如何从对象获取哈希/ md5？

Question

I have a class like this

Class A{

  private String string1;
  private String string2;
  ..
  ..

  private String string19;

}

Some of the strings may be empty.

I want to get an hash/identifier (string or number) for the instances of this class. I can use hashCode but i don't know if i can get some collision, i have a lot of instances of this class (about 4-5 million).

I need a fast way to get this hash.

Thanks for the help.

Answer 1

You could write a custom hashCode method that uses a slightly altered version of the hashCode algorithm in the String class. From the Oracle documentation, the String hashCode is computed as follows:

 s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

You could implement a modified version where you compute n over all the String objects in your class. This way you are not wasting space and time creating a concatenated String to do the same. In some circumstances this might be fine, but with 4-5 million objects, you probably want to avoid that much churn.

Source: http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/String.html#hashCode%28%29

Answer 2

You can concatenate all of the strings, and then do a hash of your choosing on it. If the string is null, you can use "Null" or any other string -- it really doesn't matter as long as you're consistent.