RankoR RankoR - 14 days ago 6
Java Question

Java - strange errors with unsigned values

There is 2-bytes array:
private byte[] mData;

and method:

public void setWord(final short pData) {
mData[0] = (byte) (pData >>> 8);
mData[1] = (byte) (pData);
}


I wrote the simple test:

public void testWord() {
Word word = new Word();
word.setWord((short) 0x3FFF);

Assert.assertEquals(0x3F, word.getByte(0));
Assert.assertEquals(0xFF, word.getByte(1));
}


The second assert fails with message "Expected 255, but was -1".
I know, that 0xFF signed short is, in fact, -1, but why JUnit thinks, that they are not equal? And, what is the correct way to implement such classes?

Akh Akh
Answer

Java does not support unsigned types, so in order for a value to be 255, it must not be a signed byte, which is incapable of holding the value of 255. The 0xFF constant value will be taken as a signed int, and for the comparison, the byte value 0xFF will be converted to an int at -1 as well.

You need to type cast the literal 0xFF to be a byte. Change the assert to be Assert.assertEquals((byte)0xFF, word.getByte(1)); Then the left hand side will evaluate to -1 as well as the right.

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