如何在数组列表中显示对象的距离？ （多态性）

Question

I am trying to display all of the elements in the ArrayList plantList. My main program will add, delete, search, filter, and display all of the plants of four different child classes. Everything "seems" to be working except when I display. ~I will only including portions of my code that are relevant to the questions.

A little background: I am a student and this is my first time working with inheritance/polymorphism.

1)How do I distinguish between the different objects since they all have different parameters, at the time of displaying?

2) Any suggestions on how to improve the performance/logic of what I'm doing? A little explanation would be great.

//Parent class

public class Plant{

   private String name;
   private String id;
   private String color;

   public Plant(String name, String id, String color){
   this.name = name;
   this.id = id;
   this.color = color;

   }

   public String getName(){

      return this.name;
   }

   public void setName(String name){

      name = this.name;  
   }

   public String getId(){

      return this.id;
   }

   public void setId(String id){

      id = this.id;  
   }

   public String getColor(){

      return this.color;
   }

   public void setColor(String color){
      color = this.color;  
   }


}

//one of several child classes

   public class Flower extends Plant{

   private boolean thorns;
   private boolean smell;   

   public Flower(String name, String id, String color, boolean blnThorns, boolean blnSmell){
      super(name, id, color);
      thorns = blnThorns;
      smell = blnSmell;

   }

   public boolean isThorns(){

      return thorns;
   }

   public void setThorns(boolean blnThorns){
      thorns = blnThorns;  
   }

   public boolean isSmell(){

      return smell;
   }

   public void setSmell(boolean blnSmell){
      smell = blnSmell;  
   }


}

// portion of the main driver

ArrayList<Plant> plantList = new ArrayList<Plant>();

//adding a flower to the plantList
System.out.println("\nEnter the name of the flower to add: ");
            name = add.nextLine();

            System.out.println("\nEnter the ID code: ");
            id = add.nextLine(); 

            System.out.println("\nEnter the color: ");
            color = add.nextLine();

            System.out.println("\nAre there thorns present? (True/False) ");
            blnThorns = add.nextBoolean();

            System.out.println("\nDoes the flower smell? (True/False) ");
            blnSmell = add.nextBoolean();

            plantList.add(new Flower(name, id, color, blnThorns, blnSmell));

            System.out.println("Flower inserted.");
            System.out.println();
            break;

//displaying all plants
for( int i = 0; i < plantList.size(); i++){
      System. out.println("\t" + (i+1) + ":");
            System.out.print("\n\tName: " + plantList.get(i).getName());
            System.out.print("\n\tName: " + plantList.get(i).getId());
            System.out.print("\n\tColor: " + plantList.get(i).getColor());
            if(plantList instanceof Flower){ // HERE I am not sure what I'm doing or how to do it
            System.out.print("\n\tThorns presence: " + plantList.get(i).isThorns()); /* this is an example of what is not working properly */
            System.out.print("\n\tSmell presence: " + plantList.get(i).isSmell());  /* this is an example of what is not working properly*/
            System.out.println("\n");
            }
         }

Answer 1

If by "display" you mean "print some sort of string to the console or other output", then the answer is fairly simple: there's no need to use instanceof at all. All you need to do is override the toString method in each different class that you want to be displayable, then when you want to display an object (even if you don't know exactly what type it is), just call toString on it and print the result. Polymorphism will do the job of picking which toString method implementation to call.

Here's how it would look in your specific example.

In the Plant class:

@Override
public String toString() {
   return "\n\tName: " + getName()
        + "\n\tName: " + getId()
        + "\n\tColor: " + getColor();
}

Then, in the Flower class:

@Override
public String toString() {
   return super.toString()
        + "\n\tThorns presence: " + isThorns()
        + "\n\tSmell presence: " + isSmell();
}

Finally, to display all plants:

for (Plant plant : plantList) {
   System.out.println(plant);
}

Note that toString is called automatically when you pass any Object to System.out.println .

Answer 2

You were really close. You just needed to check against the element of the list, not the list itself, when you did the instanceof check. Then, if it is in fact an instance of Flower , then you need to cast the list element to a Flower and make the method calls from there.

Like this:

for(int i = 0; i < plantList.size(); i++){
    System.out.println("\t" + (i+1) + ":");
    System.out.print("\n\tName: " + plantList.get(i).getName());
    System.out.print("\n\tName: " + plantList.get(i).getId());
    System.out.print("\n\tColor: " + plantList.get(i).getColor());
    if (plantList.get(i) instanceof Flower) {
        Flower flower = (Flower)plantList.get(i);
        System.out.print("\n\tThorns presence: " + flower.isThorns());
        System.out.print("\n\tSmell presence: " + flower.isSmell());
        System.out.println("\n");
    }
}