Question

I am using jdk 1.6
My main method :

exmp3<String , Integer> exmp3 = new exmp3( "ravi" , new ArrayList<String>());
                exmp3.put(5);
                exmp3.put(15);
                exmp3.put(20);
                ArrayList<Integer> a =  exmp3.getObject();
                System.out.println(a.size());

class exmp3 :

public class exmp3<ABC , XYZ>
{
    private ABC abc ;
    private ArrayList<XYZ> xyz;

        public exmp3(ABC abc , ArrayList<XYZ> xyz)
        {
            this.xyz =  xyz;
        }

        public void put(int i)
        {
            ArrayList<Integer> a =  (ArrayList<Integer>) this.xyz;
            a.add(i);
        }

        public ArrayList<XYZ> getObject()
        {
            return xyz;
        }
    }

My query is while creating instance of exmp3 in main method why String is accepted as parametric type of ArrayList where as i have mentioned exmp3<String , Integer> exmp3 in main method.

Answer 1

Because you are using this :

new exmp3( "ravi" , new ArrayList<String>());

which is almost the same as

new exmp3<Object,Object>( "ravi" , new ArrayList<String>());

Arguments in methods or constructors never defines the types when instancing. the compiler only sees (if you don’t think about erasure) :

new exmp3( String , ArrayList);

while your instance is declared to use this definition :

public class exmp3<Object, Object>
{
    private Object abc ;
    private ArrayList<Object> xyz;

        public exmp3(Object abc , ArrayList<Object> xyz)
        {
            this.xyz =  xyz;
        }

        public void put(int i)
        {
            ArrayList<Integer> a =  (ArrayList<Integer>) this.xyz;
            a.add(i);
        }

        public ArrayList<Object> getObject()
        {
            return xyz;
        }
    }