🔧 Top 25 Utility Functions Every Java Project Needs - Part 2

 🧾 Introduction

In Part 1, we explored 10 essential utility functions covering validation, string handling, retry logic, and date/time formatting.

In this Part 2, we’ll go deeper and cover 15 more practical and modern utility methods — covering collections, files, optional handling, system info, and more — using Java 17–21 syntax and conventions.

Let’s continue!

11. coalesce(String... values)

Returns the first non-null and non-blank string.

✅ Implementation:

public static String coalesce(String... values) {
    return Arrays.stream(values)
        .filter(s -> s != null && !s.isBlank())
        .findFirst()
        .orElse("");
}

🔄 Usage:

String name = coalesce(inputName, defaultName, "Unknown");

---

12. toListOrEmpty(List<T> list)

Returns the given list or an empty list if null.

✅ Implementation:

public static <T> List<T> toListOrEmpty(List<T> list) {
    return list == null ? List.of() : list;
}

🔄 Usage:

List<Order> orders = toListOrEmpty(user.getOrders());

---

🔢 13. joinWithComma(List<String> items)

Joins a list of strings into a comma-separated string.

✅ Implementation:

public static String joinWithComma(List<String> items) {
    return items == null ? "" : String.join(", ", items);
}

🔄 Usage:

String tagLine = joinWithComma(List.of("Java", "Spring", "Docker"));

---

14. wrapWithQuotes(String value)

Wraps a string with double quotes.

✅ Implementation:

public static String wrapWithQuotes(String value) {
    return "\"" + value + "\"";
}

🔄 Usage:

System.out.println(wrapWithQuotes("Ramesh")); // Output: "Ramesh"

---

15. asOptional(T value)

Wraps a nullable value into an Optional.

✅ Implementation:

public static <T> Optional<T> asOptional(T value) {
    return Optional.ofNullable(value);
}

🔄 Usage:

Optional<String> maybeName = asOptional(user.getName());

---

16. isEven(int number)

Checks if a number is even.

✅ Implementation:

public static boolean isEven(int number) {
    return number % 2 == 0;
}

🔄 Usage:

if (isEven(10)) System.out.println("Even number!");

---

17. readFileContent(Path path)

Reads the content of a file into a string.

✅ Implementation:

public static String readFileContent(Path path) throws IOException {
    return Files.readString(path);
}

🔄 Usage:

String content = readFileContent(Path.of("config.txt"));

---

18. writeToFile(Path path, String content)

Writes content to a file, overwriting if exists.

✅ Implementation:

public static void writeToFile(Path path, String content) throws IOException {
    Files.writeString(path, content, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);
}

🔄 Usage:

writeToFile(Path.of("log.txt"), "System started at " + LocalDateTime.now());

---

19. isPalindrome(String input)

Checks if a string is a palindrome.

✅ Implementation:

public static boolean isPalindrome(String input) {
    if (input == null) return false;
    String clean = input.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();
    return new StringBuilder(clean).reverse().toString().equals(clean);
}

🔄 Usage:

boolean result = isPalindrome("RaceCar"); // true

---

20. roundToTwoDecimals(double value)

Rounds a double value to two decimal places.

✅ Implementation:

public static double roundToTwoDecimals(double value) {
    return Math.round(value * 100.0) / 100.0;
}

🔄 Usage:

double price = roundToTwoDecimals(199.999); // 200.0

---

21. getFileExtension(String filename)

Extracts the file extension from a filename.

✅ Implementation:

public static String getFileExtension(String filename) {
    return Optional.ofNullable(filename)
        .filter(f -> f.contains("."))
        .map(f -> f.substring(f.lastIndexOf('.') + 1))
        .orElse("");
}

🔄 Usage:

String ext = getFileExtension("notes.txt"); // txt

---

22. isValidURL(String url)

Validates a URL string using regex.

✅ Implementation:

public static boolean isValidURL(String url) {
    return Optional.ofNullable(url)
        .map(u -> u.matches("^(http|https)://[\\w.-]+(?:\\.[\\w.-]+)+.*$"))
        .orElse(false);
}

🔄 Usage:

if (!isValidURL(inputUrl)) {
    System.out.println("Invalid URL!");
}

---

23. generateRandomAlphanumeric(int length)

Generates a random alphanumeric string of the given length.

