Don't build the wheel again. It is recommended to use Google guava open source tool class library. It is really powerful!

Google Guava summary

1、Guava It's a group from Google At the heart of Java library , Include new set types （ Such as multimap and multiset）、 Immutable set 、 Graphics library and for concurrency 、I/O、 hash 、 cache 、 The original language 、 String and so on ！ Widely used in Google Most of Java In the project , It's also widely used by many other companies .

2、guava github Open source address ：GitHub - google/guava：

https://github.com/google/guava

3、 Official website user manual

https://github.com/google/guava/wiki

4、com.google.guava rely on ：

<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>29.0-jre</version>
</dependency>

Immutable sets and objects

1、 Making immutable copies of objects is a good defensive programming technique , Immutable objects have many advantages , Include ：

• Safe for untrusted Libraries .
• Thread safety ： Can be used by multiple threads , No contention risk .
• There is no need to support mutation , And it can save time and space , All immutable collection implementations save more memory than their mutable siblings .
• Can be used as a constant , And expect it to remain the same .

2、 The main points of ： Every Guava Immutable set implementations reject null value .Guava It is recommended to use null value , Most of the time , encounter null The value will throw an exception .

3、 An immutable ImmutableXxx Collections can be created in several ways ：

• Use copyOf Method , Such as ImmutableSet.copyOf(set)
• Use of Method , Such as ImmutableSet.of("a", "b", "c") or ImmutableMap.of("a", 1, "b", 2)
• Use Builder Method ,.

4、Guava by java jdk Each standard collection type provides an easy-to-use, immutable version , Include Guava Own set variants , by java The immutable versions provided are inherited java jdk From the interface of , So the operation is basically the same . The implementation classes of the following interfaces also have corresponding immutable versions .

Interface	JDK perhaps 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

Online demo source code ：

https://github.com/main/java/com/wmx/guava/ImmutableCollectionTest.java

Official document

https://github.com/google/guava/wiki/ImmutableCollectionsExplained

Recommend a Spring Boot Basic tutorials and practical examples ：

https://github.com/javastacks/spring-boot-best-practice

Guava New collection type

1、Guava Many new collection types have been introduced , These types are not in Java JDK in , But it's very useful , These are all for the sake of JDK The collection framework is designed to coexist happily , Instead of stuffing things into JDK In set abstraction .

Multiset Repeatable set

1、Guava Provides a new collection type Multiset, It supports adding multiple identical elements , Members can appear more than once .

2、Multiset amount to Set, The difference lies in Multiset You can add the same elements , It uses a HashMap To maintain the ,

3、Multiset It also has its own implementation class , Commonly used HashMultiset、LinkedHashMultiset、TreeMultiset etc. ,HashMultiset 、TreeMultiset Is chaotic ,LinkedHashMultiset Is ordered , The operation is exactly the same JDK Of HashSet、TreeSet、LinkedHashSet.

Online demo source code

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/MultisetTest.java

Multimap Multiple mapping

1、 Every experienced Java Programmers have implemented... Somewhere Map<K、List<V>> or Map<K、Set<V>>,Guava Of Multimap The framework makes it easy to handle mapping from keys to multiple values , Multiple mapping is a general method of associating keys with any number of values .

2、 conceptually , There are two ways to think of multiple mappings as a collection of mappings from a single key to a single value ：

3、Multimap A variety of implementations are provided ：

Multimap	Realization key It uses	value It uses
ArrayListMultimap	HashMap	ArrayList
HashMultimap	HashMap	HashSet
LinkedListMultimap *	LinkedHashMap*	LinkedList*
LinkedHashMultimap**	LinkedHashMap	LinkedHashSet
TreeMultimap	TreeMap	TreeSet
ImmutableListMultimap	ImmutableMap	ImmutableList
ImmutableSetMultimap	ImmutableMap	ImmutableSet

4、 In addition to the immutable implementation , Each implementation supports null keys and values . Not all implementations are implemented as Map<K,Collection<V>> Realized （ Especially some Multimap The implementation uses a custom hash table to minimize overhead .）

Online demo source code

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/MultimapTest.java

Recommend a Spring Boot Basic tutorials and practical examples ：

https://github.com/javastacks/spring-boot-best-practice

BiMap Bidirectional mapping

1、 The traditional way to map values back to keys is to maintain two separate mappings , And keep them synchronized , But it's easy to make mistakes , And when a value already exists in the mapping

Map<String, Integer> nameToId = Maps.newHashMap();
Map<Integer, String> idToName = Maps.newHashMap();

nameToId.put("Bob", 42);
idToName.put(42, "Bob");

2、BiMap A variety of implementations are provided ：

Key value mapping implementation	Value key mapping implementation	Corresponding BiMap
HashMap	HashMap	HashBiMap
ImmutableMap	ImmutableMap	ImmutableBiMap
EnumMap	EnumMap	EnumBiMap
EnumMap	HashMap	EnumHashBiMap

Online demo source code ：

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/BiMapTest.java

Table Table structure data

1、 When trying to index multiple keys at once , You will get something like Map<FirstName,Map<LastName,Person>> Code for , It's ugly , And it's embarrassing to use .Guava Provides a new collection type Table, It supports any “row” The type and “column” This use case of type .

