Guava 常用方法

转载：https://dalin.blog.csdn.net/article/details/109396692

1. 数据校验

/**
     * 数据校验
     */
    @Test
    public void jiaoyan() {
        String param = "who are you";
        String name = Preconditions.checkNotNull(param);
        System.out.println(name);

        String param2 = null;
        String name2 = Preconditions.checkNotNull(param2, "param2 is null");
        System.out.println(name2);
    }

结果：

who are you

java.lang.NullPointerException: param2 is null
...

2. 是否越界

 // Guava 中快速创建ArrayList
        List<String> list = Lists.newArrayList("A", "B", "C", "D");
        // 开始校验
        int index = Preconditions.checkElementIndex(5, list.size());

结果：

java.lang.IndexOutOfBoundsException: index (5) must be less than size (4)
...

3. 集合

3.1 不可变集合

@Test
    public void buKeBianCollection() {
        // 创建方式1：of
        ImmutableSet<String> immutableSet = ImmutableSet.of("a", "b", "c");
        immutableSet.forEach(System.out::println);

        // 创建方式2：builder
        ImmutableSet<String> immutableSet2 = ImmutableSet.<String>builder()
                .add("hello")
                .add("汤姆")
                .build();
        immutableSet2.forEach(System.out::println);

        // 创建方式3：从其他集合中拷贝创建
        ArrayList<String> arrayList = new ArrayList();
        arrayList.add("ccc");
        arrayList.add("xxx");
        ImmutableSet<String> immutableSet3 = ImmutableSet.copyOf(arrayList);
        immutableSet3.forEach(System.out::println);

        // 使用 JDK Collections 创建不可变 List
        List<String> list = Collections.unmodifiableList(arrayList);
        list.forEach(System.out::println);
        //list.add("vvv"); // java.lang.UnsupportedOperationException
    }

注意：

• 使用 Guava 创建的不可变集合是拒绝 null 值的，因为在 Google 内部调查中，95% 的情况下都不需要放入 null 值。
• 使用 JDK 提供的不可变集合创建成功后，原集合添加元素会体现在不可变集合中，而 Guava 的不可变集合不会有这个问题。

        List<String> arrayList2 = new ArrayList<>();
        arrayList2.add("l");
        arrayList2.add("iu");
        List<String> jdkList = Collections.unmodifiableList(arrayList2);
        ImmutableList<String> immutableList = ImmutableList.copyOf(arrayList2);
        arrayList2.add("shuai");
        jdkList.forEach(System.out::println);// result: a b ccc
        System.out.println("-------");
        immutableList.forEach(System.out::println);// result: a b

3.2 集合创建工厂

@Test
    public void createCollections() {
        List<String> list1 = Lists.newArrayList();

        List<String> list2 = Lists.newArrayList("a", "b", "c");

        HashMap<Object, Object> hashMap = Maps.newHashMap();
        ConcurrentMap<Object, Object> concurrentMap = Maps.newConcurrentMap();
        TreeMap<Comparable, Object> treeMap = Maps.newTreeMap();

        HashSet<Object> hashSet = Sets.newHashSet();
        HashSet<String> newHashSet = Sets.newHashSet("a", "a", "b", "c");
    }

3.3 交集、并集、差集

	@Test
    public void compileCollections() {
        Set<String> newHashSet1 = Sets.newHashSet("a", "a", "b", "c");
        Set<String> newHashSet2 = Sets.newHashSet("b", "b", "c", "d");

        // 交集
        Sets.SetView<String> intersection = Sets.intersection(newHashSet1, newHashSet2);
        System.out.println(intersection); // [b, c]

        // 并集
        Sets.SetView<String> unionSet = Sets.union(newHashSet1, newHashSet2);
        System.out.println(unionSet); // [a, b, c, d]

        // newHashSet1 中存在，newHashSet2 中不存在
        Sets.SetView<String> setView = Sets.difference(newHashSet1, newHashSet2);
        System.out.println(setView); // [a]
    }

3.4 集合统计

1. 集合统计(计数): 统计相同元素出现的次数

@Test
    public void countCollections() {
        
        // use JDK
        List<String> words = Lists.newArrayList("a", "b", "c", "a", "e");

        Map<String, Integer> countMap = Maps.newHashMap();
        for (String word : words) {
            Integer count = countMap.get(word);
            count = (count == null) ? 1 : ++count;
            countMap.put(word, count);
        }

        countMap.forEach((k, v) -> System.out.println(k + " : " + v));

