In Java, how do I convert a byte array to a string of hex digits while keeping leading zeros?

Option with Flair: Apache Commons Codec

A connoisseur of third-party libraries? Apache Commons Codec is your go-to. First, add its magic in your POM file:

<dependency>
    <groupId>commons-codec</groupId>
    <artifactId>commons-codec</artifactId>
    <version>1.15</version>  <!-- Everything's classy with 1.15 -->
</dependency>
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Then, just call upon the mighty Hex.encodeHexString to do your bidding:

import org.apache.commons.codec.binary.Hex;

byte[] bytes = {0, 15, 5};  // Array parked and ready
String hex = Hex.encodeHexString(bytes);
System.out.println(hex);  // Outputs: "000f05", oh yeah!
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Home-made Hex converter: For DIY lovers

For the star bakers who prefer home-cooked code, here's a loop with bitwise operations:

byte[] bytes = {0, 15, 5};  // Huddle up, bytes, we're going in!
StringBuilder sb = new StringBuilder();
for (byte b : bytes) {
    sb.append(String.format("%02x", b));
}
System.out.println(sb.toString()); // Outputs: "000f05", yum-yum!
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This manual method is your playground. Remix and season it to your tastes!

High-stakes scenarios: Byte array on an epic scale

When you're battling with giant byte arrays, every optimization makes a difference:

Halving time, doubling style with StringBuilder and hex characters

byte[] bytes = {0, 15, 5};
StringBuilder hexBuilder = new StringBuilder(bytes.length * 2); // Pre-allocated capacity, knock-knock memory leak, who's there!
final char[] hexArray = "0123456789ABCDEF".toCharArray();

for (byte b: bytes) {
    int v = b & 0xFF;
    hexBuilder.append(hexArray[v >>> 4]);
    hexBuilder.append(hexArray[v & 0x0F]);
}
String hex = hexBuilder.toString();  // Full-throttle conversion!
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In the high-performance lane

To make it in time for dinner:

• Ditch new object creation in loops.
• Pre-size your StringBuilder, because wisdom is knowing when to book your table.
• Use bitwise shift instead of modulo and division - faster than a speeding bullet!

Dark alley scenarios: When byte order matters

If you're playing in the Major League of data accuracy, beware the byte order wolf:

• Get your endianness straight: Big endian or little endian, it's reading order not a burger!
• If endianness rears its head, quickly bring out your ByteBuffer shield.

A touch of modern with Java 8+

Java 8, anyone? The dashing DatatypeConverter class is here to rescue:

import javax.xml.bind.DatatypeConverter;

byte[] bytes = {0, 15, 5};
String hex = DatatypeConverter.printHexBinary(bytes);  // Instant glamour!
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Don't forget, post-Java 9, this class is in the Java witness protection program:

--add-modules java.xml.bind
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Pro tips: The golden nuggets

Keep it clean, keep it consistent

Ensure consistency:

• For case-sensitive environments, String.format with "X" gives uppercase and "x" outputs lowercase hex.
• Zero-padding: Length matters, especially when on a strict fixed format diet.

Troubleshooting 101

• Input size: Is your input byte array sized for the runway or sitting at home in pajamas?
• Data corruption: Always perform a round-trip test (hex string to byte array and back) to prevent "Lost in Translation" scenarios.