How can I generate a random hash function in java ?

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Java

hash

random

I'm working on a java project where I have a method that returns a hash function as a
pair of integers a , b that exist in {1 , 2 , ... , 10000} and are picked randomly . To
pick the random numbers I have an "rnd" variable . My problem is that I have learned that prime numbers are better for hashing a function which means that the number b must be a prime number . With the above information given how can a hash function be randomly
generated ?
I have posted my existing code below .I want to implement the hasfuncgen() function.
Thank you for your time .

What I have tried:

import java.util.Random;

class IntegerPair {
	private int a;
	private int b;
	
	public IntegerPair(int a, int b) {
		this.a = a; 
		this.b = b;
	}	
	
	public int getA() {
		return a;
	}
	public void setA(int a) {
		this.a = a;
	}
	public int getB() {
		return b;
	}
	public void setB(int b) {
		this.b = b;
	}
}

public class Document {
	private int id = -1;
	private int[] tokens = null;
	private String filename = "";
	private double sim = 0.0;
	private static Random rnd = new Random(System.currentTimeMillis());

	public Document(Integer[] tokens, String filename, int id) {
		if (tokens != null) {
			if (this.tokens == null)
				this.tokens = new int[tokens.length];
			for (int i=0 ; i < tokens.length ; i++) {
				this.tokens[i] = tokens[i];
			}
		}
		this.filename = filename;
		this.id = id;
	}

	private IntegerPair hashFuncGen() 
	{		
		return null;
	}