/**
* Returns a <code>double</code> value with a positive sign, greater
* than or equal to <code>0.0</code> and less than <code>1.0</code>.
* Returned values are chosen pseudorandomly with (approximately)
* uniform distribution from that range.
*
* <p>When this method is first called, it creates a single new
* pseudorandom-number generator, exactly as if by the expression
* <blockquote><pre>new java.util.Random</pre></blockquote> This
* new pseudorandom-number generator is used thereafter for all
* calls to this method and is used nowhere else.
*
* <p>This method is properly synchronized to allow correct use by
* more than one thread. However, if many threads need to generate
* pseudorandom numbers at a great rate, it may reduce contention
* for each thread to have its own pseudorandom-number generator.
*
* @return a pseudorandom <code>double</code> greater than or equal
* to <code>0.0</code> and less than <code>1.0</code>.
* @see java.util.Random#nextDouble()
*/
等于1 得自己判断。