Java创建银行账号类SavingAccount，用静态变量存储年利率，用私有实例变量存储存款额。提供计算年利息的方法和计算月利息（年利息/12）的方法。编写一个测试程序测试该类，建立SavingAccount的对象saver，存款额是30000，设置年利率是3%，计算并显示saver的存款额、年利息和月利息；

下面是Java代码实现：

public class SavingAccount {
    private double balance; // 存款额
    private static double annualInterestRate; // 年利率

    public SavingAccount(double balance) {
        this.balance = balance;
    }

    // 设置年利率
    public static void setAnnualInterestRate(double rate) {
        annualInterestRate = rate;
    }

    // 计算月利息
    public double getMonthlyInterest() {
        return balance * annualInterestRate / 12;
    }

    // 计算年利息
    public double getAnnualInterest() {
        return balance * annualInterestRate;
    }

    // 存款
    public void deposit(double amount) {
        balance += amount;
    }

    // 取款
    public void withdraw(double amount) {
        balance -= amount;
    }

    // 获取存款余额
    public double getBalance() {
        return balance;
    }
}

public class TestSavingAccount {
    public static void main(String[] args) {
        SavingAccount saver = new SavingAccount(30000);
        SavingAccount.setAnnualInterestRate(0.03); // 设置年利率为3%

        double balance = saver.getBalance();
        double annualInterest = saver.getAnnualInterest();
        double monthlyInterest = saver.getMonthlyInterest();

        System.out.println("存款额：" + balance);
        System.out.println("年利息：" + annualInterest);
        System.out.println("月利息：" + monthlyInterest);
    }
}

运行结果：

存款额：30000.0
年利息：900.0
月利息：75.0