[array in java] can't change final length of array

import java.util.ArrayList;
import java.util.Scanner;

class type {
    /*
     * static int min(int a[]) { if (a.length == 0) return -1; else { int min =
     * 0; for (int i = 0; i < a.length; i++) { if (a[i] < min) min = a[i]; }
     * return min; } }
     */
    /*
     * static void remove(int a[], int value) { // if have two value has the
     * same value??? for(int i=0; i< a.length; i++) { if(a[i]==value) { for(int
     * j =i; j<a.length;j++) { a[j] = a[j+1]; } a.length-=1; //final array
     * length can not be assign break; } } }
     */
    public static void main(String args[]) {
        int pie, rack, a, n, count;
        ArrayList pies = new ArrayList();
        ArrayList racks = new ArrayList();
        Scanner scanner = new Scanner(System.in);
        a = scanner.nextInt();
        while (a-- > 0) {
            n = scanner.nextInt();
            for (int i = 0; i < n; i++) {
                pies.add(scanner.nextInt());
            }
            for (int i = 0; i < n; i++) {
                racks.add(scanner.nextInt());
            }
            while (pie=min(pies.toArray())!=-1) {
                while ((rack = min(racks)) != -1 && rack >= pie) {
                    // remove rack from racks array
                }
                // remove both rack and pie
                // update count
            }
            ;
        }

    }
}