Java 如何从另一个类调用方法函数？

How to call a method function from another class?

提问by Eddie Wong

I'm writing a java project that has three different classes. This is what i have have so far. I'm just stuck on how do you call a method function from another class to another class. I have written 2 classes already. I got the "Date" class and "TemperatureRange" class done; now i'm trying to call those 2 classes into "WeatherRecord" class. I'm not sure if i'm explaining this right.

我正在编写一个包含三个不同类的 Java 项目。这是我迄今为止所拥有的。我只是被困在如何从另一个类调用方法函数到另一个类。我已经写了 2 个类。我完成了“Date”类和“TemperatureRange”类；现在我试图将这两个类称为“WeatherRecord”类。我不确定我是否正确解释了这一点。

public class WeatherRecord //implements Record
{

    private String TemperatureRangetoday;
    private String TemperatureRangenormal;
    private String TemperatureRangerecord;


    public static void main (String[] args){

    }
}

This is another class

这是另一堂课

public class Date
{
    public static String date(String date, String month, String year){
        String rdate = date + " " +month + " " +year;
        return rdate;   
    }
}

And here's another class

这是另一堂课

public class TemperatureRange
{
    public static String TempRange (String high, String low){

        String rTempRange = high +"high" + " "+low+"low";
        return rTempRange;

    }
}

回答by MarsAtomic

You need a reference to the class that contains the method you want to call. Let's say we have two classes, A and B. B has a method you want to call from A. Class A would look like this:

您需要对包含要调用的方法的类的引用。假设我们有两个类，A 和 B。B 有一个你想从 A 调用的方法。类 A 看起来像这样：

public class A
{
    B b; // A reference to B

    b = new B(); // Creating object of class B

    b.doSomething();  // Calling a method contained in class B from class A
}

B, which contains the doSomething() method would look like this:

B，其中包含 doSomething() 方法将如下所示：

public class B
{
    public void doSomething()
    {
        System.out.println("Look, I'm doing something in class B!");
    }
}

回答by Chad

You need to instantiate the other classes inside the main class;

您需要在主类中实例化其他类；

Date d = new Date(params);
TemperatureRange t = new TemperatureRange(params);

You can then call their methods with:

然后你可以调用他们的方法：

object.methodname(params);
d.method();

You currently have constructors in your other classes. You should not return anything in these.

您目前在其他类中有构造函数。你不应该返回这些中的任何东西。

public Date(params){
    set variables for date object
}

Next you need a method to reference.

接下来你需要一个方法来引用。

public returnType methodName(params){
  return something;
}

回答by Romski

enter link description hereYou need to understand the difference between classes and objects. From the Java tutorial:

在此处输入链接描述您需要了解类和对象之间的区别。从 Java 教程：

An object is a software bundle of related state and behavior

A class is a blueprint or prototype from which objects are created

对象是相关状态和行为的软件包

类是创建对象的蓝图或原型

You've defined the prototypes but done nothing with them. To use an object, you need to create it. In Java, we use the newkeyword.

您已经定义了原型，但没有对它们做任何事情。要使用对象，您需要创建它。在 Java 中，我们使用new关键字。

new Date();

new Date();

You will need to assign the object to a variable of the same type as the class the object was created from.

您需要将对象分配给与创建对象的类具有相同类型的变量。

Date d = new Date();

Date d = new Date();

Once you have a reference to the object you can interact with it

一旦你有了对象的引用，你就可以与它进行交互

d.date("01", "12", "14");

d.date("01", "12", "14");

The exception to this is static methods that belong to the class and are referenced through it

例外是属于该类并通过它引用的静态方法

public class MyDate{ public static date(){ ... } }

public class MyDate{ public static date(){ ... } }

... MyDate.date();

... MyDate.date();

In case you aren't aware, Java already has a class for representing dates, you probably don't want to create your own.

如果您不知道，Java 已经有一个用于表示日期的类，您可能不想创建自己的类。

回答by Asis

In class WeatherRecord:

First import the class if they are in different package else this statement is not requires

在类中WeatherRecord：

如果它们在不同的包中，则首先导入类，否则不需要此语句

Import <path>.ClassName

Then, just referene or call your object like:

然后，只需引用或调用您的对象，如：

Date d;
TempratureRange tr;
d = new Date();
tr = new TempratureRange;
//this can be done in Single Line also like :
// Date d = new Date();

But in your code you are not required to create an object to call function of Date and TempratureRange. As both of the Classes contain Static Function , you cannot call the thoes function by creating object.

但是在您的代码中，您不需要创建一个对象来调用 Date 和 TempratureRange 的函数。由于两个类都包含静态函数，因此您不能通过创建对象来调用 thoes 函数。

Date.date(date,month,year);   // this is enough to call those static function 

Have clear concept on Object and Static functions. Click me

对对象和静态函数有清晰的概念。点击我

回答by Moni

For calling the method of one class within the second class, you have to first create the object of that class which method you want to call than with the object reference you can call the method.

要在第二个类中调用一个类的方法，您必须首先创建该类的对象，您要调用哪个方法，而不是使用可以调用该方法的对象引用。

Class A {

Public void fun(){

//do something

}

Class B
{
public static void main(String args[]){
A obj = new A();
obj.fun();
}
}

But in your case you have the static method in Date and TemperatureRange class. You can call your static method by using the class name directly like below code or by creating the object of that class like above code but static method ,mostly we use for creating the utility classes, so best way to call the method by using class name. Like in your case -

但是在您的情况下，您在 Date 和 TemperatureRange 类中有静态方法。您可以像下面的代码一样直接使用类名来调用静态方法，也可以像上面的代码一样创建该类的对象，但静态方法主要用于创建实用程序类，因此最好使用类名调用该方法. 就像你的情况 -

public static void main (String[] args){

String dateVal = Date.date("01","11,"12"); // calling the date function by passing some parameter.

String tempRangeVal = TemperatureRange.TempRange("80","20"); 
}