Trouble over riding and using equals method in java

问题

I'm have problems trying to figure out how to compare selectedRadio between AM FM AND XM so that I can determine which one it is currently on and then return the station of the particular radio. I know that i need the equals method i'm just not sure the correct way to use it to get the result i'm looking for.

public class AutoRadioSystem
    {
      private Radio selectedRadio;
      private AMRadio radioAM;
      private FMRadio radioFM; 
      private XMRadio radioXM;

      //I'm not sure if this is the correct way to do this so that selectedRadio equals the object of radioAM
      public AutoRadioSystem()
      {
        selectedRadio = radioAM;
      }
     //trying to figure out how to compare if selectedRadio is one of these. 
      public double getCurrentStation()
      {
        if (selectedRadio == radioAM)
        {
          return radioAM.getCurrentStaion();
        }
        else if (selectedRadio == radioFM)
        {
          return radioFM.getCurrentStaion();
        }
        return 0.0;
      }
      // its supposed to switch the radio from AM to FM when this method is called 
      public void selectRadio()
      {
        if (selectedRadio.equals(radioAM))
          selectedRadio = radioFM;
      }
      public boolean equals (Object o)    
     {        
       if (o == null)           
         return false;        
       if (! (o instanceof AutoRadioSystem))           
         return false;       
       AutoRadioSystem other = (AutoRadioSystem) o;       
       return this.selectedRadio == other.selectedRadio;    
     }
      public static void main (String [] args) { 
        AutoRadioSystem c = new AutoRadioSystem();
        c.selectRadio();
        double b = c.getCurrentStation();
        System.out.println(b);
      }
    }


public abstract class Radio 
{
 double currentStation;

 RadioSelectionBar radioSelectionBar;
 public Radio()
 {
   this.currentStation = getMin_Station();
 }
 public abstract double getMax_Station();
 public abstract double getMin_Station();
 public abstract double getIncrement();
 public void up()
 {

 }
 public void down()
 {

 }
 public double getCurrentStaion()
 {
   return this.currentStation;
 }
 public void setCurrentStation(double freq)
 {
   this.currentStation = freq;
 }
 public void setStation(int buttonNumber, double station)
 {
 }
 public double getStation(int buttonNumber)
 {
   return 0.0;
 }
 public String toString()
 {
   String message = ("" + currentStation);
   return message;
 } 

 public boolean equals (Object o)    
 {        
   if (o == null)           
     return false;        
   if (! (o instanceof Radio))           
     return false;       
   Radio other = (Radio) o;       
   return this.currentStation == other.currentStation;    
 }

 public static void main(String[] args)
 {
   Radio amRadio = new AMRadio();

   System.out.println(amRadio);

   Radio fmRadio = new FMRadio();

   System.out.println(fmRadio);

   Radio xmRadio = new XMRadio();

   System.out.println(xmRadio);

 }  
}



public class FMRadio extends Radio
{
  private static final double Max_Station = 108.0;
  private static final double Min_Station = 88.0;
  private static final double Increment = .01;
  public FMRadio()
  {
    currentStation = Min_Station;
  }
  public  double getMax_Station()
  {
    return this.Max_Station;
  }
  public  double getMin_Station()
  {
    return this.Min_Station;
  }
  public  double getIncrement()
  {
    return this.Increment;
  }
  public String toString()
  {
    String message = ("FM " + this.currentStation);
    return message;
  } 
} 

回答1:

Main point: (moved from bottom of post)

You'll want to learn the difference between the tests ObjA == ObjB, ObjA.equals(ObjB), and ObjA instanceof ClassB.

---

This method

public double getCurrentStation()
{
    if (selectedRadio == radioAM)
    {
        return radioAM.getCurrentStaion();
    }
    else if (selectedRadio == radioFM)
    {
        return radioFM.getCurrentStaion();
    }
    return 0.0;
}

Probably won't work as you expect. If you implement the double getCurrentStation() method inside the Radio base class (which you did), you can just do something like:

public double getCurrentStation()
{
    return selectedRadio.getCurrentStation();
}

But that's probably overkill, so you should just replace the getCurrentStation() calls in AutoRadioSystem with selectedRadio.getCurrentStation().

Similarly,

public void selectRadio()
{
    if (selectedRadio.equals(radioAM))
        selectedRadio = radioFM;
}

Won't work as you expect. You probably want something like

public void selectRadio()
{
    if (selectedRadio. instanceOf AMRadio))
        selectedRadio = new FMRadio();
}

Also,

if (o == null)           
    return false;        
if (! (o instanceof AutoRadioSystem))           
    return false;  

Can be simplified to

if (! (o instanceof AutoRadioSystem))
    return false;

Since o instanceof SomeClass returns false if o is null.

There are other issues as well.

回答2:

Define equals method in all subclasses like AMRadio, FMRadio and so on. You can use EqualsBuilder from Apache commons for this. http://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/builder/EqualsBuilder.html

Typical equals method using EqualsBuilder looks like this. It uses reflection to compare equality.

 public boolean equals(Object obj) {
     return EqualsBuilder.reflectionEquals(this, obj);
  }

And while comparing, do not use == since it checks for reference equality, not logical equality. Use equals method like

   if (selectedRadio.equals(radioAM))
    {
      return radioAM.getCurrentStaion();
    }

来源：https://stackoverflow.com/questions/18991232/trouble-over-riding-and-using-equals-method-in-java