Fast ByteBuffer to CharBuffer or char[]

What is the fastest method to convert a java.nio.ByteBuffer a into a (newly created) CharBuffer b or char[] b.

By doing this it is important, that a[i] == b[i]. This means, that not a[i] and a[i+1] together make up a value b[j], what getChar(i) would do, but the values should be "spread".

byte a[] = { 1,2,3, 125,126,127, -128,-127,-126 } // each a byte (which are signed)
char b[] = { 1,2,3, 125,126,127,  128, 129, 130 } // each a char (which are unsigned)

Note that byte:-128 has the same (lower 8) bits as char:128. Therefore I assume the "best" interpretation would be as I noted it above, because the bits are the same.

After that I also need the vice versa translation: The most efficient way to get a char[] or java.nio.CharBuffer back into a java.nio.ByteBuffer.