How to encode and decode Base64

Short answer

Base64 turns bytes into text made of Latin letters, digits and the +, / and = characters. Turn the string into bytes first:

void main() {
    String text = "Hello, Java!";
    String encoded = Base64.getEncoder().encodeToString(text.getBytes(StandardCharsets.UTF_8));
 
    IO.println(encoded);
}

Output:

SGVsbG8sIEphdmEh

Decoding

The decoder returns a byte array — make a string out of it again:

void main() {
    byte[] bytes = Base64.getDecoder().decode("SGVsbG8sIEphdmEh");
    IO.println(new String(bytes, StandardCharsets.UTF_8));
}

Output:

Hello, Java!

For URLs and tokens

Regular Base64 contains + and /, which have a special meaning in URLs. The URL-safe variant replaces them with - and _:

void main() {
    byte[] data = {(byte) 0xfb, (byte) 0xff, (byte) 0xbf};
 
    IO.println(Base64.getEncoder().encodeToString(data));
    IO.println(Base64.getUrlEncoder().withoutPadding().encodeToString(data));
}

Output:

+/+/
-_-_

Common mistake

Treating Base64 as encryption. It is just another way to write data: anyone can decode the string back without a key. And a string with invalid characters throws an exception:

void main() {
    Base64.getDecoder().decode("not base64!");
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: Illegal base64 character 20

Thanks for reading!

Short answer

Decoding

For URLs and tokens

Common mistake

Thanks for reading!