IRandom is a random number generator. The class can only be used when you have an instance of a World (Such as in an Event handler), it can not be in recipes.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.

import crafttweaker.util.IRandom;

You can get an instance of IRandom by using an IWorld instance.

Once you have an IWorld instance, you can simply do:

ZenScript
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world.random

• nextInt() returns the next pseudorandom int, ranging from Integer.MIN_VALUE to Integer.MAX_VALUE
• nextInt(int bound) returns the next pseudorandom int, ranging from [0, bound)
• nextInt(int min, int max) returns the next pseudorandom int, ranging from [min, max]
• nextFloat() returns the next pseudorandom float, ranging from [0.0f, 1.0f)
• nextDouble() returns the next pseudorandom double, ranging from [0.0, 1.0)
• nextFloat(float min, float max) returns the next pseudorandom float, ranging from [min, max]
• nextDouble(double min, double max) returns the next pseudorandom double, ranging from [min, max]
• nextBoolean() returns the next pseudorandom boolean
• String getRandomUUID() returns a random UUID

The ranges above are presented in using the mathematical interval notation, you can read more about it here