Boost Your Java Productivity: Essential Utility Classes You Should Know

1. Collections

The java.util.Collections class provides a rich set of static methods for operating on collections. Below are some frequently used features.

1.1 Sorting

List<Integer> list = new ArrayList<>();
list.add(2);
list.add(1);
list.add(3);
Collections.sort(list); // ascending
System.out.println(list);
Collections.reverse(list); // descending
System.out.println(list);

Output:

[1, 2, 3]
[3, 2, 1]

1.2 Max / Min

List<Integer> list = new ArrayList<>();
list.add(2);
list.add(1);
list.add(3);
Integer max = Collections.max(list);
Integer min = Collections.min(list);
System.out.println(max);
System.out.println(min);

Output:

3
1

1.3 Empty List

private List<Integer> fun(List<Integer> list) {
    if (list == null || list.size() == 0) {
        return Collections.emptyList();
    }
    return list;
}

1.4 Unmodifiable List

List<Integer> list = new ArrayList<>();
list.add(2);
list.add(1);
list.add(3);
List<Integer> integers = Collections.unmodifiableList(list);
integers.add(4); // throws UnsupportedOperationException

1.5 Synchronized List

List<Integer> list = new ArrayList<>();
list.add(2);
list.add(1);
list.add(3);
List<Integer> integers = Collections.synchronizedList(list);
System.out.println(integers);

The underlying implementation creates SynchronizedRandomAccessList or SynchronizedList, which lock each method.

2. CollectionUtils

The Apache Commons CollectionUtils class offers additional collection utilities. Add the dependency:

<dependency>
    <groupId>commons-collections</groupId>
    <artifactId>commons-collections</artifactId>
    <version>3.2.2</version>
</dependency>

2.1 Empty / Not Empty

if (CollectionUtils.isEmpty(list)) {
    System.out.println("Collection is empty.");
}
if (CollectionUtils.isNotEmpty(list)) {
    System.out.println("Collection is not empty");
}

2.2 Set Operations

List<Integer> list2 = new ArrayList<>();
list2.add(2);
list2.add(4);
Collection<Integer> unionList = CollectionUtils.union(list, list2);
System.out.println(unionList);
Collection<Integer> intersectionList = CollectionUtils.intersection(list, list2);
System.out.println(intersectionList);
Collection<Integer> disjunctionList = CollectionUtils.disjunction(list, list2);
System.out.println(disjunctionList);
Collection<Integer> subtractList = CollectionUtils.subtract(list, list2);
System.out.println(subtractList);

Result:

[1, 2, 3, 4]
[2]
[1, 3, 4]
[1, 3]

3. Lists (Guava)

Guava’s com.google.common.collect.Lists provides convenient list utilities.

3.1 Quick Initialization

List<Integer> list = Lists.newArrayList(1, 2, 3);
System.out.println(list);

Output:

[1, 2, 3]

3.2 Cartesian Product

List<Integer> list1 = Lists.newArrayList(1, 2, 3);
List<Integer> list2 = Lists.newArrayList(4, 5);
List<List<Integer>> productList = Lists.cartesianProduct(list1, list2);
System.out.println(productList);

Output:

[[1, 4], [1, 5], [2, 4], [2, 5], [3, 4], [3, 5]]

3.3 Partition

List<Integer> list = Lists.newArrayList(1, 2, 3, 4, 5);
List<List<Integer>> partitionList = Lists.partition(list, 2);
System.out.println(partitionList);

Output:

[[1, 2], [3, 4], [5]]

3.4 Transform

List<String> list = Lists.newArrayList("a", "b", "c");
List<String> transformList = Lists.transform(list, x -> x.toUpperCase());
System.out.println(transformList);

Output:

[A, B, C]

3.5 Reverse

List<Integer> list = Lists.newArrayList(3, 1, 2, 5, 4);
List<Integer> reverseList = Lists.reverse(list);
System.out.println(reverseList);

Output:

[4, 5, 2, 1, 3]

