Why does printing bytes (a numeric data type) using a print stream give me characters?

Why does the compilation of the code below results in an ASCII code value: GHI? I thought byte is a numerical data type? See code below:

import java.io.*;

public class PrintStreamDemo {

   public static void main(String[] args) {
    
      byte c[] = {70, 71, 72, 73, 74, 75, 76};
      // create printstream object
      PrintStream ps = new PrintStream(System.out);
    
      // write bytes 1-3
      ps.write(c, 1, 3);
    
      // flush the stream
      ps.flush();
  }
}

> I thought byte is a numerical data type?

As far as the Java Language Specification is concerned, byte is indeed a numeric data type, and so is char:

> The types of the Java programming language are divided into two categories: primitive types and reference types. The primitive types (§4.2) are the boolean type and the numeric types. The numeric types are the integral types byte, short, int, long, and char, and the floating-point types float and double.

The meaning that "char represents a character" is a useful abstraction, its range just so happens to be the range of a UTF-16 character. But to the computer, char is just 16 ones and zeroes, and byte is just 8 ones and zeroes. How they are interpreted is up to the interpreter.

When you use System.out.println() to print the byte, the byte first gets converted to int and the method interprets them as a number¹, that's why System.out.println(70b) prints "70" rather than G.

On the other hand, the console that you are running this program interprets the bytes it receives as a string of characters encoded in UTF-8. System.out, to which your ps is connected to, writes the bytes to the console. This is why 70 is printed as "G" etc.

<hr>

¹ If you looked at the source code of what println actually does, you will see that at the lowest level, it also calls write(byte[], int, int), just not writing the byte 70. Rather, it writes the bytes 55 and 48, which represents the character "7" and "0".

First of all, System.out is already a PrintStream. So wrapping it in another PrintStream is pointless.

Secondly, print and write are not the same thing in Java. Not even on a PrintStream.

Thirdly, even though byte is a numeric type in Java, so is char. So any reasoning along the lines of "it is a number so it must be printed as a number" doesn't hold in Java. Instead, you need to look at the details of the API specification.

What is actually happening here is that PrintStream.write(byte[], int, int) is writing bytes from the array to the underlying OutputStream without doing any formatting or character set encoding, as described in the javadoc for this method.

In many character set encodings, the byte values 70, 71 and 72 encode the characters G, H and I. (The codes 70, 71 and 72 represent these letters in ASCII, and many encodings "borrow" the ASCII encodings for their first 128 codes.)

Now, presumable, the default character set encoding used by your JVM is one of the above ... so you see "GHI" as output on the console.

However ... not all character encodings are like that. If your platform's default encoding was set to 16 bit encoding for example, the bytes 70 and 71 would actually represent a single Unicode codepoint U+4647 ... which is a Chinese character.

If you use ps.write(c[i]) or ps.print(c[i]) instead, you get the same behavior. This will be calling respectively PrintStream.write(int) or PrintStream.print(char). And both end up treating the byte as a representation of a character¹.

If you want to print a byte formatted as a number using PrintStream, then you need to cast it to an int:

  ps.print((int) c[1]);

The PrintStream.print(int) method prints a decimal representation of its argument.

^{1 - In different ways. PrintStream.write(int) simply writes the byte without any respect to the encoding. PrintStream.print(char) applies the correct encoding. Depending on byte you are printing and the character encoding, these two calls may give different results.}

Byte and char are numeric data types, that doesn't mean they are similar to int or float... Bytes store data in Byte format.

To read data from a byte array, use ByteArrayInputStream.

import java.io.*;

public class PrintStreamDemo {

   public static void main(String[] args) {
    
      byte c[] = {70, 71, 72, 73, 74, 75, 76};

      // Create the new byte array input stream  
      ByteArrayInputStream byt = new ByteArrayInputStream(buf);  
    
      // Print data
      int k = 0;  
      while ((k = byt.read()) != -1) {  
         //Conversion of a byte into character  
         char ch = (char) k;  
         System.out.println("ASCII value of Character is:" + k + "; Special character is: " + ch);  
      }  
      
      // flush the stream
      byt.flush();
  }
}