Boost Java Productivity: Essential Utility Libraries and Code Tricks

1. Java built‑in utility methods

1.1 Join a List into a comma‑separated string

List<String> list = Arrays.asList("a", "b", "c");
// Using streams
String join = list.stream().collect(Collectors.joining(","));
System.out.println(join); // a,b,c
// Using String.join
join = String.join(",", list);
System.out.println(join); // a,b,c

1.2 Case‑insensitive string comparison

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

1.3 Object equality with null‑safety

Use Objects.equals(a, b) to avoid NullPointerException. Objects.equals(strA, strB); Source implementation:

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

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 null/blank 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 len = cs == null ? 0 : cs.length();
    if (len == 0) return true;
    for (int i = 0; i < len; 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 capitalized = StringUtils.capitalize(str);
System.out.println(capitalized); // Yideng

2.1.3 Repeat a string

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

2.1.4 Date formatting utilities

// 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 Temporary pair/triple objects

ImmutablePair<Integer, String> pair = ImmutablePair.of(1, "yideng");
System.out.println(pair.getLeft() + "," + pair.getRight()); // 1,yideng
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 null/empty checks

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

2.2.2 Set operations

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 (bean manipulation)

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
// Bean ↔ Map conversion
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(list);
List<List<Integer>> partitions = Lists.partition(list, 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 (bijective map)

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

3.2.4 Multiset (bag for counting)

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();
System.out.println(distinct); // [orange, apple]
multiset.setCount("apple", 5);

These libraries and snippets provide ready‑made solutions for many everyday Java development tasks, dramatically reducing boilerplate code and improving readability.