Brainfuck interpreter misbehaving

Question

I am writing an interpreter for the language Brainfuck

I used the command line

java bfinterpreter/BFInterpreter >output.bin

the bf program is supposed to put out 00 to FF in hex, but it puts out

00 01 02 03 04 05 06 ... 7f 3f 3f 3f 3f 3f ... a0 a1 a2 ... ff

where the ... represent a sequence of +1 in hex

the 3f replaces 80 to 9f, and i have no idea why

Source:

package bfinterpreter;

/**
 *
 * @author Nicki von Bulow
 */
public class BFInterpreter {
private char[] program;
private StringBuilder output;
private byte[] input;
private short inputPosition;
private short[] data;
private short dataPosition;
private boolean debug;
private boolean valid;

public static final char INPUT = ',';
public static final char OUTPUT = '.';
public static final char INCREASEPOINTER = '>';
public static final char DECREASEPOINTER = '<';
public static final char INCREASE = '+';
public static final char DECREASE = '-';
public static final char LOOPSTART = '[';
public static final char LOOPEND = ']';
public static final char DEBUG = '#';

public BFInterpreter(char[] program, byte[] input, boolean debug) {
    this.program = program;
    this.input = input;
    this.debug = debug;
    output = new StringBuilder();;
    valid = true;
    data = new short[32767];
}

public void execute(){
    execute(program);
}

private void execute(char[] program) {
    short programPosition = 0;
    for (char b:program){
        if (b == INPUT) {
            if(inputPosition >= input.length) break;
            data[dataPosition] = input[inputPosition];
            inputPosition++;
        }
        else if (b == OUTPUT) {
            if (data[dataPosition] >=0) output.append((char) (short) data[dataPosition]);
            else {
                short a = data[dataPosition];
                System.err.println(a + 256);
                output.append((char) (a + 256));
            }
        }
        else if (b == INCREASEPOINTER) {
            if (dataPosition == 32766) dataPosition = 0;
            else dataPosition++;
        }
        else if (b == DECREASEPOINTER) {
            if (dataPosition == 0) dataPosition = 32766;
            else dataPosition--;
        }
        else if (b == INCREASE) {
            if (data[dataPosition] != 255) data[dataPosition]++;
            else data[dataPosition] = 0;
        }
        else if (b == DECREASE) {
            if (data[dataPosition] != 0) data[dataPosition]--;
            else data[dataPosition] = 255;
        }
        else if (b == LOOPSTART) {
            short loopcount = 1;
            char[] newProg = new char[program.length - programPosition];
            System.arraycopy(program, programPosition, newProg, 0, newProg.length);
            int at = 0;
            for (char d:newProg) {
                if(loopcount == 0) break;
                if (d == '[') loopcount++;
                else if(d == ']') loopcount--;
                at++;
            }
            char[] finalNewProg = new char[at];
            System.arraycopy(newProg, 1, finalNewProg, 0, at-2);
            while (data[dataPosition] != 0) execute(finalNewProg);
        }
        else if (b == LOOPEND);
        programPosition++;
    }
}

private boolean analyze(char[] prog) {
    return true;
}

public String output() {
    return output.toString();
}

public static void main(String args[]){
    BFInterpreter a = new BFInterpreter("+[.+]".toCharArray(), "".getBytes(), false);
    a.execute();
    System.out.println(a.output());
    for(byte b:a.output().getBytes()) System.err.print(b + " ");
}
}

the output on the command prompt is

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127
63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63
63 63 63 63 63 -96 -95 -94 -93 -92 -91 -90 -89 -88 -87 -86 -85 -84 -83 -82 -81
-80 -79 -78 -77 -76 -75 -74 -73 -72 -71 -70 -69 -68 -67 -66 -65 -64 -63 -62 -61
-60 -59 -58 -57 -56 -55 -54 -53 -52 -51 -50 -49 -48 -47 -46 -45 -44 -43 -42 -41
-40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21
-20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

Answer 1

Change your main method as follows and I believe you will find it works fine -

public static void main(String args[]) {
  BFInterpreter a = new BFInterpreter(
      "+[.+]".toCharArray(), new byte[1], false);
  a.execute();
  String output = a.output();
  for (Character c : output.toCharArray()) {
    System.out.print(Integer.toString((int) c)); // You want this method.
    System.out.print(" ");
  }
  System.out.println();
}

Which gives me

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 0