Extracting hierarchy of objects from Vector

[Nigel Shrin]

I am elaborating on a Whizlabs example. Superclass shape, subclasses circle square & rhombus. One instance of each subclass, each has a string name attribute.

I wish to output in a loop two columns ie

[b]Classtype        Name[/b]
Circle              curved
Square            straight
Rhombus         leaning

How can I access the name of each vector element in a loop, do I need to cast them into object?

import java.util.Iterator;
import java.util.Vector;

public class Q2_Vector_shapes {
	
	public static void main(String[] args)
	{
		Vector picture = new Vector<Q2_Vector_shapes>(); 
		
		Circle myCirc = new Circle("curved");
		picture.add(myCirc);
		Rectangle myRect = new Rectangle("straight");
		picture.add(myRect); 
		Rhombus myRhom = new Rhombus("leaning");
		picture.add(myRhom); 
		
		Rectangle rect = (Rectangle)picture.get(1); 
		System.out.println(rect.name);
		
		Iterator iter = picture.iterator();
		System.out.println("\nWhile loop:\n");
		while(iter.hasNext()){
			System.out.println(iter.next());
                         // Cannot find a method to access 'name' ???
		}
		
		System.out.println("\nFor loop:\n");
		for(int i=0; i<picture.size(); i++){
			System.out.println(picture.get(i).getClass());
			System.out.println("get(i): " + picture.get(i));
//			System.out.println("get(i): " + picture.get(i).name; // Cannot find a method to access 'name' ???
		}
	}
}

// constructors for subclasses, to set names
class Circle extends Q2_Vector_shapes{
	String name="default circle";
	Circle(){}; Circle(String msg){this.name = msg;}
}

class Rectangle extends Q2_Vector_shapes{
	String name="default rectangle";
	Rectangle(){};	Rectangle(String msg){this.name = msg;}
}

class Rhombus extends Q2_Vector_shapes{
	String name="default rhombus";
	Rhombus(){};	Rhombus(String msg){this.name = msg;}
}

Thank you!

[Prithvi Sehgal]

Hello Nigel,

Please try to change the for loop this way

 

   for(int i=0; i<picture.size(); i++){  
       //     System.out.println(picture.get(i).getClass());
            Object obj = picture.get(i);
            
            if( obj instanceof Circle){
            	Circle c = (Circle) obj ;
            	System.out.println("Circle: " + c.name);
            }
            
            if( obj instanceof Rectangle){
            	Rectangle rect1 = (Rectangle) obj ;
            	System.out.println("Rectangle: " + rect1.name);
            }
            
            if( obj instanceof Rhombus){
            	Rhombus romb = (Rhombus) obj ;
            	System.out.println("Rhombus: " + romb.name);
            }
  }

You can do certain other modifications as well, that override the toString() method in each class and simply print out the object like
this

   
System.out.println("Circle: " + cirlce);

Hope this helps,

[Prithvi Sehgal]

Hi,

I have slightly modified the code more, and have a look at it.

  

package chapter2;

import java.util.Iterator;  
import java.util.Vector;  
  
public class Q2_Vector_shapes {  
      
    public static void main(String[] args)  
    {  
        Vector picture = new Vector<Q2_Vector_shapes>();   
          
        Circle myCirc = new Circle("curved");  
        picture.add(myCirc);  
        Rectangle myRect = new Rectangle("straight");  
        picture.add(myRect);   
        Rhombus myRhom = new Rhombus("leaning");  
        picture.add(myRhom);   
          
        Rectangle rect = (Rectangle)picture.get(1);   
        
          
       
          
        System.out.println("[b]ClassType" + "              " +"[b]Name");  
        for(int i=0; i<picture.size(); i++){  
      
            Object obj = picture.get(i);
            
            if( obj instanceof Circle){
            	Circle c = (Circle) obj ;
            	
            	System.out.println("Circle" +  "              " + c.getName());
            }
            
            else if( obj instanceof Rectangle){
            	Rectangle rect1 = (Rectangle) obj ;
            	System.out.println("Rectangle" +  "              " + rect1.getName());
            }
            
            else if( obj instanceof Rhombus){
            	Rhombus romb = (Rhombus) obj ;
            	System.out.println("Rhombus" +  "              " + romb.getName());
            }
            
            else{
            	System.out.println("Q2_Vector_Shapes" +  "              " + obj);
            }
            
        }
        
        
        
    } 
    
    public String toString(){
    	return "Parent";
    }
}  
  
// constructors for subclasses, to set names  
class Circle extends Q2_Vector_shapes{  
    String name="default circle";  
    public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	Circle(){}; Circle(String msg){this.name = msg;}  
}  
  
class Rectangle extends Q2_Vector_shapes{  
    String name="default rectangle";  
    Rectangle(){};  Rectangle(String msg){this.name = msg;}  
    public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}  
  
class Rhombus extends Q2_Vector_shapes{  
    String name="default rhombus";  
    Rhombus(){};    Rhombus(String msg){this.name = msg;}
    public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
} 

Hope this helps,

[Karthik Shiraly]

Hi,

I suggest an improvement: Push up the definitions of name, getName() and setName() into the superclass Q2_Vector_Shapes, instead of duplicating them in each subclass.