4. Objects (JDK 7+)

4.1 Null Checks

Integer i = new Integer(10);
if (Objects.isNull(i)) {
    System.out.println("Object is null");
}
if (Objects.nonNull(i)) {
    System.out.println("Object is not null");
}

4.2 requireNonNull

Integer i = new Integer(128);
Objects.requireNonNull(i);
Objects.requireNonNull(i, "Parameters cannot be null");
Objects.requireNonNull(i, () -> "Parameters cannot be null");

4.3 Equality

Integer i1 = new Integer(1);
Integer i2 = new Integer(1);
System.out.println(Objects.equals(i1, i2)); // true
Long l = new Long(1);
System.out.println(Objects.equals(i1, l)); // false

4.4 Hash Code

String str = new String("abc");
System.out.println(Objects.hashCode(str));

5. StringUtils (Apache Commons Lang)

5.1 Empty / Blank Checks

String str1 = null;
String str2 = "";
String str3 = " ";
String str4 = "abc";
System.out.println(StringUtils.isEmpty(str1));
System.out.println(StringUtils.isEmpty(str2));
System.out.println(StringUtils.isEmpty(str3));
System.out.println(StringUtils.isEmpty(str4));
System.out.println(StringUtils.isBlank(str1));
System.out.println(StringUtils.isBlank(str2));
System.out.println(StringUtils.isBlank(str3));
System.out.println(StringUtils.isBlank(str4));

5.2 Split

String str1 = null;
System.out.println(StringUtils.split(str1, ",")); // returns null, no NPE

5.3 Numeric Check

System.out.println(StringUtils.isNumeric("123")); // true
System.out.println(StringUtils.isNumeric("123abc")); // false
System.out.println(StringUtils.isNumeric("0.33")); // false

5.4 Join

List<String> list = Lists.newArrayList("a", "b", "c");
List<Integer> list2 = Lists.newArrayList(1, 2, 3);
System.out.println(StringUtils.join(list, ","));
System.out.println(StringUtils.join(list2, " "));

6. BeanUtils (Spring)

6.1 Copy Properties

User user1 = new User();
user1.setId(1L);
user1.setName("Dylan");
user1.setAddress("Hong Kong");
User user2 = new User();
BeanUtils.copyProperties(user1, user2);
System.out.println(user2);

6.2 Find Declared Method

Method declaredMethod = BeanUtils.findDeclaredMethod(User.class, "getId");
System.out.println(declaredMethod.getName());

6.3 Find Property for Method

PropertyDescriptor pd = BeanUtils.findPropertyForMethod(declaredMethod);
System.out.println(pd.getName());

7. ReflectionUtils (Spring)

7.1 Find Method

Method method = ReflectionUtils.findMethod(User.class, "getId");

7.2 Find Field

Field field = ReflectionUtils.findField(User.class, "id");

7.3 Invoke Method

ReflectionUtils.invokeMethod(method, beanInstance, args);

7.4 Check Constant

System.out.println(ReflectionUtils.isPublicStaticFinal(field));

7.5 Check Equals Method

System.out.println(ReflectionUtils.isEqualsMethod(method));

8. DigestUtils (Apache Commons Codec)

8.1 MD5

String md5Hex = DigestUtils.md5Hex("Dylan");
System.out.println(md5Hex);

8.2 SHA‑256

String sha256Hex = DigestUtils.sha256Hex("Dylan");
System.out.println(sha256Hex);

9. HttpStatus

Instead of defining custom status codes, use the standard enums from org.springframework.http.HttpStatus or org.apache.http.HttpStatus which already provide constants such as 200 (OK), 404 (NOT_FOUND), and 500 (INTERNAL_SERVER_ERROR).

Conclusion

The nine utility classes presented—Collections, CollectionUtils, Lists, Objects, StringUtils, BeanUtils, ReflectionUtils, DigestUtils, and HttpStatus—cover a wide range of common development scenarios. Leveraging them can dramatically reduce boilerplate code, improve readability, and let developers focus on business logic rather than low‑level implementation details.