Converting bytebuffer to string not working in java

You can do something like this:

String val = new String(paramByteBuffer.array());

OR

String val = new String(paramByteBuffer.array(),"UTF-8");

Here is a list of supported charsets

Buffers are a little tricky to use as they have a current state, which you need to take into account when accessing them.

you want to put

 paramByteBuffer.flip();

before each decode to get the buffer into the state you want for the decode to read.

e.g.

ByteBuffer paramByteBuffer = ByteBuffer.allocate(100);
paramByteBuffer.put((byte)'a');  // write 'a' at next position(0)
paramByteBuffer.put((byte)'b');  // write 'b' at next position(1)
paramByteBuffer.put((byte)'c');  // write 'c' at next position(2)

// if I try to read now I will read the next byte position(3) which is empty
// so I need to flip the buffer so the next position is at the start
paramByteBuffer.flip();          

// we are at position 0 so we can do our read function
CharBuffer charBuffer = StandardCharsets.UTF_8.decode(paramByteBuffer);
String text = charBuffer.toString();
System.out.println("UTF-8" + text);

// because the decoder has read all the written bytes we are back to the
// state (position 3) we had just after we wrote the bytes in the first 
// place so we need to flip again 
paramByteBuffer.flip();

// we are now at position 0 so we can do our read function
charBuffer = StandardCharsets.UTF_16.decode(paramByteBuffer);
text = charBuffer.toString();
System.out.println("UTF_16"+text);

The toString method of HeapByteBuffer simply

Returns a string summarizing the state of this buffer.

In other words, it returns

java.nio.HeapByteBuffer[pos=0 lim=3 cap=3]

which shows the byte buffer's position, limit, and capacity.

In this case, your ByteBuffer has capacity for 3 more bytes. Each Charset#decode call consumes the ByteBuffer and you don't rewind/reset it so there are no more bytes to consume for subsequent calls. In other words, all those strings will be empty.