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CHAPTER 5 STORING AND RETRIEVING DATA
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Figure 5-1. Two complete boards of the Dungeon game It s not hard to compact data if you know a bit about byte arithmetic. And Java Micro Edition (Java ME) programmers naturally need to use byte arithmetic more than the average Java programmer. Recall that you use the bitwise or operator for placing strings and images (see the Using the Graphics and Canvas Classes section in 2) and for layout directives (see the Using the Form and Item Classes section, also in 2), and you use a bitwise and operator to get all the information about the current key states (see the Using the GameCanvas Class section in 3). Another useful tool for bitwise arithmetic is the shift operator. The shift operators are written in Java code as >> and <<, where the operand on the left is the byte to shift and the operand on the right is the number of places to shift it. They have the effect of multiplying the value of the byte by a power of 2. For example, the result of myval >> 1 is the same as dividing myval by 2. All the shift operator is really doing is moving the values of the byte shifting them all in unison to the left or to the right so they can be used to make it easier to access each individual value (1 or 0) in the byte. So to stock eight Booleans into a byte, all you need to do is add the ones and zeros to the byte, one by one, and use the shift operator to shift the result up one bit between each new addition. To get the data out again, you just perform a bitwise and between the byte and a series of appropriate flag bytes. For example, 128 corresponds to the top bit of a byte, so to find out if the top bit of a given byte is set, all you have to do is perform a bitwise and with your chosen byte and a flag byte whose value is equal to 128. The value returned by the operation will be nonzero if and only if the top bit of your byte was set. You can get all the other flag bytes easily from the initial flag by shifting down the flag byte. The class in Listing 5-3 is the complete version of my integer compression utility class (called DataConverter). It contains methods to pack eight Booleans into a byte as described previously, as well as methods to convert integers in various size ranges to bytes. I ve even included methods that will convert an int to an array of 4 bytes giving exactly the same values as you d get using DataInputStream. The only tricky part in any of the algorithms in Listing 5-3 is dealing with when a byte of data is considered to be signed and when it s considered to be unsigned. Signed bytes range in value from 128 to 127, and unsigned bytes range in value from 0 to 255. Any byte can be regarded
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CHAPTER 5 STORING AND RETRIEVING DATA
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as signed or unsigned; it s just a question of whether you consider the top bit to indicate a negative sign or 128. If you cast a byte to an int, Java will consider the byte to be signed when returning the value. If you wanted its value as an unsigned byte, then you can fix it by adding 256 if the value is negative (that is, add 128 to get the value into the positive range and then add another 128 for the value of the top bit that was set). An integer obviously needs only one sign, so when Java represents an integer internally as 4 bytes, only the high byte (which is the first of the 4 bytes) is regarded as signed. Dealing with the interplay between signed and unsigned bytes is a little bit confusing, but I hope that the code example in Listing 5-3 will help clarify how it works. I ve written the code to check for positive or negative values because it s a fairly intuitive way of making it clear that the program is converting between viewing the values as signed or unsigned. Keep in mind, however, that as discussed earlier, you can get the value of the sign bit through bitwise arithmetic: just perform a bitwise and (&) with the byte whose value is 128. Listing 5-3 shows the code for DataConverter.java. Listing 5-3. DataConverter.java package net.frog_parrot.util; import java.io.*; /** * This class is a set of simple utility functions that * can be used to convert standard data types to bytes * and back again. It is used especially for data storage, * but also for sending and receiving data. * * @author Carol Hamer */ public class DataConverter { //-------------------------------------------------------// utilities to encode small, compactly stored small ints. /** * Encodes a coordinate pair into a byte. * @param coordPair a pair of integers to be compacted into * a single byte for storage. * WARNING: each of the two values MUST BE * between 0 and 15 (inclusive). This method does not * verify the length of the array (which must be 2!) * and it doesn't verify that the ints are of the right size. */ public static byte encodeCoords(int[] coordPair) { // get the byte value of the first coordinate: byte retVal = (byte)(coordPair[0]); // move the first coordinate's value up to the top // half of the storage byte: retVal = (byte)(retVal << 4);
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