buffer = read bytes ...

String value = new String(buffer, "some encoding");

[][] [][] [][] []  | [] [][] [][]
[ a] [b ] [c ] [ d |  ]  [e]  [f ]
{первая порция}    | {вторая порция}

import java.io.UnsupportedEncodingException;
import java.nio.charset.StandardCharsets;

public class Newc {
	public static void main(String[] args) throws UnsupportedEncodingException {
		
		String str = "Hello"; // Utf-8 is default.
		
		byte[] buffer = str.getBytes(StandardCharsets.UTF_16);

		System.out.println("Buffer length: " +  buffer.length + " bytes");
		
		String s2=null;
		
		for (int i=0; i<buffer.length; i++) {	
			s2 = new String(buffer, 0, buffer.length-i, StandardCharsets.UTF_16);
			System.out.println(i + " - " + s2);
		}
	}
}

Buffer length: 12 bytes
0 - Hello
1 - Hell�
2 - Hell
3 - Hel�
4 - Hel
5 - He�
6 - He
7 - H�
8 - H
9 - �
10 - 
11 - �

Buffer length: 12 bytes
0 -  Hello
1 - ＀䠀攀氀氀�
2 -  Hello
3 -  䠀攀氀氀�
4 -   ello
5 -   攀氀氀�
6 -    llo
7 -    氀氀�
8 -     lo
9 -      氀�
10 -     o
11 -     