✅ Implementation:

public static String generateRandomAlphanumeric(int length) {
    final String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    return new Random().ints(length, 0, chars.length())
        .mapToObj(chars::charAt)
        .map(Object::toString)
        .collect(Collectors.joining());
}

🔄 Usage:

String otp = generateRandomAlphanumeric(6);

---

24. getEnv(String key, String defaultValue)

Fetches an environment variable or returns a default value.

✅ Implementation:

public static String getEnv(String key, String defaultValue) {
    return Optional.ofNullable(System.getenv(key)).orElse(defaultValue);
}

🔄 Usage:

String dbHost = getEnv("DB_HOST", "localhost");

---

25. safeCast(Object obj, Class<T> targetType)

Safely casts an object using Java 21’s pattern matching.

✅ Implementation (Java 21):

public static <T> Optional<T> safeCast(Object obj, Class<T> type) {
    return (obj != null && type.isInstance(obj)) ? Optional.of(type.cast(obj)) : Optional.empty();
}

🔄 Usage:

Optional<String> name = safeCast(input, String.class);

✅ Avoids ClassCastException in flexible APIs.

---

✅ Final Summary

Here are the remaining 15 utility functions we covered in this part:

1. coalesce
2. toListOrEmpty
3. joinWithComma
4. wrapWithQuotes
5. asOptional
6. isEven
7. readFileContent
8. writeToFile
9. isPalindrome
10. roundToTwoDecimals
11. getFileExtension
12. isValidURL
13. generateRandomAlphanumeric
14. getEnv
15. safeCast

Combined with Part 1, these 25 utility functions can become a reliable part of your shared Java library, making development faster, cleaner, and more modern.

Here is the complete code in a single Java class for your reference:

package com.example.utils;

import java.io.IOException;
import java.nio.file.*;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.*;
import java.util.stream.Collectors;

public class JavaUtilsPart2 {

    public static String coalesce(String... values) {
        return Arrays.stream(values)
                .filter(s -> s != null && !s.isBlank())
                .findFirst()
                .orElse("");
    }

    public static <T> List<T> toListOrEmpty(List<T> list) {
        return list == null ? List.of() : list;
    }

    public static String joinWithComma(List<String> items) {
        return items == null ? "" : String.join(", ", items);
    }

    public static String wrapWithQuotes(String value) {
        return """ + value + """;
    }

    public static <T> Optional<T> asOptional(T value) {
        return Optional.ofNullable(value);
    }

    public static boolean isEven(int number) {
        return number % 2 == 0;
    }

    public static String readFileContent(Path path) throws IOException {
        return Files.readString(path);
    }

    public static void writeToFile(Path path, String content) throws IOException {
        Files.writeString(path, content, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);
    }

    public static boolean isPalindrome(String input) {
        if (input == null) return false;
        String clean = input.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();
        return new StringBuilder(clean).reverse().toString().equals(clean);
    }

    public static double roundToTwoDecimals(double value) {
        return Math.round(value * 100.0) / 100.0;
    }

    public static String getFileExtension(String filename) {
        return Optional.ofNullable(filename)
                .filter(f -> f.contains("."))
                .map(f -> f.substring(f.lastIndexOf('.') + 1))
                .orElse("");
    }

    public static boolean isValidURL(String url) {
        return Optional.ofNullable(url)
                .map(u -> u.matches("^(http|https)://[\w.-]+(?:\.[\w.-]+)+.*$"))
                .orElse(false);
    }

    public static String generateRandomAlphanumeric(int length) {
        final String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        return new Random().ints(length, 0, chars.length())
                .mapToObj(chars::charAt)
                .map(Object::toString)
                .collect(Collectors.joining());
    }

    public static String getEnv(String key, String defaultValue) {
        return Optional.ofNullable(System.getenv(key)).orElse(defaultValue);
    }

    public static <T> Optional<T> safeCast(Object obj, Class<T> type) {
        return (obj != null && type.isInstance(obj)) ? Optional.of(type.cast(obj)) : Optional.empty();
    }
}