Boost Java Productivity: Essential Utility Libraries and Code Tricks

1. Java Built‑in Utility Methods

1.1 Join List to a Comma‑Separated String

// How to join a List into a comma‑separated string
List<String> list = Arrays.asList("a", "b", "c");
String join = list.stream().collect(Collectors.joining(",")); // a,b,c
// Or using String.join
String join2 = String.join(",", list); // a,b,c

1.2 Compare Two Strings Ignoring Case

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

1.3 Compare Two Objects Safely

Use Objects.equals to avoid NullPointerException when comparing objects.

Objects.equals(strA, strB);

1.4 Intersection of Two Lists

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); // [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>

2.1.1 String Empty Checks

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) {
    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);
}

2.1.2 Capitalize First Letter

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

2.1.3 Repeat String

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

2.1.4 Date Formatting

// Date → String
String date = DateFormatUtils.format(new Date(), "yyyy-MM-dd HH:mm:ss");
System.out.println(date);
// String → Date
Date parsed = DateUtils.parseDate("2021-05-01 01:01:01", "yyyy-MM-dd HH:mm:ss");
// Add one hour
Date plusOneHour = DateUtils.addHours(new Date(), 1);

2.1.5 Pair & Triple for Multiple Return Values

// Pair (two values)
ImmutablePair<Integer, String> pair = ImmutablePair.of(1, "yideng");
System.out.println(pair.getLeft() + "," + pair.getRight()); // 1,yideng
// Triple (three values)
ImmutableTriple<Integer, String, Date> triple = ImmutableTriple.of(1, "yideng", new Date());
System.out.println(triple.getLeft() + "," + triple.getMiddle() + "," + triple.getRight());

2.2 commons‑collections4 (Collection Utilities)

Maven dependency:

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

2.2.1 Collection Empty Checks

public static boolean isEmpty(final Collection<?> coll) {
    return coll == null || coll.isEmpty();
}
public static boolean isNotEmpty(final Collection<?> coll) {
    return !isEmpty(coll);
}
// Intersection, union, subtraction
Collection<String> intersect = CollectionUtils.retainAll(listA, listB);
Collection<String> union = CollectionUtils.union(listA, listB);
Collection<String> diff = CollectionUtils.subtract(listA, listB);

2.3 commons‑beanutils (Object‑Map Operations)

Maven dependency:

<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")); // yideng
// Object ↔ Map
Map<String, String> map = BeanUtils.describe(user);
User newUser = new User();
BeanUtils.populate(newUser, map);

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>

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);

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 Creating Collections

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

3.2 Advanced Collection Types

3.2.1 Multimap (One key → Multiple values)

Multimap<String, Integer> multimap = ArrayListMultimap.create();
multimap.put("key", 1);
multimap.put("key", 2);
System.out.println(multimap); // {key=[1, 2]}
Map<String, Collection<Integer>> asMap = multimap.asMap();

3.2.2 BiMap (Bidirectional map, values must be unique)

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

3.2.3 Table (Two‑dimensional map)

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

3.2.4 Multiset (Set that counts occurrences)

Multiset<String> multiset = HashMultiset.create();
multiset.add("apple");
multiset.add("apple");
multiset.add("orange");
System.out.println(multiset.count("apple")); // 2
Set<String> distinct = multiset.elementSet(); // [apple, orange]
Iterator<String> it = multiset.iterator();
while (it.hasNext()) {
    System.out.println(it.next());
}
multiset.setCount("apple", 5);

These libraries and snippets can dramatically reduce boilerplate code, improve readability, and increase development speed for Java developers.