Java Language Features

Java provides certain functions, classes and algorithms by default which oftentimes aren’t available in other languages. This page explains how they work, alongside relatively generic, language-agnostic implementations (in this case, C++).

  1. hashCode
  2. Random
    1. Initilization
    2. Next
  3. Further Reading

hashCode

HashCode is for turning a string into an integer. It’s used in Minecraft to transform text-based seeds into a number.

A basic implementation looks like this:

int h = 0;
// Hash is 0 if text is empty or invalid
for (int i = 0; i < text.size(); i++) {
    h = 31 * h + text[i];
}
// Returns a 32-Bit Integer
return h;

Random

This section will explain how the Random Number Generator for Beta Minecraft/Java 8 worked, as that is what Minecrafts old generator was largely utilizing for its data.

Initilization

The Random class always starts with an initial seed, consisting of a signed 64-Bit Integer. If nothing is provided, the current timestamp is used instead. The actual seed is calculated from there via combining it with a few other numbers.

Our final seed value can be calculated as follows.

seed = (inputSeed ^ 0x5DEECE66D) & 0xffffffffffff;

In other words, our input seed, for example 3257840388504953787 is XOR’d with 0x5DEECE66D, then limited to 48-Bits.

Our initial seed would become 0x2C974D2EC5D6/49028346594774.

Next

Whenever a new random number is needed, the next functon is utilized, which generates a new random number based on the seed. It takes in the number of bits that need to be generated and limits the result to that number of bits, capping out at 32.

seed = (seed * 0x5DEECE66D + 11) & 0xffffffffffff;
return int(seed >> (48 - bits));

Our seed, after being initialized, would now become 0x7F9FD7F08029/140324499390505.

How the next function is used to generate other numers

Further Reading