# 计算时间差，你的方式或许不是最佳实践！

long startTime = System.currentTimeMillis();
// 执行代码
long endTime = System.currentTimeMillis();
System.out.println(endTime - startTime);


/**
* Returns the current time in milliseconds.  Note that
* while the unit of time of the return value is a millisecond,
* the granularity of the value depends on the underlying
* operating system and may be larger.  For example, many
* operating systems measure time in units of tens of
* milliseconds.
*
*  See the description of the class Date for
* a discussion of slight discrepancies that may arise between
* "computer time" and coordinated universal time (UTC).
*
* @return  the difference, measured in milliseconds, between
*          the current time and midnight, January 1, 1970 UTC.
* @see     java.util.Date
*/
public static native long currentTimeMillis();


long startTime = System.nanoTime();
// 执行代码
long endTime = System.nanoTime();
System.out.println(endTime - startTime);


/**
* Returns the current value of the running Java Virtual Machine's
* high-resolution time source, in nanoseconds.
*
* This method can only be used to measure elapsed time and is
* not related to any other notion of system or wall-clock time.
* The value returned represents nanoseconds since some fixed but
* arbitrary origin time (perhaps in the future, so values
* may be negative).  The same origin is used by all invocations of
* this method in an instance of a Java virtual machine; other
* virtual machine instances are likely to use a different origin.
*
* This method provides nanosecond precision, but not necessarily
* nanosecond resolution (that is, how frequently the value changes)
* - no guarantees are made except that the resolution is at least as
* good as that of {@link #currentTimeMillis()}.
*
* Differences in successive calls that span greater than
* approximately 292 years (263 nanoseconds) will not
* correctly compute elapsed time due to numerical overflow.
*
* The values returned by this method become meaningful only when
* the difference between two such values, obtained within the same
* instance of a Java virtual machine, is computed.
*
*  For example, to measure how long some code takes to execute:
*   {@code
* long startTime = System.nanoTime();
* // ... the code being measured ...
* long estimatedTime = System.nanoTime() - startTime;}
*
*
To compare two nanoTime values
*
{@code
* long t0 = System.nanoTime();
* ...
* long t1 = System.nanoTime();}
*
* one should use {@code t1 - t0 < 0}, not {@code t1 < t0},
* because of the possibility of numerical overflow.
*
* @return the current value of the running Java Virtual Machine's
*         high-resolution time source, in nanoseconds
* @since 1.5
*/
public static native long nanoTime();