Implementing hashcode() in java

Question

In the guidelines to write a good hashCode() written in Effective java, the author mentions the following step if the field is long.

If the field is a long, compute (int) (f ^ (f >>> 32)).

I am not able to get why this is done. Why are we doing this ?

Answer 1

In Java, a long is 64-bit, and an int is 32-bit.

So this is simply taking the upper 32 bits, and bitwise-XORing them with the lower 32 bits.

Answer 2

Because hasCode is 32-bit integer value and long 64-bit. You need hashCode to differ for values with same lower 32-bit for each long and this function should ensure it.