How to convert byte to Boolean[8] and convert back?

Question

I need to record some booleans in a File, and then retrieve them. I can record and read one or more bytes with the Classes java.io.FileInputStream and java.io.FileOutputStream, so I need to convert 1 byte to an array of 8 booleans, and, the conversion of an array of 8 booleans to 1 byte, so:

public static boolean[] byteToBooleanArray(byte b) {
    boolean[] array = new boolean[8];
    //O que colocar aqui?
    return array;
}

public static byte booleanArrayToByte(boolean[] array) {
    if (array.length != 8) throw new IllegalArgumentException();
    byte b;
    //O que colocar aqui?
    return b;
}

Note: I wish I didn’t have to download a library.

Answer 1

I believe that there will be various ways of doing this.
What I can think of now is:

• Use expression (b & (1 << i)) != 0 to check whether the bit in position i byte is "set" and store the result at its position in the array.

  public static boolean[] byteToBooleanArray(byte b) {
      boolean[] array = new boolean[8];
      for (int i=0; i<8 ;i++){
          array[i] = (b & (1 << i)) != 0;
      }
      return array;
  }
  

• Use expression b |= 1 << i for "setar", in byte, the bit in the position that corresponds to the position i array, if its value is true.

  public static byte booleanArrayToByte(boolean[] array) {
      if (array.length != 8) throw new IllegalArgumentException();
      byte b = 0;
      for (int i = 0; i < 8; i++) {
          if (array[i]) {
              b |= 1 << i;
          }
      }
      return b;
  }
  

Answer 2

I believe that converting the boolean array to a single byte involves many Bite to Bite (arithmetic shift) drive operations, which are usually complex to understand and maintain, unless you have extensive experience with it. Probably the result but simple is an array of bytes, which you can use in the stream you want, as in the example below:

@Test
public void testConvert() throws Exception{
    boolean[] esperado = {true, false, false, true, true, false, false, true};
    byte[] result;
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutputStream oos = new ObjectOutputStream(bos);

    oos.writeObject(esperado);

    oos.flush(); 
    result = bos.toByteArray();

    Assert.assertNotNull(result);
    System.out.println(result.length);

    ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(result));

    boolean[] obtido = (boolean[]) ois.readObject();

    for(int i = 0; i < esperado.length; i++){
        Assert.assertEquals(esperado[i], obtido[i]);
        System.out.println(esperado[i]);
    }
}

Remembering that instead of Bytearrayoutputstream you can use Fileoutputstream directly or, as stated above, any type of stream you want.

However, although I don’t know exactly what you need this solution for, I believe there may be more elegant ways to solve your problem.

I hope I’ve helped.