为什么这不引发NullPointerException？

null

为了简单起见，假设它

sample

Object#hashCode()

StringBuilder sample = new StringBuilder(); //sample refers to //StringBuilder at 12345StringBuilder referToSample = sample; //referToSample refers to //the same StringBuilder at 12345 //SEE DIAGRAM 1referToSample = null; //referToSample NOW refers to 00000, //so accessing it will throw a NPE. //The other reference is not affected.//SEE DIAGRAM 2sample.append("A"); //sample STILL refers to the same StringBuilder at 12345 System.out.println(sample);

从标记的线

See diagram

图1：

[StringBuilder sample]    -----------------> [java.lang.StringBuilder@00012345]          ↑[StringBuilder referToSample] ------------------------/

图2：

[StringBuilder sample]    -----------------> [java.lang.StringBuilder@00012345][StringBuilder referToSample] ---->> [null pointer]

图2显示，废止

referToSample

sample

00012345

1 GC的考虑使其难以置信。