我正在学习Java初学者课程，我正在做我的第一个面向对象的作业。我确实了解OOP的基础知识，但我不具备将其付诸实践所需的知识。如果有人能指出任何新手的错误或给我一些建议，我将非常感激。

class savings{
    public static void main(String[] args){
        double annualInterestRate = 0;
        double monthlyInterest = 0;
        SavingsAccount saver1 = new SavingsAccount(2000, .03);
        SavingsAccount saver2 = new SavingsAccount(4000, .03);
        calculateMonthlyInterest(saver1);
        System.out.println("Monthly Interest at 3%: " + monthlyInterest);
        calculateMonthlyInterest(saver2);
        System.out.println("Monthly Interest at 3%: " + monthlyInterest);
        modifyInterestRate(saver1);
        modifyInterestRate(saver2);
        calculateMonthlyInterest(saver1);
        System.out.println("Monthly Interest at 5%: " + monthlyInterest);
        calculateMonthlyInterest(saver2);
        System.out.println("Monthly Interest at 5%: " + monthlyInterest);
    }
}

class SavingsAccount{
    static double annualInterestRate;
    static private double savingsBalance;
    public static double calculateMonthlyInterest(double annualInterestRate){
                double monthlyInterest = 0;
                monthlyInterest = savingsBalance * annualInterestRate / 12;
                return monthlyInterest;
        }
    public static void modifyInterestRate(double annualInterestRate){
        annualInterestRate = .05;
    }
    SavingsAccount(double savingsBalance, double annualInterestRate){
        this.savingsBalance = savingsBalance;
        this.annualInterestRate = annualInterestRate;
    }
}

F:\Archive\Homework_Spring_2014\Java\Project 2\Savings\savings.java:7: error: cannot find symbol
        calculateMonthlyInterest(saver1);
        ^
  symbol:   method calculateMonthlyInterest(SavingsAccount)
  location: class savings
F:\Archive\Homework_Spring_2014\Java\Project 2\Savings\savings.java:9: error: cannot find symbol
        calculateMonthlyInterest(saver2);
        ^
  symbol:   method calculateMonthlyInterest(SavingsAccount)
  location: class savings
F:\Archive\Homework_Spring_2014\Java\Project 2\Savings\savings.java:11: error: cannot find symbol
        modifyInterestRate(saver1);
        ^
  symbol:   method modifyInterestRate(SavingsAccount)
  location: class savings
F:\Archive\Homework_Spring_2014\Java\Project 2\Savings\savings.java:12: error: cannot find symbol
        modifyInterestRate(saver2);
        ^
  symbol:   method modifyInterestRate(SavingsAccount)
  location: class savings
F:\Archive\Homework_Spring_2014\Java\Project 2\Savings\savings.java:13: error: cannot find symbol
        calculateMonthlyInterest(saver1);
        ^
  symbol:   method calculateMonthlyInterest(SavingsAccount)
  location: class savings
F:\Archive\Homework_Spring_2014\Java\Project 2\Savings\savings.java:15: error: cannot find symbol
        calculateMonthlyInterest(saver2);
        ^
  symbol:   method calculateMonthlyInterest(SavingsAccount)
  location: class savings
6 errors

Tool completed with exit code 1

我相信这些错误是由跨类引用的方法造成的，但我不知道正确的方法。