        // use Guava
        ArrayList<String> arrayList = Lists.newArrayList("a", "b", "c", "d", "a", "c");
        HashMultiset<String> multiset = HashMultiset.create(arrayList);
        multiset.elementSet().forEach(s -> System.out.println(s + ":" + multiset.count(s)));
    }

结果：

a : 2
b : 1
c : 1
e : 1
a:2
b:1
c:2
d:1

2. 集合统计(分类): 把很多动物按照种类进行分类

	@Test
    public void countCollectionsMore() {
        // JDK 原生写法
        HashMap<String, Set<String>> animalMap = new HashMap<>();
        HashSet<String> dogSet = new HashSet<>();
        dogSet.add("旺财");
        dogSet.add("大黄");
        animalMap.put("狗", dogSet);
        HashSet<String> catSet = new HashSet<>();
        catSet.add("加菲");
        catSet.add("汤姆");
        animalMap.put("猫", catSet);
        System.out.println(animalMap.get("猫")); // [加菲, 汤姆]

        // use guava
        // HashMultimap 可以扔进去重复的 key 值，最后获取时可以得到所有的 value 值
        HashMultimap<String, String> multimap = HashMultimap.create();
        multimap.put("狗", "大黄");
        multimap.put("狗", "旺财");
        multimap.put("猫", "加菲");
        multimap.put("猫", "汤姆");
        System.out.println(multimap.get("猫")); // [加菲, 汤姆]
    }

4. 字符串

4.1 字符拼接

	@Test
    public void StrJoin(){

        // JDK 方式一
        ArrayList<String> list = Lists.newArrayList("a", "b", "c", null);
        String join = String.join(",", list);
        System.out.println(join);

        // JDK 方式二
        String result = list.stream().collect(Collectors.joining(","));
        System.out.println(result);

        // JDK 方式三
        StringJoiner joiner = new StringJoiner(",");
        list.forEach(joiner::add);
        System.out.println(joiner.toString());

        // use guava
        ArrayList<String> list2 = Lists.newArrayList("a","b","c",null);
        String join1 = Joiner.on(",").skipNulls().join(list2);
        System.out.println(join1);

        String join2 = Joiner.on(",").useForNull("空空如也").join("jack","lucy","Lili",null);
        System.out.println(join2);

    }

4.2 字符分隔

 	@Test
    public void StrSplit(){
        // use JDK
        String str = ",a,,b,";
        String[] splitArr = str.split(",");
        Arrays.stream(splitArr).forEach(System.out::println);

        // use guava
        Iterable<String> split = Splitter.on(",")
                .omitEmptyStrings() // 忽略空值
                .trimResults() // 过滤结果中的空白
                .split(str);
        split.forEach(System.out::println);
    }

5. 临时缓存

package cn.luis.guava;

import com.google.common.cache.*;
import org.junit.jupiter.api.Test;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

public class GuavaCacheDemo {

    @Test
    public void test() throws ExecutionException, InterruptedException {
        CacheLoader cacheLoader = new CacheLoader<String, Animal>() {
            @Override
            public Animal load(String s) throws Exception {
                return new Animal();
            }
        };

        /*
          容量、过期时间、失效监听器
         */
        LoadingCache<String, Animal> loadingCache = CacheBuilder.newBuilder()
                .maximumSize(1000)
                .expireAfterWrite(3, TimeUnit.SECONDS)
                .removalListener(new MyRemovalListener())
                .build(cacheLoader);
        loadingCache.put("狗", new Animal("旺财", 1));
        loadingCache.put("猫", new Animal("汤姆", 3));
        loadingCache.put("狼", new Animal("灰太狼", 4));
        loadingCache.invalidate("猫"); // 手动失效

        Animal animal = loadingCache.get("狼");
        System.out.println(animal);
        Thread.sleep(4 * 1000);
        // 狼已经自动过去，获取为 null 值报错
        System.out.println(loadingCache.get("狼"));

    }


    /**
     * 缓存移除监听器
     */
    class MyRemovalListener implements RemovalListener<String, Animal> {

        @Override
        public void onRemoval(RemovalNotification<String, Animal> notification) {
            String reason = String.format("key=%s,value=%s,reason=%s", notification.getKey(), notification.getValue(), notification.getCause());
            System.out.println(reason);
        }


    }
}

package cn.luis.guava;

public class Animal {

    private String name;
    private Integer age;

    @Override
    public String toString() {
        return "Animal{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public Animal(){}

    public Animal(String name, Integer age) {
        this.name = name;
        this.age = age;
    }
}