String to byte and viceversa in java

Question

I am converting a byte[] array to string . And then converting the string back to byte[] array. Then on checking if both the arrays are equal - I get that they are not equal:

 byte[] ciphertext2=c.doFinal(username.getBytes("utf-8"));
          //JUST CHECKING IGNORE
          String qaz=new String(ciphertext2,"utf-8");
          //qaz=qaz+"1";
          System.out.println("just chekcing------------------------"+qaz);
          byte[] ciphertext3=qaz.getBytes("utf-8");
          if(Arrays.equals(ciphertext2,ciphertext3))
          {
           System.out.println("just chekcing they are equal------------------------");
          }
          else
          System.out.println("just chekcing they are not equal------------------------");<br>

OUTPUT :

just chekcing they are not equal--------------------

Why doesn't it work?
Edit
It works perfectly fine when using Base64 of java. But why doesn't it work when converting byte to string and vice-versa directly? What actually happens when you convert a string a byte array and vice versa?

Your bytes are probably not valid utf-8. When they are parsed as utf-8 strings, invalid sequences are turned into valid utf-8 sequences, and so the byte arrays are different. Base64 is guaranteed to be valid utf-8 (it is, after all, a superset of ascii). — user684934
– user684934, Commented Apr 18, 2012 at 5:50
@bdares : how does the conversion of a byte array to string take place? What happens inside? — Ashwin
– Ashwin, Commented Apr 18, 2012 at 5:54
I am getting this as true if I hard code the value for username. — UVM
– UVM, Commented Apr 18, 2012 at 5:57
@UNNI : Did you encrypt it? the encrypted username is converted into string. — Ashwin
– Ashwin, Commented Apr 18, 2012 at 6:00
yes very much....say "hello".getBytes("utf-8").this will make your comparison true — UVM
– UVM, Commented Apr 18, 2012 at 6:03

Jon Skeet · Accepted Answer · 2012-04-18 05:52:18Z

4

If this is the result of encryption, you should not be using a string constructor. You don't have encoded text - you have arbitrary binary data. You need to understand the difference between data which is fundamentally text and which needs to be represented in binary (e.g. writing a text file to disk) and data which is fundamentally binary and which needs to be represented in text (e.g. including encrypted data in an XML document).

You should use base64 (e.g. with this public domain library) or possibly hex. This will result in ASCII data which is guaranteed to roundtrip to the original binary data.

answered Apr 18, 2012 at 5:52

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Ashwin Over a year ago

how does the conversion of a byte array to string take place? What happens inside?

Jon Skeet Over a year ago

@Ashwin: It uses whatever encoding has been specified, or the platform default if you don't specify one. The whole point of an encoding is to convert from bytes to text and vice versa - exactly how it works is up to the implementation. For example, there are some simple encodings (e.g. ASCII) where each byte represents a single character and vice versa, but various bytes aren't valid and various characters can't be recognized. But the binary data you use to construct a string should always be a byte sequence which could be the result of encoding some text in the same encoding.

Ashwin Over a year ago

So when you convert a byte array to a string directly using utf-8, each byte in the byte array is checked it it has a corresponding text in the utf-8 mapping and that corresponding byte is replaced with the character. What happens if the mapping is not there in utf-8 - as is the case in my case?

Jon Skeet Over a year ago

@Ashwin: Not quite, because UTF-8 isn't a fixed-width encoding. A single character can be encoded in multiple bytes. (It gets even more complicated with surrogate pairs.) If the sequence isn't valid UTF-8, any unknown parts of the sequence are replaced with "?" IIRC. But basically, you simply shouldn't do this. Use base64 instead, as per my answer.

Ashwin Over a year ago

So base 64 will convert the group the incoming bits into to groups of 6 and then using its own character encoding scheme, it will represent the groups with its characters . In this way there is not loss of data right?

|

Collectives™ on Stack Overflow

String to byte and viceversa in java

1 Answer 1

8 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

8 Comments

Your Answer

Sign up or log in

Post as a guest

Related