在 java 中理解类

Question

所以我对

的概念有点困惑

Instance methods of a class being called without first instantiating an object

这在 java 中如何运作？我是否必须实例化一个对象，以便我可以调用该对象的方法。

例如，下面是我的类。

public class Date{
        public String month;
        public int day;
        public int year;
    public void writeOutput()
    {
        System.out.println("Today is : " + this.month + " " + this.day + " , " + this.year);
    }
}

public class DateTest{
    public static void main(String[] yolo){
        Date today;
        today = new Date();
        today.month = "January";
        today.day = 31;
        today.year = 2015;
        today.writeOutput();
    }
}

因此我必须先实例化一些日期？我可以在不实例化某种 "date" 对象的情况下调用实例方法吗？

Answer 1

关于：

Hence I would have to instantiate some date first? Can I call an instance method without instantiating a "date" object of some kind?

正确。要调用 Date 的实例方法，您必须首先创建一个 Date 对象，您目前在 main 方法中正在这样做。

    Date today;  // declare Date variable today
    today = new Date();  // create instance and assign it to today

请注意您的代码直接操作日期字段，应该避免这种情况。

Answer 2

语句 today = new Date(); 实例化 class Date 的一个实例，并将对该实例的引用分配给变量 today.

这允许通过 today 变量引用实例变量和方法。没有实例，这些成员将不存在。

Answer 3

是的，您可以通过使用静态字段和工厂方法在不实例化的情况下调用方法。

public class Date{
    public static String month;
    public static int day;
    public static int year;
public static void writeOutput()
{
    System.out.println("Today is : " + this.month + " " + this.day + " , " + this.year);
}}

然后您可以进行以下操作：

Date.month = "January";
Date.day = 1;
Date.year=2015;
Date.writeOutput();

这样你就不需要实例化了。但是，这不一定是好的做法。如果不需要 class 变量，静态工厂方法很好，而是将必要的参数传递给方法。例如：

public class Date
{

public void writeOutput(String year, int day, int month)
{
    System.out.println("Today is : " + month + " " + day + " , " + year);
}
}

那你可以打电话给

Date.writeOutput("January", 1, 1);

Answer 4

这是我评论的副本：

Can you make a dog bark if it doesn't exists? (and only the concept of the dogs exists). Now imagine the concept of the dog as the class, bark as a method and Rex as an instance of a dog. So yes, you need to instanciate a class (Dog Rex = new Dog();) In order to use a method (Rex.bark()). Of course you can use static methods that allow you to do something like Dog.bark() but that it's not really OOP.

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