实例化对象为什么出错ya？

Question

package com;
import com.Human;
import com.Bank;
public class Exchange {
    public static void main(String[] args) {
        Human human = new Human();
        Bank bank = new Bank();
        bank.setName("光明银行城东支行");
        int[] c =new int[] {1,2,5};
        bank.setSmallChanges(c);
        human.setAmount(186);
        human.setName("李大伯");
        int h=bank.change(human);
        System.out.println(bank.getName()+"为"+human.getName()+"兑换 "+h+" 个零钱");
    }

}

package com;

public class Human {

private String name;//人名，构造时注意下
public void setName(String name){
  this.name =name ;
}
public String getName() {return this.name;}
private int amount=15;//兑换的金额
public void setAmount(int amount){
  this.amount =amount ;
}
public int getAmount() {return this.amount;}
}

package com;
import com.Human;
import java.util.Arrays;
public class Bank {
      private String name;//银行名字，构造时注意下
      public void setName(String name){
        this.name =name ;
      }
      public String getName() {return this.name;}

      private int[] smallChanges; //零钱种类，任意
      public void setSmallChanges(int[] smallChanges){
        this.smallChanges =smallChanges;
      }
      public int[] getSmallChanges() {return this.smallChanges;}

      public int change(Human human) {//实例化
      int a=human.getAmount();          //得到零钱数量,amount
      int i,c=1,b=0;
      int[] z = null;
      Arrays.sort(smallChanges);
      for(int d=0;d<=(a-a%smallChanges[0])/smallChanges[0];d++)//从最大的开始，如果最大的用不到，则修改，从第二大的开始，一直到最小的，同时比较所有a=0时的整数b，取最小
      {for(i=c;i<=smallChanges.length;i++) {
          b=(a-a%smallChanges[smallChanges.length-i])/smallChanges[smallChanges.length-i]+b;
          a=a%smallChanges[smallChanges.length-i];//

      }
      if(a!=0) {c++;}
      else {z[d]=b;}}
      Arrays.sort(z);
      return z[0];

      }
    }

找不到错误原因，求大佬解答

Answer 1

int[] z = null;
这的问题吧