Why java hashcode implementation 31 * x + y is better than x + y?

If you use x+y then how to distinguish points (3,4) and (4,3)? Both will have the same hashcode...

Now while 31 * x + y will not be perfect, in the same case, it will be much much better.

Note: by definition of hashing there is no perfect hashing. The only thing is to analyse what kind of collisions occurs for a given hash function. In the geometrical case the first one introduces collisions for a very simple and usual symmetry property. Thus in very common cases there could be too much collisions.

Imagine you have two string properties prop1 and prop2, and two objects:

A: {prop1="foo", prop2="bar"}
B: {prop1="bar", prop2="foo"}

These are clearly different values, and it's useful to set up the hash code to distinguish between them. If you simply add the properties' hash codes together, you'll get the same value for both A and B. Instead, by multiplying and adding, the hash code will be different based on the property sequence.

It seems you may be slightly misinterpreting the advice: The purpose of the multiply-and-add is to create a dependence on the semantic order of properties within an object, not the execution order of the calculation.