Java Working with bits

Let me start by saying I have never really worked with bits before in programming. I have an object that can be in 3 states and I want to represent those states using a 3 bit array.

For example:

I have a race car and it can go forward,left, and right at a stand still the bits would be 000

If the car was moving forward the bits would be 010 if forward and left it would be 110 etc...

开发者_Python百科How would I set the bits and how could I read them back to get the values?

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I would suggest using BitSet along with enum's

enum State { LEFT, RIGHT, FORWARD,STAND_STILL}

BitSet stat=new BitSet(4);

void setLeft() // and so on for each state
{
 stat.set(State.LEFT);
}
boolean isLeft()
{
 stat.get(State.LEFT);
}
void reset() //reset function to reset the state
{
  stat.clear();
}

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If size and speed is important, use bits in a byte. (Read the links posted in the other answer as there are non-obvious complications when using and casting signed datatypes.)

This encodes for the speeds: stand, left, left_forward, forward, right_forward, and right.

public class Moo {

final static byte FORWARD = 0x1; // 00000001
final static byte LEFT     =0x2; // 00000010
final static byte RIGHT    =0x4; // 00000100

/**
 * @param args
 */
public static void main(String[] args) {

    byte direction1 = FORWARD|LEFT;  // 00000011
    byte direction2 = FORWARD|RIGHT; // 00000101
    byte direction3 = FORWARD|RIGHT|LEFT; // 00000111

    byte direction4 = 0;

    // someting happens:
    direction4 |= FORWARD;
    // someting happens again.
    direction4 |= LEFT;

    System.out.printf("%x: %s\n", direction1, dirString(direction1));
    System.out.printf("%x: %s\n", direction2, dirString(direction2));
    System.out.printf("%x: %s\n", direction3, dirString(direction3));
    System.out.printf("%x: %s\n", direction4, dirString(direction4));


}

public static String dirString( byte direction) {
    StringBuilder b = new StringBuilder("Going ");

    if( (direction & FORWARD) > 0){
        b.append("forward ");
    }

    if( (direction & RIGHT) > 0){
        b.append("turning right ");
    }
    if( (direction & LEFT) > 0){
        b.append("turning left ");
    }
    if( (direction &( LEFT|RIGHT)) == (LEFT|RIGHT)){
        b.append(" (conflicting)");
    }

    return b.toString();
}

}

Output:

3: Going forward turning left 
5: Going forward turning right 
7: Going forward turning right turning left  (conflicting)
3: Going forward turning left 

Note also that Left and Right is mutually exclusive, so its possible to create an illegal combination. (7 = 111 )

If you actually meant that a thing can only move LEFT, FORWARD or RIGHT, then you don't need flags, just enums.

This enum is possible to transport in only two bits.

    enum Direction{
    NONE, FORWARD, RIGHT, LEFT;

}


Direction dir = Direction.FORWARD;
byte enc = (byte) dir.ordinal();

The final two bits in enc will become:

00 : none  
01 : forward;
10 : right
11 : left

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The least you'll need to store these three bits is one byte.

Read this tutorial on bitwise operators to get started.

Edit: this page on bit masks may also be very helpful.

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You say three states, but you've actually got six: forward, forward-left, forward-right, left, right, stand-still. Unless your race car doesn't move sideways ofcourse, then you've got four.

You should really use an enum for this:

enum State { FORWARD, FORWARD_LEFT, FORWARD_RIGHT, STAND_STILL }

Since left, right and forward are mutually exclusive, this isn't a very good fit for a bit-fiddling program. You'll get into all kinds of consistency problems.

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In java.util there is a class called BitSet that makes bit manipulation very simple.

In your case you could create a BitSet of size 3 and then use the get() and set() methods to set a check the bits.