创建线程遇到一些小问题

Question

public class CustomerTest {
    public static void main(String[] args) {
    Customer c1 = new Customer();
   Thread t1 = new Thread(c1);
   Thread t2 = new Thread(c1);
   t1.setName("线程一：");
   t2.setName("线程二：");
   t1.start();
   t2.start();


    }
}
class Account{
    private double balance;
    public Account(double balance){
        this.balance = balance;
    }

    public synchronized void deposit(double amt){
        if(amt > 0){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            balance += amt;
        }
        System.out.println(Thread.currentThread().getName() + ": 存钱成功，余额为：" + balance);
    }
}
class Customer implements Runnable{
    private Account account;
    public Customer(){};
    public Customer(Account account){
        this.account = account;
    }
    @Override
    public void run() {
        for (int i = 0; i < 3; i++) {
            account.deposit(1000);

        }
    }
}public class CustomerTest {
    public static void main(String[] args) {
   ** Customer c1 = new Customer();****我这里调用空参构造器，运行时为什么会出现空指针__**。
   Thread t1 = new Thread(c1);
   Thread t2 = new Thread(c1);
   t1.setName("线程一：");
   t2.setName("线程二：");
   t1.start();
   t2.start();


    }
}
class Account{
    private double balance;
    public Account(double balance){
        this.balance = balance;
    }

    public synchronized void deposit(double amt){
        if(amt > 0){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            balance += amt;
        }
        System.out.println(Thread.currentThread().getName() + ": 存钱成功，余额为：" + balance);
    }
}
class Customer implements Runnable{
    private Account account;
    public Customer(){};
    public Customer(Account account){
        this.account = account;
    }
    @Override
    public void run() {
        for (int i = 0; i < 3; i++) {
            account.deposit(1000);

        }
    }
}

Answer 1

第4行改成
Customer c1 = new Customer(new Account(100));