如何从另一个 class 调用方法 function？

Question

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.

public class WeatherRecord //implements Record
{

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


    public static void main (String[] args){

    }
}

This is another class

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

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

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

    }
}

Answer 1

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:

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:

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

Answer 2

In class WeatherRecord :

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

Import <path>.ClassName

Then, just referene or call your object like:

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.

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

Have clear concept on Object and Static functions. Click me

Answer 3

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.

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 -

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"); 
}

Answer 4

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

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

You can then call their methods with:

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

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

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

Next you need a method to reference.

public returnType methodName(params){
  return something;
}

Answer 5

You need to understand the difference between classes and objects. From theJava tutorial :

An object is a software bundle of related state and behavior

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

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

new Date();

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

Date d = new Date();

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

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

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

public class MyDate{
  public static 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.

Answer 6

import <path>;
 My main java program:

import method;
import java.util.Scanner;
public class Main {
    public static void main(String[] args){
        Scanner n = new Scanner(System.in);
        int a,b,c;
        double area,s;
        System.out.print("Enter the 1st side->");
        a = n.nextInt();
        System.out.print("Enter the 2nd side->");
        b = n.nextInt();
        System.out.print("Enter the 3rd side->");
        c = n.nextInt();
        s=(a+b+c)/2.0;
        area = Math.sqrt(s*(s-a)*(s-b)*(s-c));
        System.out.println("Area is = "+area);
        //factorial function is contained in the class method
        System.out.print(method.factorial(a));
    }
}

Answer 7

There's a hack .

When you're creating the activity classes of your app, chose one (most logically your start-up activity class) as the " master ", and have all other activity classes extend the master. Any public method you create in the master should be callable from any of the other activity classes, provided the variables and parameters that method uses are also known to the other activity classes.

Answer 8

This is for public method and in same package both classes

1) create object that class and call method
e.g. A obj = new A();
       obj.m();

2) using inheritance e.g. extends 

public class A extends B
 {
 obj.m();
 }

3) if static method then directly call via class name
   A.m();