访问具有多个对象类型的ArrayList的方法

Question

I've got a superclass that multiple different subclasses extend from. Each subclass then has specific methods for that concrete class that other subclasses may not follow. ie:

• public class NMEA ... methods that all subclasses hold
• public class GPGGASentence extends NMEA .. has some specific method to this type
• public class GPGPSSentence extends NMEA .. has some specific method to this type

I create a subclass by checking the first few letters in the String and then switching between the possibilities.

ArrayList<NMEA> sentences = new ArrayList<>();

switch (s.split(",")[0]){
     case "$GPGGA":
          sentences.add(new GPGGASentence(s));
          break;
     case "$GPGPS":
          sentences.add(new GPGPSSentence(s));
          break;
}

I'm now getting to the point I want to loop through this ArrayList. Currently I'm doing the following:

for(NMEA nmea : sentences){
     if(nmea instanceof GPGGASentence){
          system.out.println((GPGGASentence) nmea).someSpecificMethod());
     }
 }

I was wondering if their is some nicer way of doing this.

Answer 1

我对这个问题的解决方案是按照建议将NMEA类更改为抽象类，并列出该类中的所有方法，然后覆盖它们。

Answer 2

Apart from writing abstract class / interface and declaring all the methods, this is what you can do to improve the redability of the code:

• Create an enum of all the possible class types
• Write a method that returns enum value for object
• Use switch with different cases.

Here is an example. Declare enum like below:

private enum ClassType {
    NMEA,
    GPGGASentence,
    SomeOtherType,
    Unknown;
}

Now, a method to return type :

private ClassType getClassType(NMEA object){
    ClassType type = null;
    try{
        type = ClassType.valueOf(object.getClass().getSimpleName());
    }catch(IllegalArgumentException e){
        type = ClassType.Unknown;
    }
    return type;
}

Now, use it with switch inside a for loop:

for(NMEA nmea : sentences){
    switch(getClassType(nmea)){
    case ClassType.NMEA :
        //do something
        break;
    }
}

If a new Impl class gets added in future (or name of any class is changed), all you need to do is to add an enum value and a switch case .