Guava：google公司开发的一款Java类库扩展工具包

<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>31.1-jre</version>
</dependency>

package com.example;
import com.google.common.base.Joiner;
import java.util.Arrays;
public class Demo {
    public static void main(String[] args) {
        // JDK 1.8
        String var1 = String.join("-", Arrays.asList("A", null, "C", "D"));
        System.out.println(var1);
        // A-null-C-D
        // guava
        Joiner joiner = Joiner.on("-").skipNulls();
        String var2 = joiner.join("A", null, "C", "D");
        System.out.println(var2);
        // A-C-D
    }
}

package com.example;
import com.google.common.collect.HashMultiset;
import com.google.common.collect.Multiset;
import java.util.Arrays;
public class Demo {
    public static void main(String[] args) {
        String[] words = {"A", "B", "C", "B"};
        // JDK 1.8
        Map<String, Integer> counts = new HashMap<String, Integer>();
        for (String word : words) {
            Integer count = counts.get(word);
            if (count == null) {
                counts.put(word, 1);
            } else {
                counts.put(word, count + 1);
            }
        }
        System.out.println(counts);
        // {A=1, B=2, C=1}
        // guava
        Multiset<String> multiset = HashMultiset.create();
        multiset.addAll(Arrays.asList(words));
        System.out.println(multiset);
        // [A, B x 2, C]
        System.out.println(multiset.count("B"));
        // 2
    }
}

package com.example;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;
public class Demo {
    public static void main(String[] args) {
        Multimap<String, Integer> multimap = ArrayListMultimap.create();
        multimap.put("A", 1);
        multimap.put("A", 2);
        multimap.put("B", 4);
        multimap.put("C", 5);
        System.out.println(multimap);
        // {A=[1, 2], B=[4], C=[5]}
    }
}