How to convert java long to 4 bytes data

By , last updated February 17, 2016

Java doesn’t have unsigned types. In order to convert C++ unsigned values to Java you may need to use larger types. Authors of the language explain it as the Java should be a simple language and be easy to understand for all developers.

Problems occur when you need to convert to or from Java numeric types. Let us look at C++ vs Java types:

C++ Type *) Java Type
8-bit unsigned byte (signed) byte
16-bit unsigned short (signed) short
32-bit unsigned int (signed) int
32-bit/64-bit unsigned long (signed) long
32-bit float float
64-bit double double

*) With C++, the types aren’t as rigidly defined in Java. The only requirement is that a char is not larger than a short, a short is not larger than an int, and an int is not larger than a long, where a short may not be smaller than 16-bits, an int may not be smaller than 32-bits and a long long at least 64-bits. It’s perfectly valid for a C++ system where are types are 64-bits.

Most of the time you don’t need to use larger numeric types.

Convert unsigned int to Integer and back

Example of the simplest case of conversion between C++ unsigned int coded bytes to Java Integer:

1. Read 4 bytes into an Integer

int length = ByteBuffer.wrap(bytes, 0, 4).getInt();

2. Convert Integer value to 4 bytes:

final byte[] bytes = intToByte(length);

private static byte[] intToByte(int value) {
	return ByteBuffer.allocate(4).putInt(value).array();

Convert unsigned int to long and back to 4 bytes data

In case when you need to account for all possible values of unsigned values in Java, you should convert from unsigned type to a larger numeric type and back.

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Example of converting a C++ unsigned int to Java long: C++ layer converts unsigned int values to 4 bytes and sends it to Java layer. Java layer converts bytes to long type, analyses the data and sends the response, converting long back to 4 bytes data.

1. Read the 4 bytes and store them as long:

long length= ByteBuffer.wrap(bytes).order(ByteOrder.LITTLE_ENDIAN).getInt() & 0xFFFFFFFFL;

2. Convert Java long value back to 4 bytes data array:

final byte[] bytes = longToByte(length);

public static byte[] longToByte(long value)
	byte [] data = new byte[4];
	data[3] = (byte) value;
	data[2] = (byte) (value >>> 8);
	data[1] = (byte) (value >>> 16);
	data[0] = (byte) (value >>> 32);

	return data;

Note, you can’t use a ByteBuffer here to convert Java long to 4 bytes. Java long is stored as 8 bytes and ByteBuffer will return an 8-bytes array. You need to use bitwise operations in order to set values back to 4 bytes array.

Little endian and big endian

This post doesn’t take into account there are at least three types of endian-ness. Little endian as most Intel CPUs, big endian and then there is middle endian. Implementing those are left as an excercise for the reader.


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