava is a set of core libraries that includes new collection types (such as multimap and multiset), immutable collections, a graph library, functional types, an in-memory cache, and APIs/utilities for concurrency, I/O, hashing, primitives, reflection, string processing, and much more!

Guava comes in two flavors.

Adding Guava to your build

Guava's Maven group ID is com.google.guava and its artifact ID is guava. Guava provides two different "flavors": one for use on a (Java 8+) JRE and one for use on Android or Java 7 or by any library that wants to be compatible with either of those. These flavors are specified in the Maven version field as either 27.0.1-jre or 27.0.1-android. For more about depending on Guava, see using Guava in your build.

To add a dependency on Guava using Maven, use the following:

<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>27.0.1-jre</version>
  <!-- or, for Android: -->
  <version>27.0.1-android</version>
</dependency>

To add a dependency using Gradle:

dependencies {
  compile 'com.google.guava:guava:27.0.1-jre'
  // or, for Android:
  api 'com.google.guava:guava:27.0.1-android'
}


Immutable Collections


Example



public static final ImmutableSet<String> COLOR_NAMES = ImmutableSet.of(
  "red",
  "orange",
  "yellow",
  "green",
  "blue",
  "purple");

class Foo {
  final ImmutableSet<Bar> bars;
  Foo(Set<Bar> bars) {
    this.bars = ImmutableSet.copyOf(bars); // defensive copy!
  }
}

Why?


Immutable objects have many advantages, including:




Making immutable copies of objects is a good defensive programming technique. Guava provides simple, easy-to-use immutable versions of each standard Collection type, including Guava's own Collection variations.


The JDK provides Collections.unmodifiableXXX methods, but in our opinion, these can be




When you don't expect to modify a collection, or expect a collection to remain constant, it's a good practice to defensively copy it into an immutable collection.


Important: Each of the Guava immutable collection implementations rejects null values. We did an exhaustive study on Google's internal code base that indicated that null elements were allowed in collections about 5% of the time, and the other 95% of cases were best served by failing fast on nulls. If you need to use null values, consider using Collections.unmodifiableList and its friends on a collection implementation that permits null. More detailed suggestions can be found here.


How?


An ImmutableXXX collection can be created in several ways:





public static final ImmutableSet<Color> GOOGLE_COLORS =
   ImmutableSet.<Color>builder()
       .addAll(WEBSAFE_COLORS)
       .add(new Color(0, 191, 255))
       .build();

Except for sorted collections, order is preserved from construction time. For example,



ImmutableSet.of("a", "b", "c", "a", "d", "b")

will iterate over its elements in the order "a", "b", "c", "d".


copyOf is smarter than you think


It is useful to remember that ImmutableXXX.copyOf attempts to avoid copying the data when it is safe to do so -- the exact details are unspecified, but the implementation is typically "smart". For example,



ImmutableSet<String> foobar = ImmutableSet.of("foo", "bar", "baz");
thingamajig(foobar);

void thingamajig(Collection<String> collection) {
   ImmutableList<String> defensiveCopy = ImmutableList.copyOf(collection);
   ...
}

In this code, ImmutableList.copyOf(foobar) will be smart enough to just return foobar.asList(), which is a constant-time view of the ImmutableSet.


As a general heuristic, ImmutableXXX.copyOf(ImmutableCollection) tries to avoid a linear-time copy if




This helps minimize the performance overhead of good defensive programming style.


asList


All immutable collections provide an ImmutableList view via asList(), so -- for example -- even if you have data stored as an ImmutableSortedSet, you can get the kth smallest element with sortedSet.asList().get(k).


The returned ImmutableList is frequently -- not always, but frequently -- a constant-overhead view, rather than an explicit copy. That said, it's often smarter than your average List -- for example, it'll use the efficient contains methods of the backing collection.


Details


Where?


InterfaceJDK or Guava?Immutable Version
Collection JDK ImmutableCollection
List JDK ImmutableList
Set JDK ImmutableSet
SortedSet/NavigableSet JDK ImmutableSortedSet
Map JDK ImmutableMap
SortedMap JDK ImmutableSortedMap
Multiset Guava ImmutableMultiset
SortedMultiset Guava ImmutableSortedMultiset
Multimap Guava ImmutableMultimap
ListMultimap Guava ImmutableListMultimap
SetMultimap Guava ImmutableSetMultimap
BiMap Guava ImmutableBiMap
ClassToInstanceMap Guava ImmutableClassToInstanceMap
Table Guava ImmutableTable

Google Guava官方教程(中文版)

Google Guava官方教程(英文版)

标签:ImmutableSet,Core,use,collection,Libraries,copyOf,Guava,example
来源: https://www.cnblogs.com/ncore/p/10470569.html