Question

我在ReadSource类中设置了String消息，我想从Automotive类方法在Driver类中访问它，但是它显示的是null而不是消息。我试图在Automotive setName（String）中打印消息，它显示正确的String，但我无法在我的主类中访问它。

我的问题可以通过测试下面的代码来说明。

有人可以帮我解释一下吗？非常感谢你。

package Driver;

import Util.ReadSource;
import Model.Automotive;
public class Driver {
    public static void main(String args[]){

        Automotive auto = new Automotive();
        ReadSource read =  new ReadSource();

        String main = "Message";
        //set the message and display
        read.set(main);

        //get the message that was set in the read.set(String)
        auto.getMessage();
    }

}
package Util;

import Model.Automotive;
public class ReadSource {


    public void set(String message){
        Automotive auto = new Automotive();
        System.out.println("This messasge is in getMessage in the
  class ReadSource "+ message);
        //Set the name for automotive
        auto.setName(message);
    }
}
//This the the ouput
/*This messasge is in getMessage in the class ReadSource Message
Message is Message
This messasge is in getMessage in the class Automotive null*/

Answer 1

在ReadSource.set（）方法中。您正在创建一个新的汽车实例。然后在创建一个新的Automotive实例（从未再次使用）之后，通过set方法再次传递消息的值。你想要做的是在类体的开头创建一个变量（在技术上是一个 field ）。

public class ReadSource {
     private String message;

     public void setMessage(String message) {
         this.message = message;
     }

     // make a getter to return it somewhere else
}

现在可以使用auto.getMessage（）（或类似的东西）在主类中访问消息。

然后，如果在主要你是：

System.out.println(auto.getMessage());

它将打印传递给setMessage（）方法的任何内容。

Answer 2

public class Driver {
public static void main(String args[]){

    Automotive auto = new Automotive();
    ReadSource read =  new ReadSource(auto);

    String main = "Message";
    //set the message and display
    read.set(main);

    //get the message that was set in the read.set(String)
    auto.getMessage();

    System.out.println(auto.getMessage());
}

public class ReadSource {

Automotive auto;

public ReadSource(Automotive auto) {
    this.auto = auto;
}

public void set(String message) {

    System.out.println("This messasge is in getMessage in the  class ReadSource: " + message);
    //Set the name for automotive
    auto.setName(message);
}

}

public class Automotive {

String name;

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getMessage() {
    return name;
}

}

使用Java访问包

2 个答案: