Question

我不太擅长编程，也不知道为什么这不起作用。无论我输入什么，它总是直接进入else语句。

    public void pizzaIntro()
    {
    Scanner user_input = new Scanner(System.in);
    String user_command = "null";
    String apology = "I'm sorry it appears there has been some kind of mistake in your order.";
    System.out.println("Welcome to " + cM + " here we strive to deliver excellent services to all our customers!");
    System.out.println("The current prize for pizza is $" + bP + " and an extra $" + tP + " per topping.");
    System.out.println(); System.out.println();
    while(user_command != "exit")
    {
        System.out.print("Would you like toppings?(yes/no):");
        user_command = user_input.next();
        if(user_command.toLowerCase() == "yes")
        {
            System.out.println("Good Eat Your Pizza.");
        }
        else if (user_command.toLowerCase() == "no")
        {
            System.out.println("Well Ok Then!");
        }
        else
        {
            System.out.println(apology);
            System.exit(1);
        }
    }
    pic1.show();
}

Answer 1

使用equals方法比较字符串。要了解==和equals之间的区别，请阅读https://stackoverflow.com/questions/7311451/difference-between-equals-and-instanceof 您将清楚地知道何时使用。

while(!(user_command.equals("exit")) {
    System.out.print("Would you like toppings?(yes/no):");
    user_command = user_input.next();
    if(user_command.toLowerCase().equals("yes"))
    {
        System.out.println("Good Eat Your Pizza.");
    }
    else if (user_command.toLowerCase().equals("no"))
    {
        System.out.println("Well Ok Then!");
    }
    else
    {
        System.out.println(apology);
        System.exit(1);
    }
}

Answer 2

使用equals方法代替==

user_command.toLowerCase().equals("yes")

在Java中，==总是只比较两个引用。可以将它用于原始数据类型。 String不是原始数据类型。 String是一个对象。你应该使用equals方法。

在您的情况下，您可以考虑使用equalsIgnoreCase方法忽略案例。

Answer 3

使用equalsIgnoreCase。它更安全

public void pizzaIntro()
    {
        Scanner user_input = new Scanner(System.in);
        String user_command = "null";
        String apology = "I'm sorry it appears there has been some kind of mistake in your order.";
        System.out.println("Welcome to " + cM
                           + " here we strive to deliver excellent services to all our customers!");
        System.out.println("The current prize for pizza is $" + bP
                           + " and an extra $" + tP + " per topping.");
        System.out.println();
        System.out.println();
        while (!user_command.equalsIgnoreCase("exit"))
        {
            System.out.print("Would you like toppings?(yes/no):");
            user_command = user_input.next();
            if (user_command.equalsIgnoreCase("yes"))
            {
                System.out.println("Good Eat Your Pizza.");
            }
            else if (user_command.equalsIgnoreCase("no"))
            {
                System.out.println("Well Ok Then!");
            }
            else
            {
                System.out.println(apology);
                System.exit(1);
            }
        }
        pic1.show();
    }

Answer 4

永远不要将任何Java对象与“==”进行比较（只有原语如int，long，double等）。它应该是：

if(user_command.toLowerCase().equals("yes")) {
  ...
}

否则你检查对象的位置是否相同，而不是内容。

在这个特定示例中，您可能只想使用String.equalsIgnoreCase（...）。

我的Java程序无效

4 个答案: