通过java.util.concurrent.TimeUnit来学习枚举

<h2>一、背景</h2>
<p>今天无意间看到了java.util.concurrent.TimeUnit枚举类。</p>
<p>想通过这个类谈谈枚举，以及编程规范问题。</p>
<p></p>
<h2>二、源码</h2>
<p>源码：</p>
<pre class="blockcode"><code class="language-java">
package java.util.concurrent;

/**
* A {@code TimeUnit} represents time durations at a given unit of
* granularity and provides utility methods to convert across units,
* and to perform timing and delay operations in these units.  A
* {@code TimeUnit} does not maintain time information, but only
* helps organize and use time representations that may be maintained
* separately across various contexts.  A nanosecond is defined as one
* thousandth of a microsecond, a microsecond as one thousandth of a
* millisecond, a millisecond as one thousandth of a second, a minute
* as sixty seconds, an hour as sixty minutes, and a day as twenty four
* hours.
*
* <p>A {@code TimeUnit} is mainly used to inform time-based methods
* how a given timing parameter should be interpreted. For example,
* the following code will timeout in 50 milliseconds if the {@link
* java.util.concurrent.locks.Lock lock} is not available:
*
*  <pre> {@code
* Lock lock = ...;
* if (lock.tryLock(50L, TimeUnit.MILLISECONDS)) ...}</pre>
*
* while this code will timeout in 50 seconds:
*  <pre> {@code
* Lock lock = ...;
* if (lock.tryLock(50L, TimeUnit.SECONDS)) ...}</pre>
*
* Note however, that there is no guarantee that a particular timeout
* implementation will be able to notice the passage of time at the
* same granularity as the given {@code TimeUnit}.
*
* @since 1.5
* @author Doug Lea
*/
public enum TimeUnit {
/**
   * Time unit representing one thousandth of a microsecond
   */
NANOSECONDS {
      public long toNanos(long d) { return d; }
      public long toMicros(long d)  { return d/(C1/C0); }
      public long toMillis(long d)  { return d/(C2/C0); }
      public long toSeconds(long d) { return d/(C3/C0); }
      public long toMinutes(long d) { return d/(C4/C0); }
      public long toHours(long d) { return d/(C5/C0); }
      public long toDays(long d) { return d/(C6/C0); }
      public long convert(long d, TimeUnit u) { return u.toNanos(d); }
      int excessNanos(long d, long m) { return (int)(d - (m*C2)); }
},

/**
   * Time unit representing one thousandth of a millisecond
   */
MICROSECONDS {
      public long toNanos(long d) { return x(d, C1/C0, MAX/(C1/C0)); }
      public long toMicros(long d)  { return d; }
      public long toMillis(long d)  { return d/(C2/C1); }
      public long toSeconds(long d) { return d/(C3/C1); }
      public long toMinutes(long d) { return d/(C4/C1); }
      public long toHours(long d) { return d/(C5/C1); }
      public long toDays(long d) { return d/(C6/C1); }
      public long convert(long d, TimeUnit u) { return u.toMicros(d); }
      int excessNanos(long d, long m) { return (int)((d*C1) - (m*C2)); }
},

/**
   * Time unit representing one thousandth of a second
   */
MILLISECONDS {
      public long toNanos(long d) { return x(d, C2/C0, MAX/(C2/C0)); }
      public long toMicros(long d)  { return x(d, C2/C1, MAX/(C2/C1)); }
      public long toMillis(long d)  { return d; }
      public long toSeconds(long d) { return d/(C3/C2); }
      public long toMinutes(long d) { return d/(C4/C2); }
      public long toHours(long d) { return d/(C5/C2); }
      public long toDays(long d) { return d/(C6/C2); }
      public long convert(long d, TimeUnit u) { return u.toMillis(d); }
      int excessNanos(long d, long m) { return 0; }
},

/**
   * Time unit representing one second
   */
SECONDS {
      public long toNanos(long d) { return x(d, C3/C0, MAX/(C3/C0)); }
      public long toMicros(long d)  { return x(d, C3/C1, MAX/(C3/C1)); }
      public long toMillis(long d)  { return x(d, C3/C2, MAX/(C3/C2)); }
      public long toSeconds(long d) { return d; }
      public long toMinutes(long d) { return d/(C4/C3); }
      public long toHours(long d) { return d/(C5/C3); }
      public long toDays(long d) { return d/(C6/C3); }
      public long convert(long d, TimeUnit u) { return u.toSeconds(d); }
      int excessNanos(long d, long m) { return 0; }
},

/**
   * Time unit representing sixty seconds
   */
MINUTES {
      public long toNanos(long d) { return x(d, C4/C0, MAX/(C4/C0)); }
      public long toMicros(long d)  { return x(d, C4/C1, MAX/(C4/C1)); }
      public long toMillis(long d)  { return x(d, C4/C2, MAX/(C4/C2)); }
      public long toSeconds(long d) { return x(d, C4/C3, MAX/(C4/C3)); }
      public long toMinutes(long d) { return d; }
      public long toHours(long d) { return d/(C5/C4); }
      public long toDays(long d) { return d/(C6/C4); }
      public long convert(long d, TimeUnit u) { return u.toMinutes(d); }
      int excessNanos(long d, long m) { return 0; }
},

/**
   * Time unit representing sixty minutes
   */
HOURS {
      public long toNanos(long d) { return x(d, C5/C0, MAX/(C5/C0)); }
      public long toMicros(long d)  { return x(d, C5/C1, MAX/(C5/C1)); }
      public long toMillis(long d)  { return x(d, C5/C2, MAX/(C5/C2)); }
      public long toSeconds(long d) { return x(d, C5/C3, MAX/(C5/C3)); }
      public long toMinutes(long d) { return x(d, C5/C4, MAX/(C5/C4)); }