Java 8+

Encode or decode byte arrays:

byte[] encoded = Base64.getEncoder().encode("Hello".getBytes());
println(new String(encoded));   // Outputs "SGVsbG8="

byte[] decoded = Base64.getDecoder().decode(encoded);
println(new String(decoded))    // Outputs "Hello"

Or if you just want the strings:

String encoded = Base64.getEncoder().encodeToString("Hello".getBytes());
println(encoded);   // Outputs "SGVsbG8="

String decoded = new String(Base64.getDecoder().decode(encoded.getBytes()));
println(decoded)    // Outputs "Hello"

For more info, see Base64.

Java < 8

Base64 is not bundled with Java versions less than 8. I recommend using Apache Commons Codec.

For direct byte arrays:

Base64 codec = new Base64();
byte[] encoded = codec.encode("Hello".getBytes());
println(new String(encoded));   // Outputs "SGVsbG8="

byte[] decoded = codec.decode(encoded);
println(new String(decoded))    // Outputs "Hello"

Or if you just want the strings:

Base64 codec = new Base64();
String encoded = codec.encodeBase64String("Hello".getBytes());
println(encoded);   // Outputs "SGVsbG8="

String decoded = new String(codec.decodeBase64(encoded));
println(decoded)    // Outputs "Hello"

Android (with Java < 8)

If you are using the Android SDK before Java 8 then your best option is to use the bundled android.util.Base64.

For direct byte arrays:

byte[] encoded = Base64.encode("Hello".getBytes());
println(new String(encoded))    // Outputs "SGVsbG8="

byte [] decoded = Base64.decode(encoded);
println(new String(decoded))    // Outputs "Hello"

Or if you just want the strings:

String encoded = Base64.encodeToString("Hello".getBytes());
println(encoded);   // Outputs "SGVsbG8="

String decoded = new String(Base64.decode(encoded));
println(decoded)    // Outputs "Hello"

Other: Spring < 6.0.5

If for some reason Java's Base64 isn't suitable and you use Spring, see Base64Utils. This package, however, is deprecated as of 6.0.5 and so I don't recommend.

To start with, this is wrong:

String base64Encoded = new String(imgData, StandardCharsets.UTF_8);

The image bytes you have do not represent UTF-8 encoded text, so it makes no sense to create a String out of the bytes, pretending that this is UTF-8 encoded text.

If you are using Java 8, then use the Base64 class available in the standard library:

import java.util.Base64;

// ...

byte[] imgData = (byte[]) base64.get("images");

String base64Encoded = Base64.getEncoder().encodeToString(imgData);

If you are not using Java 8, you can use a third-party library, for example Apache Commons Codec, which includes classes for Base64 encoding and decoding.

have you tried with the encodeBase64String method instead of using encodeBase64URLSafeString ?

encodeBase64URLSafeString:

Encodes binary data using a URL-safe variation of the base64 algorithm but does not chunk the output. The url-safe variation emits - and _ instead of + and / characters.

encodeBase64String:

Encodes binary data using the base64 algorithm but does not chunk the output. NOTE: We changed the behaviour of this method from multi-line chunking (commons-codec-1.4) to single-line non-chunking (commons-codec-1.5).

source : org.apache.commons.codec.binary.Base64 javadoc

I used this and it worked fine (contrary to the accepted answer, which uses a format not recommended for this scenario):

StringBuilder sb = new StringBuilder();
sb.append("data:image/png;base64,");
sb.append(StringUtils.newStringUtf8(Base64.encodeBase64(imageByteArray, false)));
contourChart = sb.toString();

For such a basic utility class, as there is a late implementation of Base64 in API 8, I would copy the latest source from the AOSP git repo to my own application package. It compiles with API level 3.

This way, you can code in your application with the same API as the official android Base64 implementation and stay compatible with all android versions. The day API 8 becomes the oldest version of android on the market, you just have to delete your version of the class in your package and update the imports to the official android.util.Base64.

Eclipse hint: just follow the link I provided to the source in your browser, Ctrl-A to select all the code, Ctrl-C to copy it, open Eclipse, click on the package where you want to create the class, hit Ctrl-V. The .java file is created automatically with the source directly modified with the correct package declaration.