Custom Object ArrayList Sort milk.java

/*
ID: karihee1
LANG: JAVA
TASK: milk
*/

/*  milk.java
*
*  This is a solution to the 10th USACO Training Problem "Mixing Milk".  
*  A copy of the problem statement can be found here:  https://jvonk.github.io/usaco/2018/09/29/milk.html
*
*  The solution below demonstrates sorting an ArrayList containing a custom class
*
*  NOTE: since USACO Trainiing Problems still uses files for input and output,
*  I have replaced the BufferedReader and PrintWriter lines that use stdin and stdout.
*
* The file "milk.in" looks like this:
* 100 5 
* 5 20 
* 9 40 
* 3 10 
* 8 80 
* 6 30
*/

import java.io.*;
import java.util.*;

class milk {
    public static void main (String [] args) throws IOException {

        //BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        //PrintWriter out = new PrintWriter(System.out);
        BufferedReader in = new BufferedReader(new FileReader("milk.in"));
        PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("milk.out")));

        StringTokenizer st;
    
        st = new StringTokenizer(in.readLine());        
        int N = Integer.parseInt(st.nextToken());        // Milk Demand (0 <= N <= 2,000,000)
        int M = Integer.parseInt(st.nextToken());        // Number of farmers to buy from (0 <= M <= 5,000)
        
        // out.println(N + " " + M);   // DEBUGGING PRINT STATEMENT
    
        ArrayList<Farmer> al = new ArrayList<Farmer>(M);
    
        for (int i=0;i<M;i++) {
            st = new StringTokenizer(in.readLine());
            int P = Integer.parseInt(st.nextToken());    // Price
            int A = Integer.parseInt(st.nextToken());    // Amount
            
            // out.println(P + " " + A);  // DEBUGGING PRINT STATEMENT
            
            al.add(new Farmer(P,A));
        }
    
        Collections.sort(al);
        
        Farmer f;
        int answer=0;
        
        for (int i=0;i<M; i++) {
            f = al.get(i);
            
            //out.println(f.price + " " + f.amount);  // DEBUGGING PRINT STATEMENT
            
            if ((N>0) && (f.amount>0)) {              // We want to buy from this farmer!
                if (N>=f.amount) {
                    answer += f.amount * f.price;     // buy all from this farmer
                    N -= f.amount;
                } else {
                    answer += N*f.price;              // buy some from this farmer -- demand fulfilled
                    N = 0;
                }
            }
        }
        out.println(answer);
        out.close();
    }
}

class Farmer implements Comparable<Farmer> {
    public int price;
    public int amount;
 
    public Farmer(int price,int amount) {
        this.price = price;
        this.amount = amount;
    }
    @Override
    public int compareTo(Farmer obj) {
    // sort in ascending order.  
    // compareTo returns a negative integer, zero, or a positive integer as 
    // this object is less than, equal to, or greater than the specified object. 
        if (this.price<obj.price) {
            return -1;            
        } else if (this.price==obj.price) {
            return 0;             
        } else {
            return 1;              
        }
    }
}