Common Java Utility Libraries and Their Usage Examples

1. Java Built‑in Utility Methods

Examples of common operations using the standard Java API, such as joining a List into a comma‑separated string, case‑insensitive string comparison, safe object equality checks with Objects.equals , and obtaining the intersection of two lists.

// How to join a list into a comma‑separated string
List
list = Arrays.asList("a", "b", "c");
String join = list.stream().collect(Collectors.joining(","));
System.out.println(join); // a,b,c

// Using String.join
default String join = String.join(",", list);
System.out.println(join); // a,b,c

// Case‑insensitive comparison
if (strA.equalsIgnoreCase(strB)) {
    System.out.println("相等");
}

// Safe object equality
Objects.equals(strA, strB);

// List intersection
list1.retainAll(list2);
System.out.println(list1); // [a, b]

2. Apache Commons Utility Libraries

2.1 commons‑lang3 (Enhanced java.lang)

Maven dependency:

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

Key utilities include:

StringUtils.isEmpty / isNotEmpty – null‑safe string emptiness checks.

StringUtils.isBlank / isNotBlank – also trims whitespace.

StringUtils.capitalize – capitalizes the first character.

StringUtils.repeat – repeats a string.

DateFormatUtils.format and DateUtils – simplify date formatting and arithmetic.

ImmutablePair and ImmutableTriple – lightweight tuple containers.

// String emptiness checks
boolean empty = StringUtils.isEmpty(cs);
boolean notEmpty = StringUtils.isNotEmpty(cs);

// Capitalize first letter
String capitalize = StringUtils.capitalize("yideng");
System.out.println(capitalize); // Yideng

// Repeat string
String repeated = StringUtils.repeat("ab", 2);
System.out.println(repeated); // abab

// Date formatting
String date = DateFormatUtils.format(new Date(), "yyyy-MM-dd HH:mm:ss");
System.out.println(date);

// Tuple example
ImmutablePair
pair = ImmutablePair.of(1, "yideng");
System.out.println(pair.getLeft() + "," + pair.getRight()); // 1,yideng

2.2 commons‑collections4 (Collection Utilities)

Maven dependency:

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

Provides methods such as CollectionUtils.retainAll , union , and subtract for set operations, as well as CollectionUtils.isEmpty checks.

// Collection emptiness
boolean empty = CollectionUtils.isEmpty(coll);

// Intersection, union, subtraction
Collection
intersect = CollectionUtils.retainAll(listA, listB);
Collection
union = CollectionUtils.union(listA, listB);
Collection
diff = CollectionUtils.subtract(listA, listB);

2.3 commons‑beanutils (Bean Manipulation)

Maven dependency:

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

Allows setting and getting bean properties via reflection, and converting between beans and maps.

User user = new User();
BeanUtils.setProperty(user, "id", 1);
BeanUtils.setProperty(user, "name", "yideng");
System.out.println(BeanUtils.getProperty(user, "name")); // yideng

Map
map = BeanUtils.describe(user);
User newUser = new User();
BeanUtils.populate(newUser, map);
System.out.println(newUser); // {"id":1,"name":"yideng"}

2.4 commons‑io (File I/O Utilities)

Maven dependency:

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

Simplifies reading, writing, and copying files.

File file = new File("demo1.txt");
List
lines = FileUtils.readLines(file, Charset.defaultCharset());
FileUtils.writeLines(new File("demo2.txt"), lines);
FileUtils.copyFile(srcFile, destFile);

3. Google Guava Utility Library

Maven dependency:

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

3.1 Collection Creation

List
list = Lists.newArrayList();
List
numbers = Lists.newArrayList(1, 2, 3);
List
reversed = Lists.reverse(numbers);
System.out.println(reversed); // [3, 2, 1]
List
> partitions = Lists.partition(numbers, 10);
Map
map = Maps.newHashMap();
Set
set = Sets.newHashSet();

3.2 Advanced Collection Types

3.2.1 Multimap (multiple values per key)

Multimap
map = ArrayListMultimap.create();
map.put("key", 1);
map.put("key", 2);
System.out.println(map); // {"key":[1,2]}
Map
> asMap = map.asMap();

3.2.2 BiMap (bijective map)

BiMap
biMap = HashBiMap.create();
biMap.put("key", "value");
System.out.println(biMap); // {"key":"value"}
BiMap
inverse = biMap.inverse();
System.out.println(inverse); // {"value":"key"}

3.2.3 Table (two‑dimensional map)

Table
table = HashBasedTable.create();
table.put(18, "男", "yideng");
table.put(18, "女", "Lily");
System.out.println(table.get(18, "男")); // yideng
Map
row = table.row(18);
System.out.println(row); // {"男":"yideng","女":"Lily"}

3.2.4 Multiset (set with element counts)

Multiset
multiset = HashMultiset.create();
multiset.add("apple");
multiset.add("apple");
multiset.add("orange");
System.out.println(multiset.count("apple")); // 2
Set
distinct = multiset.elementSet();
System.out.println(distinct); // ["orange","apple"]

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