分享 Guava 的一些常见方法

前几天同事分享了一些关于 Guava 的一起基础用法，我之前没用过，感觉挺好的，所以记一些常见的方法。

一. 基础工具类，字符串相关的

其实这些在 apache commons-lang3，算是重复造轮子吧，简单说一下。

1. 判断字符串是否为空，之前看到很多人自己定义，这些可能是一些老程序员吧，apache commons-lang3，Guava 的如下：

boolean nullOrEmpty = Strings.isNullOrEmpty("");

2. 补全字符串(在前面补全和后面补全)

String padStart0 = Strings.padStart("3", 2, 'a');
System.out.println("padStart0 = " + padStart0);

String padStart1 = Strings.padStart("333", 2, 'a');
System.out.println("padStart1 = " + padStart1);

3. 拆分和合并字符串

List<String> list = Splitter.on(",").splitToList("Denny,BridgeLi,CCC");
System.out.println("list = " + list);

String join = Joiner.on(",").join(list);
System.out.println("join = " + join);

4. 对象相等判断和 ToStringHelper 类

boolean equal = Objects.equal("", "");
MoreObjects.toStringHelper();

二. 集合类相关的，这些个人感觉还是非常常用的

1. 不可变集合

@Test
public void testJDK() {
  List<String> list = new ArrayList<>();
  list.add("BridgeLi");
  list.add("DennyLi");

  List<String> unmodifiableList = Collections.unmodifiableList(list);
  System.out.println(unmodifiableList);

  unmodifiableList.add("Blog");
  System.out.println(unmodifiableList);
  list.add("Blog");
  System.out.println(unmodifiableList);

}

@Test
public void testImmutableList() {
  List<String> list = new ArrayList<>();
  list.add("BridgeLi");
  list.add("DennyLi");

  List<String> unmodifiableList = ImmutableList.copyOf(list);
  System.out.println(unmodifiableList);

  unmodifiableList.add("Blog");
  System.out.println(unmodifiableList);
  list.add("Blog");
  System.out.println(unmodifiableList);

}

2. 可重复集合，这个其实很常用可重复 set 可以用来基数，而可重复 Map 则可以实现 Map<K, List> 或者 Map<K, Set> 这样比较复杂的集合类型的数据结构

@Test
public void testMultiset() {
  List<String> strings = Lists.newArrayList("aa", "bb", "aa", "cc");

  Multiset<String> wordsMultiset = HashMultiset.create();
  wordsMultiset.addAll(strings);

  for (String key : wordsMultiset.elementSet()) {
    System.out.println(key + " count：" + wordsMultiset.count(key));
  }
}

@Test
public void teststuScoreMultimap() {
  Multimap<String, StudentScore> scoreMultimap = ArrayListMultimap.create();
  StudentScore studentScore = null;
  for (int i = 10; i < 20; i++) {
    studentScore = new StudentScore();
    studentScore.courseId = 1001 + i;
    studentScore.score = 100 - i;
    scoreMultimap.put("peida", studentScore);
  }
  System.out.println("scoreMultimap:" + scoreMultimap.size());
  System.out.println("scoreMultimap:" + scoreMultimap.keys());

  Collection<StudentScore> studentScores = scoreMultimap.get("peida");
  StudentScore studentScore1 = new StudentScore();
  studentScore1.courseId = 1034;
  studentScore1.score = 67;
  studentScores.add(studentScore1);

  StudentScore studentScore2 = new StudentScore();
  studentScore2.courseId = 1045;
  studentScore2.score = 56;
  scoreMultimap.put("jerry", studentScore2);

  System.out.println("scoreMultimap:" + scoreMultimap.size());
  System.out.println("scoreMultimap:" + scoreMultimap.keys());

  for (StudentScore stuScore : scoreMultimap.values()) {
    System.out.println("stuScore one:" + stuScore.courseId + " score:" + stuScore.score);
  }

  scoreMultimap.remove("jerry", studentScore2);
  System.out.println("scoreMultimap:" + scoreMultimap.size());
  System.out.println("scoreMultimap:" + scoreMultimap.get("jerry"));

  scoreMultimap.put("harry", studentScore2);
  scoreMultimap.removeAll("harry");
  System.out.println("scoreMultimap:" + scoreMultimap.size());
  System.out.println("scoreMultimap:" + scoreMultimap.get("harry"));
}

class StudentScore{
  int courseId;
  int score;
}

3. 双向 Map(BiMap)，Map 是一种键值对映射，这个映射是键到值的映射，而 BiMap 既提供键到值的映射，也提供值到键的映射，所以它是双向 Map

@Test
public void testBimap() {
  BiMap<String, String> biMap = HashBiMap.create();
  biMap.put("星期一", "Monday");
  System.out.println("by key:" + biMap.get("星期一"));
  BiMap<String, String> biMap1 = biMap.inverse();
  System.out.println("biMap1:" + biMap1.get("Monday"));
}

4. 双键 Map(Table)，有些时候需要写 Map<String, Map<String, String» 这种格式的代码。但是这种阅读起来非常的不友好，Table 提供了新的思路：通过 rowKey + columnKey + value 来支持

@Test
public void testTable() {
  Table<String, String, String> table = HashBasedTable.create();
  table.put("IBM", "101", "A");
  table.put("IBM", "102", "B");
  table.put("IBM", "103", "C");

  table.put("sun", "111", "D");
  table.put("sun", "112", "E");
  table.put("sun", "113", "F");

  table.put("Google", "121", "G");
  table.put("Google", "102", "H");
  table.put("Google", "123", "L");

  System.out.println(table.get("IBM", "102"));
  System.out.println(table.row("IBM"));
  System.out.println(table.column("102"));
}

部分参考：https://www.cnblogs.com/peida/tag/Guava%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0/