Benefits:
> Code reuse

> Avoids code duplication

> Avoids type based checks. Instead of code like

if (obj instanceof Rectangle) {
  ...
} else if (obj instanceof Rhombus) {
  ...
}

you can utilize polymorphism like this:

 for(int i=0; i<picture.size(); i++){  
        Q2_Vector_shapes shape = picture.get(i);
        System.out.println(shape.getClass().getSimpleName() + "\t" + shape.getName());  
}

The advantage is that if you introduce a new shape subclass like Triangle in future, you won't have to change this for-loop.

Cheers
Karthik

[Prithvi Sehgal]

Hi,

I thought the same, but he doesn't have a name definition inside the shapes class. I thought he want the
modification, but sticking to same line of action.

Cheers,

[Nigel Shrin]

Hi Prithvi,

Very helpful, that works really well. It seems one has to write a lot of code, to copy it back into a new object.

toString makes such a difference:
I haven't overridden toString before, even if reduced to:

public toString(){}

it works, without it, it does not.

Are you able to explain how the toString works, I guess it is overriding the version in the Vector class?

Thank you

[Prithvi Sehgal]

Hi,

toString() is a method defined in the Object class. Basically, every class inherits a toString() method.
Basically toString() method is used, if you want to know something meaningful about the object you are using.
If you don't override a version present in the the Object class
and do something like this

   
System.out.println("My object is" + object ) ;
 

It will basically print the class name and some hexadecimal value with it. So this kind of object representation is not
really meaningful. Now if you override the toString method, something like this

    

public class Car{

private String name ;
private String color ;
private String vehicleNumber ;

// Getters and Setters of the above properties

//overriding the toString() method

public String toString()
{
  return name + vehicleNumber ; 
}

public static void main(String[] args){
Car car  = new Car();
System.out.println("My car is : " + car );

}

}

Now in the above mentioned statement, it will print out the vechicle name + vehicle number.
Basically it is more meaningful information then the previous snippet of code. Very helpful
in debugging and spitting some state about your object.

Hope this helps,

[Prithvi Sehgal]

Nigel Shrin wrote:Hi Prithvi,

I guess it is overriding the version in the Vector class?

Thank you

No nigel, it is overriding the version from the Object class. Think, you are not extending from Vector class.



HTH,

[Nigel Shrin]

Thank you for your great replies Prithvi and Karthik, I now have it working very neatly using your suggestions.

Superclass:

public class Q2c_SuperVectorShapes {
	private String name = "default name";
	public void setName(String name) {
		this.name = name;
	}
	public String getName() {
		return name;
	}
}

Subclass:

public class Q2c_SubVectorShapes extends Q2c_SuperVectorShapes {

	public static void main(String[] args)
	{
		Vector picture = new Vector<Q2_Vector_shapes>();
		
		Circle3 myCirc = new Circle3(); 
		myCirc.setName("curved3"); 
		
		Rectangle3 myRect = new Rectangle3(); 
		myRect.setName("straight3"); 

		Rhombus3 myRhom = new Rhombus3(); 
	    myRhom.setName("leaning3");    

	    picture.add(myCirc);
	    picture.add(myRect);
	    picture.add(myRhom);
	    
	    System.out.format("%1$-15s %2$-15s\n\n","Class Type","Name");
	    for(int i=0; i<picture.size(); i++){    
	    	Q2c_SubVectorShapes shape = (Q2c_SubVectorShapes)picture.get(i);  
//	        System.out.format("%1$-15s %2$-15s\n",shape.getClass().getSimpleName(), shape.getName()); // OR with toString
	        System.out.format("%1$-15s %2$-15s\n",shape.toString(), shape.getName()); 
	    }
	} 
	    
	public String toString(){
		return getClass().getSimpleName();
	}
	
}  
	  
	class Circle3 extends Q2c_SubVectorShapes{}  
	class Rectangle3 extends Q2c_SubVectorShapes{}
	class Rhombus3 extends Q2c_SubVectorShapes{} 

Thanks again

[Prithvi Sehgal]

Good work Nigel. Exactly right approach and demonstrate polymorphism as well.

Good luck,