在日常的开发过程中，很多时候我们需要枚举类(enum)来表示对象的各种状态，并且每个状态往往会关联到指定的数字，如：
private enum Color {
RED(11), GREEN(21), YELLOW(31), BLACK(160);
...
};或者用枚举类来表示一系列状态的转变关系：
enum Week{
SUNDAY(1), MONDAY(2), TUESDAY(3), WEDNESDAY(4), THRUSDAY(5), FRIDAY(6), SATRUDAY7);
...
};那么，如何用最少的存储来实现这类需求，答案很简单，位存储。如 1bit 表示 0,1 两种状态，2bit 表示 00,01,10,11 四种状态，所以我们可以用一个 long 类型(64bit)/int类型(32bit)存储多种状态，如下图：
位存储示例
但是每新建一个枚举类都需要自己操作 bit：
导致程序不易理解
容易出错，耗费精力
Hadoop hdfs 的实现中，也遇到类似的问题，它借助于 LongBitFormat.java 类封装了 bit操作：
public class LongBitFormat implements Serializable {
private static final long serialVersionUID = 1L;
private final String NAME;
/** Bit offset */
private final int OFFSET;
/** Bit length */
private final int LENGTH;
/** Minimum value */
private final long MIN;
/** Maximum value */
private final long MAX;
/** Bit mask */
private final long MASK;
public LongBitFormat(String name, LongBitFormat previous, int length,
long min) {
NAME = name;
OFFSET = previous == null ? 0 : previous.OFFSET + previous.LENGTH;
LENGTH = length;
MIN = min;
MAX = ((-1L) >>> (64 - LENGTH));//移动的位数，右移64-Leng位，相当于保留length位
MASK = MAX << OFFSET;
}
/** Retrieve the value from the record. */
public long retrieve(long record) {
return (record & MASK) >>> OFFSET;
}
/** Combine the value to the record. */
public long combine(long value, long record) {
if (value < MIN) {
throw new IllegalArgumentException(
"Illagal value: " + NAME + " = " + value + " < MIN = " + MIN);
}
if (value > MAX) {
throw new IllegalArgumentException(
"Illagal value: " + NAME + " = " + value + " > MAX = " + MAX);
}
return (record & ~MASK) | (value << OFFSET);
}
public long getMin() {
return MIN;
}
}当然，你也可以实现 IntBigFormat,ShortBitFormat 等
首先分析该类的构造方法：
NAME = name; OFFSET = previous == null? 0: previous.OFFSET + previous.LENGTH; LENGTH = length; MIN = min; MAX = ((-1L) >>> (64 - LENGTH));//移动的位数，右移64-Leng位，相当于保留length位 MASK = MAX << OFFSET;
字段：NAME：状态名，可自定义
OFFSET：该状态在 long 字节中的偏移
LENGTH：用多少位存储该状态关联的数字
MIN：该状态关联的最小值
MAX：该状态关联的最大值
MASK：掩码，（OFFSET～OFFSET+LENGTH - 1） == 1
类方法：retrieve(long record)：获得该状态关联的数字
combine(long value, long record)：将一个 value 加到 record 中，例如：将 value 值对应的枚举类存储在 32-40，则先将 32-40bits 清零，再将value 对应的二进制加入到 32-40
那么如何使用该类：
public class LongFormatTest {
static enum HeaderFormat {
PREFERRED_BLOCK_SIZE(null, 48, 1),
REPLICATION(PREFERRED_BLOCK_SIZE.BITS, 12, 1),
STORAGE_POLICY_ID(REPLICATION.BITS, 4, 0);
private final LongBitFormat BITS;
HeaderFormat(LongBitFormat previous, int length, long min) {
BITS = new LongBitFormat(name(), previous, length, min);
}
static short getReplication(long header) {
return (short) REPLICATION.BITS.retrieve(header);
}
static long getPreferredBlockSize(long header) {
return PREFERRED_BLOCK_SIZE.BITS.retrieve(header);
}
static byte getStoragePolicyID(long header) {
return (byte) STORAGE_POLICY_ID.BITS.retrieve(header);
}
static long toLong(long preferredBlockSize, long replication,
long storagePolicyID) {
long h = 0;
h = PREFERRED_BLOCK_SIZE.BITS.combine(preferredBlockSize, h);
h = REPLICATION.BITS.combine(replication, h);
h = STORAGE_POLICY_ID.BITS.combine(storagePolicyID, h);
return h;
}
}
public static void main(String[] args) {
long blockSize = 512;
long replication = 3L;
long storagePolicyID = 2L;
long combine = HeaderFormat.toLong(blockSize,replication,storagePolicyID);
System.out.println("block size: " + HeaderFormat.getPreferredBlockSize(combine));
System.out.println("replication: " + HeaderFormat.getReplication(combine));
System.out.println("storagePolicyID: " + HeaderFormat.getStoragePolicyID(combine));
}
}tolong 方法的返回值也就是我们状态存储的封装
http://www.cnblogs.com/huzuoliang/p/7095865.html
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