Java : calculate Fibonacci number by using Java's ForkJoinPool and RecursiveTask

fibonacci2.java

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package neo.app;

import java.math.BigInteger;
import java.util.concurrent.*;

class Fibonacci extends RecursiveTask<BigInteger> {
    final long n;
    public static BigInteger[] result = new BigInteger[2];
    public static int max_num = 1;
    public static final BigInteger zero = new BigInteger("0");
    
    public static void set_max(int num) {
        result[0] = new BigInteger("0");
        result[1] = new BigInteger("1");
        if ( num > max_num ) {
            BigInteger[] new_result = new BigInteger[num+1];
            
            for (int i=0; i<=max_num; i++) {
                new_result[i] = result[i];
            }                
            result = new_result;
            for (int i=max_num+1; i<=num; i++) {
                result[i] = new BigInteger("-1");
            }
            max_num = num;            
        }
    }
    
    Fibonacci(long n) {
        this.n = n;
        if ( n > 1 )
            set_max((int)n);
    }
        
    public BigInteger compute() {
        BigInteger ret;
        if ( result[(int)n].compareTo(zero) > 0 ) {
            return result[(int)n];
        }
        if (n<=10) {
            return do_fibonacci(n);
        }
        ForkJoinTask<BigInteger> subTask = new Fibonacci(n-1).fork();
        ret = new Fibonacci(n-2).compute().add( subTask.join() );
        result[(int)n] = new BigInteger(ret.toString());
        return ret;
    }
    
    static BigInteger do_fibonacci(long n) {
        BigInteger ret;
        if ( result[(int)n].compareTo(zero) >= 0 ) {
            ret = result[(int)n];
        } else {
            BigInteger ret_n_1 = do_fibonacci(n-1);
            BigInteger ret_n_2 = do_fibonacci(n-2);
            ret = new BigInteger(ret_n_1.add(ret_n_2).toString());
            result[(int)n] = ret;
        }
        return ret;
    }
}
/**
 *
 * @author neojou
 */
public class FibonacciForkJoin {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        for (int i=2; i<=300; i++) {
            Fibonacci fibonacci = new Fibonacci(i);
            ForkJoinPool pool = new ForkJoinPool();
            System.out.printf("%d : ", i);
            System.out.println(pool.invoke(fibonacci).toString());
        }
    }
    
}

FibonacciForkJoin.java

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package neo.app;

import java.util.concurrent.*;

class Fibonacci extends RecursiveTask<Long> {
    final long n;
    
    Fibonacci(long n) {
        this.n = n;
    }
    
    public Long compute() {
        if (n<=10) {
            return do_fibonacci(n);
        }
        ForkJoinTask<Long> subTask = new Fibonacci(n-1).fork();
        return new Fibonacci(n-2).compute() + subTask.join();
    }
    
    static long do_fibonacci(long n) {
        if ( n<=1 ) return n;
        return do_fibonacci(n-1) + do_fibonacci(n-2);
    }
}
/**
 *
 * @author neojou
 */
public class FibonacciForkJoin {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Fibonacci fibonacci = new Fibonacci(45);
        ForkJoinPool pool = new ForkJoinPool();
        System.out.println(pool.invoke(fibonacci));
    }
    
}

FibonacciForkJoin1.java

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package neo.app;

import java.util.concurrent.*;

class Fibonacci extends RecursiveTask<Long> {
    final long n;
    public static Long[] result = new Long[2];
    public static int max_num = 1;
    
    public static void set_max(int num) {
        result[0] = new Long(1);
        result[1] = new Long(1);
        if ( num > max_num ) {
            Long[] new_result = new Long[num+1];
            
            for (int i=0; i<=max_num; i++) {
                new_result[i] = result[i];
            }                
            result = new_result;
            for (int i=max_num+1; i<=num; i++) {
                result[i] = new Long(0);
            }
            max_num = num;            
        }
    }
    
    Fibonacci(long n) {
        this.n = n;
        if ( n > 1 )
            set_max((int)n);
    }
        
    public Long compute() {
        Long ret;
        if ( result[(int)n].longValue() != 0 ) {
            return result[(int)n];
        }
        if (n<=10) {
            return do_fibonacci(n);
        }
        ForkJoinTask<Long> subTask = new Fibonacci(n-1).fork();
        ret = new Fibonacci(n-2).compute() + subTask.join();
        result[(int)n] = new Long(ret);
        return ret;
    }
    
    static long do_fibonacci(long n) {
        long ret;
        if (result[(int)n].longValue() != 0) {
            ret = result[(int)n].longValue();
        } else {
            long ret_n_1 = do_fibonacci(n-1);
            long ret_n_2 = do_fibonacci(n-2);
            ret = ret_n_1 + ret_n_2;
            result[(int)n] = new Long(ret);
        }
        return ret;
    }
}
/**
 *
 * @author neojou
 */
public class FibonacciForkJoin {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Fibonacci fibonacci = new Fibonacci(100);
        ForkJoinPool pool = new ForkJoinPool();
        System.out.println(pool.invoke(fibonacci));
    }
    
}

Can you help me understand your set_max method please ?