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// 如何把list集合拼接成以逗号分隔的字符串 a,b,cList<String> list = Arrays.asList("a", "b", "c");// 种方法，可以用stream流String join = list.stream().collect(Collectors.joining(","));System.out.println(join); // 输出 a,b,c// 第二种方法，其实String也有join方法可以实现这个功能String join = String.join(",", list);System.out.println(join); // 输出 a,b,c

if (strA.equalsIgnoreCase(strB)) {  System.out.println("相等");}

Objects.equals(strA, strB);

public static boolean equals(Object a, Object b) {    return (a == b) || (a != null && a.equals(b));}

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); // 输出[a, b]

<dependency>    <groupId>org.apache.commons</groupId>    <artifactId>commons-lang3</artifactId>    <version>3.12.0</version></dependency>

public static boolean isEmpty(final CharSequence cs) {    return cs == null || cs.length() == 0;}

public static boolean isNotEmpty(final CharSequence cs) {    return !isEmpty(cs);}

// 判空的时候，会去除字符串中的空白字符，比如空格、换行、制表符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);}

String str = "yideng";String capitalize = StringUtils.capitalize(str);System.out.println(capitalize); // 输出Yideng

String str = StringUtils.repeat("ab", 2);System.out.println(str); // 输出abab

// Date类型转String类型String date = DateFormatUtils.format(new Date(), "yyyy-MM-dd HH:mm:ss");System.out.println(date); // 输出 2021-05-01 01:01:01

// String类型转Date类型Date date = DateUtils.parseDate("2021-05-01 01:01:01", "yyyy-MM-dd HH:mm:ss");

// 计算一个小时后的日期Date date = DateUtils.addHours(new Date(), 1);

// 返回两个字段ImmutablePair<Integer, String> pair = ImmutablePair.of(1, "yideng");System.out.println(pair.getLeft() + "," + pair.getRight()); // 输出 1,yideng// 返回三个字段ImmutableTriple<Integer, String, Date> triple = ImmutableTriple.of(1, "yideng", new Date());System.out.println(triple.getLeft() + "," + triple.getMiddle() + "," + triple.getRight()); // 输出 1,yideng,Wed Apr 07 23:30:00 CST 2021

<dependency>    <groupId>org.apache.commons</groupId>    <artifactId>commons-collections4</artifactId>    <version>4.4</version></dependency>

public static boolean isEmpty(final Collection<?> coll) {    return coll == null || coll.isEmpty();}

public static boolean isNotEmpty(final Collection<?> coll) {    return !isEmpty(coll);}

// 两个集合取交集Collection<String> collection = CollectionUtils.retainAll(listA, listB);// 两个集合取并集Collection<String> collection = CollectionUtils.union(listA, listB);// 两个集合取差集Collection<String> collection = CollectionUtils.subtract(listA, listB);

<dependency>    <groupId>commons-beanutils</groupId>    <artifactId>commons-beanutils</artifactId>    <version>1.9.4</version></dependency>

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")); // 输出 yidengSystem.out.println(user); // 输出 {"id":1,"name":"yideng"}

// 对象转mapMap<String, String> map = BeanUtils.describe(user);System.out.println(map); // 输出 {"id":"1","name":"yideng"}// map转对象User newUser = new User();BeanUtils.populate(newUser, map);System.out.println(newUser); // 输出 {"id":1,"name":"yideng"}

<dependency>    <groupId>commons-io</groupId>    <artifactId>commons-io</artifactId>    <version>2.8.0</version></dependency>

File file = new File("demo1.txt");// 读取文件List<String> lines = FileUtils.readLines(file, Charset.defaultCharset());// 写入文件FileUtils.writeLines(new File("demo2.txt"), lines);// 复制文件FileUtils.copyFile(srcFile, destFile);

<dependency>    <groupId>com.google.guava</groupId>    <artifactId>guava</artifactId>    <version>30.1.1-jre</version></dependency>

List<String> list = Lists.newArrayList();List<Integer> list = Lists.newArrayList(1, 2, 3);// 反转listList<Integer> reverse = Lists.reverse(list);System.out.println(reverse); // 输出 [3, 2, 1]// list集合元素太多，可以分成若干个集合，每个集合10个元素List<List<Integer>> partition = Lists.partition(list, 10);

Map<String, String> map = Maps.newHashMap();Set<String> set = Sets.newHashSet();

Multimap<String, Integer> map = ArrayListMultimap.create();map.put("key", 1);map.put("key", 2);Collection<Integer> values = map.get("key");System.out.println(map); // 输出 {"key":[1,2]}// 还能返回你以前使用的臃肿的MapMap<String, Collection<Integer>> collectionMap = map.asMap();

BiMap<String, String> biMap = HashBiMap.create();// 如果value重复，put方法会抛异常，除非用forcePut方法biMap.put("key","value");System.out.println(biMap); // 输出 {"key":"value"}// 既然value不能重复，何不实现个翻转key/value的方法，已经有了BiMap<String, String> inverse = biMap.inverse();System.out.println(inverse); // 输出 {"value":"key"}

// 一批用户，同时按年龄和性别分组Table<Integer, String, String> table = HashBasedTable.create();table.put(18, "男", "yideng");table.put(18, "女", "Lily");System.out.println(table.get(18, "男")); // 输出 yideng// 这其实是一个二维的Map，可以查看行数据Map<String, String> row = table.row(18);System.out.println(row); // 输出 {"男":"yideng","女":"Lily"}// 查看列数据Map<Integer, String> column = table.column("男");System.out.println(column); // 输出 {18:"yideng"}

Multiset<String> multiset = HashMultiset.create();multiset.add("apple");multiset.add("apple");multiset.add("orange");System.out.println(multiset.count("apple")); // 输出 2// 查看去重的元素Set<String> set = multiset.elementSet();System.out.println(set); // 输出 ["orange","apple"]// 还能查看没有去重的元素Iterator<String> iterator = multiset.iterator();while (iterator.hasNext()) {    System.out.println(iterator.next());}// 还能手动设置某个元素出现的次数multiset.setCount("apple", 5);