2、Table A variety of implementations are provided ：

• HashBasedTable： It's basically from HashMap<R,HashMap<C,V>> Supported by .
• TreeBasedTable： It's basically from TreeMap<R,TreeMap<C,V>> Supported by .
• ImmutableTable
• ArrayTable： It is required to specify the full range of rows and columns during construction , Memory intensive and two-dimensional arrays are supported to improve efficiency ,ArrayTable Works a little differently from other implementations

Online demo source code ：

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/TableTest.java

ClassToInstanceMap Type mapping to instance

1、 Sometimes key Not a single type , But many types ,Guava This provides ClassToInstanceMap,key It can be of many types ,value Is an instance of this type .

2、ClassToInstanceMap The realization of ： MutableClassToInstanceMap and ImmutableClassToInstanceMap The implementation of the .

Online demo source code ：

https://github.com/wangmaoxiong/src/main/java/com/wmx/guava/ClassToInstanceMapTest.java

JDK Collection helper class

1、 Any JDK Programmers with experience in the collection framework know and like the utilities provided java.util.Collections,Guava There are many static method utilities available for collections .

Interface	Belong to JDK still Guava	Corresponding Guava API
Collection	JDK	Collections2
List	JDK	Lists
Set	JDK	Sets
SortedSet	JDK	Sets
Map	JDK	Maps
SortedMap	JDK	Maps
Queue	JDK	Queues
Multiset	Guava	Multisets
Multimap	Guava	Multimaps
BiMap	Guava	Maps
Table	Guava	Tables

Lists The online demo ：

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/ListsTest.java

Sets The online demo ：

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/SetsTest.java

JDK Basic type helper class

1、Guava by Java JDK The basic type of provides utility classes ：

Basic types	Guava Auxiliary tools
byte	Bytes, SignedBytes, UnsignedBytes
short	Shorts
int	Ints, UnsignedInteger, UnsignedInts
long	Longs, UnsignedLong, UnsignedLongs
float	Floats
double	Doubles
char	Chars
boolean	Booleans

nts Online demo source code ：

https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/IntsTest.java

doubles Online demo source code ：

https://github.com/src/main/java/com/wmx/guava/DoublesTest.java

booleans Online demo source code ：

https://github.com/src/main/java/com/wmx/guava/BooleansTest.java

The same applies to other types .

JDK String helper class

1、Strings Class provides a few commonly used string Utilities .

Online demo source code ：https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/StringsTest.java

2、Joiner It's a connector , Used to connect to java.lang.Iterable、java.util.Iterator、java.lang.Object[] The elements in .

Online demo source code ：https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/JoinerTest.java

3、Splitter It's a divider , Used to segment character sequences java.lang.CharSequence.

Online demo source code ：https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/SplitterTest.java

4、CharMatcher Character matchers , Used to match characters , Can be CharMatcher Considered to represent a specific class of characters , Such as numbers or blanks . Be careful ：CharMatcher Only deal with char value .

Online demo source code ：https://github.com/wangmaoxiong/apache-study/blob/master/src/main/java/com/wmx/guava/CharMatcherTest.java

Stopwatch Stopwatch

1、google Stop watch Stopwatch comparison Spring framewrk core package and apache commons lang3 The stopwatch in the bag is the most convenient to use .

2、 This class is not thread safe .

/**
 * Stopwatch createStarted()： establish （ And start the ） A new stopwatch , Use  System#nanoTime  As its time source .
 * Stopwatch createUnstarted()： establish （ But it doesn't start. ） A new stopwatch , Use  System#nanoTime  As its time source .
 * long elapsed(TimeUnit desiredUnit)： Returns the current elapsed time displayed on this stopwatch , Expressed in required time units , Any fraction is rounded down 
 * boolean isRunning()： If... Has been called on this stopwatch start（）}, And since the last call start（） Has not been called since stop（）, Then return to true
 * Stopwatch reset()： Set the running time of this stopwatch to zero , And put it in the stop state .
 * Stopwatch start()： Start stopwatch , If the stopwatch is already running , be  IllegalStateException
 * Stopwatch stop()： Stop the stopwatch , Future reads will return the fixed duration elapsed so far .
 * tring toString()： Returns the string representation of the current runtime , such as  2.588 s,106.8 ms
 */
@Test
public void testStopwatch() throws InterruptedException {
    SecureRandom secureRandom = new SecureRandom();
    Stopwatch stopwatch = Stopwatch.createStarted();

    int nextInt = secureRandom.nextInt(2000);
    System.out.println(" Mission 1 Budget time ：" + nextInt);// Mission 1 Budget time ：81
    TimeUnit.MILLISECONDS.sleep(nextInt);
    System.out.println("\t Mission 1 The actual time consuming ：" + stopwatch.elapsed(TimeUnit.MILLISECONDS) + "( millisecond )");//  Mission 1 The actual time consuming ：563( millisecond )

    stopwatch.reset().start();
    nextInt = secureRandom.nextInt(4000);
    System.out.println(" Mission 2 Budget time ：" + nextInt);// Mission 2 Budget time ：1591
    TimeUnit.MILLISECONDS.sleep(nextInt);
    System.out.println("\t Mission 2 The actual time consuming ：" + stopwatch.toString());//  Mission 2 The actual time consuming ：1.592 s

    stopwatch.reset().start();
    nextInt = secureRandom.nextInt(3000);
    System.out.println(" Mission 3 It's expected to take ：" + nextInt);// Mission 3 It's expected to take ：1964
    TimeUnit.MILLISECONDS.sleep(nextInt);
    System.out.println("\t Mission 3 The actual time consuming ：" + stopwatch.stop().toString());//  Mission 3 The actual time consuming ：1.965 s
}

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