android - How can a Java byte variable be 128?

I probably miss some fundamental understanding of byte in Java. The following is a simplified excerpt from an app to illustrate the problem:

public class Foo
{

 byte b1;
 byte b2;
 byte bProblem;
}

foo is an instance of Foo. The following has puzzled me for hours:

Log.d("Debug", "Before: " + String.valueOf(foo.bProblem));
if (foo.bProblem != (byte) 0x80) {

    foo.bProblem = (byte) 0x80;
    Log.d("Debug", "After: " + String.valueOf(foo.bProblem));
}

LogCat shows the following:

03-17 21:58:46.590: D/Debug(2130): Before: 128    
03-17 21:58:46.590: D/Debug(2130): After: -128

Eclipse's debugger always shows -128 (0x80) for foo.bProblem. This means the debugger cannot see what String.valueOf() reveals. How can a Java byte be 128?

I noticed this when adding foo.bProblem to a List caused:
Java.lang.ArrayIndexOutOfBoundsException: length=256; index=256

Could anyone offer some hint for me to understand this?

Edited:

I found later this happens only on an Intel Android emulator as I wrote in my comment following Joop's answer.