# Java 自带的好用的工具类
# List 集合拼接成以逗号分隔的字符串
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List<String> list = Arrays.asList("a", "b", "c");
String join = list.stream().collect(Collectors.joining(","));
System.out.println(join);
String join = String.join(",", list);
System.out.println(join);
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# 比较两个字符串是否相等，忽略大小写
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| if (strA.equalsIgnoreCase(strB)) {
System.out.println("相等");
}
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# 比较两个对象是否相等
当我们用 equals 比较两个对象是否相等的时候，还需要对左边的对象进行判空，不然可能会报空指针异常，我们可以用 java.util 包下 Objects 封装好的比较是否相等的方法。
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| Objects.equals(strA, strB);
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
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# 两个 List 取交集
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| List<String> list1 = new ArrayList<>();
list1.add("a");
list1.add("b");
list1.add("c");
List<String> list2 = new ArrayList<>();
list2.add("a");
list2.add("b");
list2.add("d");
list1.retainAll(list2);
System.out.println(list1);
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# 使用 apache commons 工具类库
# 字符串判空
传参 CharSequence 类型是 String、StringBuilder、StringBuffer 的父类，都可以直接下面方法判空，以下是源码：
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public static boolean isEmpty(final CharSequence cs) {
return cs == null || cs.length() == 0;
}
public static boolean isNotEmpty(final CharSequence cs) {
return !isEmpty(cs);
}
public static boolean isBlank(final CharSequence cs) {
final int strLen = length(cs);
if (strLen == 0) {
return true;
}
for (int i = 0; i < strLen; i++) {
if (!Character.isWhitespace(cs.charAt(i))) {
return false;
}
}
return true;
}
public static boolean isNotBlank(final CharSequence cs) {
return !isBlank(cs);
}
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# 首字母转成大写
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| String str = "yideng";
String capitalize = StringUtils.capitalize(str);
System.out.println(capitalize);
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# 重复拼接字符串
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| String str = StringUtils.repeat("ab", 2);
System.out.println(str);
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# 格式化日期
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String date = DateFormatUtils.format(new Date(), "yyyy-MM-dd HH:mm:ss");
System.out.println(date);
Date date = DateUtils.parseDate("2021-05-01 01:01:01", "yyyy-MM-dd HH:mm:ss");
Date date = DateUtils.addHours(new Date(), 1);
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# 包装临时对象
当一个方法需要返回两个及以上字段时，我们一般会封装成一个临时对象返回，现在有了 Pair 和 Triple 就不需要了。
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ImmutablePair<Integer, String> pair = ImmutablePair.of(1, "yideng");
System.out.println(pair.getLeft() + "," + pair.getRight());
ImmutableTriple<Integer, String, Date> triple = ImmutableTriple.of(1, "yideng", new Date());
System.out.println(triple.getLeft() + "," + triple.getMiddle() + "," + triple.getRight());
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# commons-collections 集合工具类
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| <dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-collections4</artifactId>
<version>4.4</version>
</dependency>
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# 集合判空
封装了集合判空的方法，以下是源码：
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| public static boolean isEmpty(final Collection<?> coll) {
return coll == null || coll.isEmpty();
}
public static boolean isNotEmpty(final Collection<?> coll) {
return !isEmpty(coll);
}
Collection<String> collection = CollectionUtils.retainAll(listA, listB);
Collection<String> collection = CollectionUtils.union(listA, listB);
Collection<String> collection = CollectionUtils.subtract(listA, listB);
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# common-beanutils 操作对象
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| <dependency>
<groupId>commons-beanutils</groupId>
<artifactId>commons-beanutils</artifactId>
<version>1.9.4</version>
</dependency>
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| public class User {
private Integer id;
private String name;
}
User user = new User();
BeanUtils.setProperty(user, "id", 1);
BeanUtils.setProperty(user, "name", "yideng");
System.out.println(BeanUtils.getProperty(user, "name"));
System.out.println(user);
Map<String, String> map = BeanUtils.describe(user);
System.out.println(map);
User newUser = new User();
BeanUtils.populate(newUser, map);
System.out.println(newUser);
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# commons-io 文件流处理
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| <dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.8.0</version>
</dependency>
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文件处理
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| File file = new File("demo1.txt");
List<String> lines = FileUtils.readLines(file, Charset.defaultCharset());
FileUtils.writeLines(new File("demo2.txt"), lines);
FileUtils.copyFile(srcFile, destFile);
```
# ``Google Guava`` 工具类库
```xml
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>30.1.1-jre</version>
</dependency>
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# 创建集合
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| List<String> list = Lists.newArrayList();
List<Integer> list = Lists.newArrayList(1, 2, 3);
List<Integer> reverse = Lists.reverse(list);
System.out.println(reverse);
List<List<Integer>> partition = Lists.partition(list, 10);
Map<String, String> map = Maps.newHashMap();
Set<String> set = Sets.newHashSet();
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# Multimap 一个 key 可以映射多个 value 的 HashMap
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Multimap<String, Integer> map = ArrayListMultimap.create();
map.put("key", 1);
map.put("key", 2);
Collection<Integer> values = map.get("key");
System.out.println(map);
Map<String, Collection<Integer>> collectionMap = map.asMap();
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多省事，多简洁，省得你再创建 Map<String, List>
# BiMap 一种连 value 也不能重复的 HashMap
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| BiMap<String, String> biMap = HashBiMap.create();
biMap.put("key","value");
System.out.println(biMap);
BiMap<String, String> inverse = biMap.inverse();
System.out.println(inverse);
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这其实是双向映射，在某些场景还是很实用的。
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# Table 一种有两个 key 的 HashMap
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Table<Integer, String, String> table = HashBasedTable.create();
table.put(18, "男", "yideng");
table.put(18, "女", "Lily");
System.out.println(table.get(18, "男"));
Map<String, String> row = table.row(18);
System.out.println(row);
Map<Integer, String> column = table.column("男");
System.out.println(column);
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# Multiset 一种用来计数的 Set
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| Multiset<String> multiset = HashMultiset.create();
multiset.add("apple");
multiset.add("apple");
multiset.add("orange");
System.out.println(multiset.count("apple"));
Set<String> set = multiset.elementSet();
System.out.println(set);
Iterator<String> iterator = multiset.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
multiset.setCount("apple", 5);
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