Generics query

[Sahil Kapoor]

In K&B pg 603 , it is explained that when we try to use legacy code (method) which modifies the type-safe list, the complier
shows warnings and runs well without exception. But i noticed that on my compiler( Dr JAVA) i am getting ClassCastException .
Could anyone please tell ??
Here goes the code :-

import java.util.*;

class TestLegacy {
  public static void main(String... args) {
    
    List<String> myList = new ArrayList<String>();
    
    myList.add("Sahil");
    myList.add("Varun");
    
    Adder a = new Adder();
    a.add("Kapoor",myList);
    
    for( Object temp : myList)                   // Line 1
      System.out.println(temp + "   ");        // Line 2
  }
}

class Adder {
 
  public void add(String s, List l) {
    
    Iterator it = l.iterator();
    
    int i=0;
    while(it.hasNext() ){
      String s1 = (String)it.next();
      s1=s1+" "+s;
      l.set(i,s1);
      i++;
    }
    
    l.add(new Integer(42));
    
          
  }
}    

[Henry Wong]

I think you misinterpreted the statement. I believe the book is trying to say that all you will get is a compiler warning -- ie. you will not get any runtime errors about the (due to the) mixing generics and non-generics.

Other runtime errors caused by problems from mixing generics and non-generics is a different story.

Henry

[Sahil Kapoor]

In accordance to K&B verbatim

pg 602,

 void insert(List list) {
list .add(new String("42"));
}   

.

Will that work? Yes,sadly, it does! It both compiles and runs. No runtime Exception. Yet, someone just stuffed a String into a supposedly type safe
Array List of type <Integer> How can that be ???

Actually i removed Line 1 and Line2 to and it worked fine , ie Runs without Exceptions. So i thought that since i am using it thats why there was an exception
But, then again another brain neuron hits my mind that, i am using Object ref in for loop then there should not be any issue like that.....

Please run the above code in your compiler ....May be some compiler issue ???

Thanks !!!

[Jim Hoglund]

Try the code below and you will see the incompatible type errors.
The List parameter of add2() is local, and previously it was not
declared as type <String>.

class TestLegacy {
   public static void main(String[] args) {
      List<String> myList = new ArrayList<String>();
      myList.add("Sahil");
      myList.add("Varun");
      add2("Kapoor", myList);
      for (Object temp : myList) {
         System.out.println(temp); 
      }
   }

   static void add2(String str, List<String> list) {
      Iterator iter = list.iterator();
      for (int idx = 0; iter.hasNext(); idx++) {
         String s1 = (String) iter.next();
         s1 += (" " + str);
         list.set(idx, s1);
      }
      list.add(new Integer(42));
      list.add(new Object());
   }
}

Jim ... ...

[Unmesh Chowdhury]

I think it is not due to legacy code; the problem is elsewhere. Here, JVM generates the following exception:

Sahil Kapoor
Varun Kapoor
Exception in thread “main” java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
At TestLegacy.main(TestLegacy.java:14)

  
	for( Object temp : myList)                   // Line 1  
           System.out.println(temp + "   ");  // Line 2
  

From the above for-each loop, we can apparently think that the element of the myList, what is either String or Integer should assign into Object reference normally. But, I feel there is an intermediate step that try to cast the element of the myList into the actual parameter type with which the myList is declared and instantiated. And then the reference of the element is assigned into the Object reference.

Another part of the story is that if we use Iterator<E> instead of for-each loop then we will get the expected result. For instance,

    
	Iterator<String> i = myList.iterator();
	 while(i.hasNext()) {
	 	Object o = i.next();
		System.out.println(o + "   ");
	 }  

        The output:

		Sahil Kapoor
		Varun Kapoor
		42
        

Vary interesting!

[Abimaran Kugathasan]

I think, there is a internal mechanism in the foreach loop, when we are using it with the Generics. Because, the code below works ...

import java.util.*;

class TestLegacy {
  public static void main(String... args) {

    List<String> myList = new ArrayList<String>();

    myList.add("Sahil");
    myList.add("Varun");

    Adder a = new Adder();
    a.add("Kapoor",myList);
    System.out.println(myList);

    //for( Object temp : myList)                   // Line 1
      //System.out.println(temp + "   ");        // Line 2
  }
}

class Adder {

  public void add(String s, List l) {

    Iterator it = l.iterator();

    int i=0;
    while(it.hasNext() ){
      String s1 = (String)it.next();
      s1=s1+" "+s;
      l.set(i,s1);
      i++;
    }

    l.add(new Integer(42));

        for( Object temp : l)                   // Line 1
      System.out.println(temp + "   ");        // Line 2


  }
}

[Unmesh Chowdhury]

In your case, for-each loop is used for raw type, that is, the list is not generic. So, in this case the compiler doesn't treat the list specially. The context of the previous for-each loop and the context of your for-each loop is not same.