Java,空指针异常.当我向数组添加一个对象时

您已将其定义accounts为构造函数的局部变量,而不是类级别成员.

public class Bank {
    Account[] accounts;
    int count;

    public Bank(String bankName, int num) {
        accounts = new Account[num];
        count = 0;
    }

    public boolean addAccount(Account acct) {
        // Do your work
    }
}