Parsing Bytes Java Efficiently

Question

so I am parsing a KNOWN amount of bytes. However, the first two bytes represent some number, then the next one represents a number that's only one byte, but then possibly the next four all represent one number that's large. Is there a better way to parse the data, other than the way I am.

    switch (i) {
            //Status
            case 2:
                temp[3] = bytes[i];
                break;
            case 3:
                temp[2] = bytes[i];
                ret.put("Status", byteArrayToInt(temp).toString());
                break;
            //Voltage
            case 4:
                temp[3] = bytes[i];
                break;
            case 5:
                temp[2] = bytes[i];
                ret.put("Voltage", byteArrayToInt(temp).toString());
                break;
            //Lowest Device Signal
            case 6:
                temp[3] = bytes[i];
                break;
            case 7:
                temp[2] = bytes[i];
                ret.put("Lowest Device Signal", byteArrayToInt(temp).toString());
                clearBytes(temp);
                break;

}

I am looping through the array of bytes and I have a switch that knows which bytes go to which location, for example I know the 2nd and third bytes go to the Status code. So I take them and combine them into an int. the temp byte array is a byte[] temp = new byte[4]. Any better way to do this?

You can either do it your way, with ad-hoc code, or try to employ some sort of generic parser. If it doesn't get much more complex than the above, and you won't be changing it a lot, then the above ad-hoc approach is probably easier than figuring out a more generic scheme. — Hot Licks
– Hot Licks, Commented Aug 1, 2013 at 19:33
Are the values of i read from the byte[] itself? Can you provide some additional information for the data structure? — c.s.
– c.s., Commented Aug 1, 2013 at 19:41
Creating the Strings and putting it in a Map is far more expensive than your parsing. Possibly as much as 10x. I would worry about avoiding creating object or setting Map first. — Peter Lawrey
– Peter Lawrey, Commented Aug 1, 2013 at 19:42

Syon · Accepted Answer · 2013-08-01 19:50:11Z

11

ByteBuffer can handle this.

byte[] somebytes = { 1, 5, 5, 0, 1, 0, 5 };
ByteBuffer bb = ByteBuffer.wrap(somebytes);
int first = bb.getShort(); //pull off a 16 bit short (1, 5)
int second = bb.get(); //pull off the next byte (5)
int third = bb.getInt(); //pull off the next 32 bit int (0, 1, 0, 5)
System.out.println(first + " " + second + " " + third);

Output
261 5 65541

You can also pull off an arbitrary number of bytes using the get(byte[] dst, int offset, int length) method, and then convert the byte array to whatever data type you need.

edited Aug 1, 2013 at 19:50

answered Aug 1, 2013 at 19:38

Syon

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4 Comments

Peter Lawrey Over a year ago

+1 I would keep reading until you have all the bytes you expect to get

Dylan Holmes Over a year ago

I am not getting the right values as I was with my old way... not sure what's happening.

parsedData.put("Status", status); 		parsedData.put("Voltage", String.valueOf(buffer.getShort())); 		parsedData.put("Lowest Device Signal", String.valueOf(buffer.getShort())); 		parsedData.put("Lowest Device Signal Address", String.valueOf(buffer.get()));

Syon Over a year ago

You also may need to skip the first 2 bytes by calling buffer.getShort() once before getting "status". Recall that your switch cases start at array index 2, not 0.

Dylan Holmes Over a year ago

Smart man, that was the problem, however, I figured that out by my self a while ago. But Nice job picking that up.

dkatzel · Accepted Answer · 2013-08-01 19:35:16Z

5

You can use a DataInputStream to read multiple bytes as ints or shorts. It looks like you only use 2 bytes at a time, so you should be reading shorts instead of ints (which in Java are always 4 bytes).

But in the code example below, I will use your description "However, the first two bytes represent some number, then the next one represents a number that's only one byte, but then possibly the next four all represent one number"

DataInputStream in = new DataInputStream(new ByteArrayInputStream(bytes));

//the first two bytes represent some number
ret.put("first", Short.toString(in.readShort()));
//next one represents a number that's only one byte
ret.put("second", Byte.toString(in.readByte()));
//next four all represent one number
ret.put("Lowest Device Signal", Integer.toString(in.readInt()));

answered Aug 1, 2013 at 19:35

dkatzel

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5 Comments

Dylan Holmes Over a year ago

I am reading from a BluetoothSocket, and sometimes I read 4 bytes, and sometimes 3 bytes, I only pasted the first 7 cases. (There are 58 cases)

dkatzel Over a year ago

It would still work about the same, just call the appropriate readXXX() method in each of your cases.

c.s. Over a year ago

@user2635648 extend the DataInputStream and add methods readStatus(), readVoltage() and finally readRecord() or whatever it is the whole block of data you are reading

Dylan Holmes Over a year ago

How does it know to only read the first 2 for in.readShort()?

dkatzel Over a year ago

readShort() means read 2 consecutive bytes as a single value. readInt() means read 4 consecutive bytes as a single value

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