Google Protobuf ByteString vs. Byte[]

You can think of ByteString as an immutable byte array. That's pretty much it. It's a byte[] which you can use in a protobuf. Protobuf does not let you use Java arrays because they're mutable.

ByteString exists because String is not suitable for representing arbitrary sequences of bytes. String is specifically for character data.

The protobuf MessageLite Interface provides toByteArray() and toByteString() methods. If ByteString is an immutable byte[], would the byte representation of a message represented by both ByteString and byte[] be the same?

Sort of. If you call toByteArray() you'll get the same value as if you were to call toByteString().toByteArray(). Compare the implementation of the two methods, in AbstractMessageLite:

public ByteString toByteString() {  try {    final ByteString.CodedBuilder out =      ByteString.newCodedBuilder(getSerializedSize());    writeTo(out.getCodedOutput());    return out.build();  } catch (IOException e) {    throw new RuntimeException(      "Serializing to a ByteString threw an IOException (should " +      "never happen).", e);  }}public byte[] toByteArray() {  try {    final byte[] result = new byte[getSerializedSize()];    final CodedOutputStream output = CodedOutputStream.newInstance(result);    writeTo(output);    output.checkNoSpaceLeft();    return result;  } catch (IOException e) {    throw new RuntimeException(      "Serializing to a byte array threw an IOException " +      "(should never happen).", e);  }}

A ByteString gives you the ability to perform more operations on the underlying data without having to copy the data into a new structure. For instance, if you wanted to provide a subset of bytes in a byte[] to another method, you would need to supply it with a start index and an end index. You can also concatenate ByteStrings without having to create a new data structure and manually copy the data.

However, with a ByteString you can give the method a subset of that data without the method knowing anything about the underlying storage. Just like a a substring of a normal String.

A String is for representing text and is not a good way to store binary data (as not all binary data has a textual equivalent unless you encode it in a manner that does: e.g. hex or Base64).