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If your answer was A. Compiler Error , you were correct.

What caused the compiler error?

The compiler error was caused by the code shown in Listing 2 .

Listing 2 . The code with the problem.
public static void fillIt(Collection ref){ ref.add(0,new MyClass(4));

The problem here is that the method named fillIt receives a reference to an object of the ArrayList class as the interface type Collection , and attempts to call the following overloaded method on that reference:

add(int index, Object element)

However, the Collection interface knows nothing about a method having that signature.

Implements Collection and List

The ArrayList class implements both the Collection interface and the List interface. As you may recall from earlier modules in this series, List is a sub-interface of Collection . The List interface declares the following overloaded versions of the add method:

  • add(Object o)
  • add(int index, Object element)

The second of these two methods, which is called in Listing 2 , is unknown to the Collection interface. The Collection interface declares only the first version of the add method shown above.


This is the result of specialization. A List object is a more-specialized collection than a Collection object.

Therefore, the version of the add method that requires two parameter cannot be called on a reference to an ArrayList object when that object is treated as the generic type Collection .

Modified program

Now, take a look at the modified version of the program as shown in Listing 3 .

What output is produced by the program shown in Listing 3 ?

  • A. Compiler Error
  • B. Runtime Error
  • C. 44321
  • D. 4321
  • E. 1234
  • F. 12344
  • G. 443521
  • H. None of the above.
Listing 3 . The program named Comparable02.
//File Comparable02.java import java.util.*;public class Comparable02{ public static void main(String args[]){ new Worker().doIt();}//end main() }//end class Comparable02class Worker{ public void doIt(){Iterator iter; Collection ref;ref = new ArrayList(); Populator.fillIt(ref);iter = ref.iterator(); while(iter.hasNext()){System.out.print(iter.next()); }//end while loopSystem.out.println(); }//end doIt()}// end class Workerclass Populator{ public static void fillIt(Collection ref){((List)ref).add(0,new MyClass(4)); ((List)ref).add(1,new MyClass(4));((List)ref).add(2,new MyClass(3)); ((List)ref).add(3,new MyClass(2));((List)ref).add(4,new MyClass(1)); ((List)ref).add(3,new MyClass(5));}//end fillIt() }//end class populatorclass MyClass{ int data;MyClass(){ data = 0;}//end noarg constructor MyClass(int data){this.data = data; }//end parameterized constructorpublic String toString(){ return "" + data;}//end overridden toString() }//end MyClass

If your answer was G. 443521 , you are correct.

The corrected code

This version of the program illustrates a mechanism for correcting the problem in the earlier program shown in Listing 1 . The updated code that corrected the problem is shown in Listing 4 .

Listing 4 . The corrected code.
class Populator{ public static void fillIt(Collection ref){((List)ref).add(0,new MyClass(4)); ((List)ref).add(1,new MyClass(4));((List)ref).add(2,new MyClass(3)); ((List)ref).add(3,new MyClass(2));((List)ref).add(4,new MyClass(1)); ((List)ref).add(3,new MyClass(5));}//end fillIt() }//end class populator

Questions & Answers

explain and give four Example hyperbolic function
Lukman Reply
The denominator of a certain fraction is 9 more than the numerator. If 6 is added to both terms of the fraction, the value of the fraction becomes 2/3. Find the original fraction. 2. The sum of the least and greatest of 3 consecutive integers is 60. What are the valu
1. x + 6 2 -------------- = _ x + 9 + 6 3 x + 6 3 ----------- x -- (cross multiply) x + 15 2 3(x + 6) = 2(x + 15) 3x + 18 = 2x + 30 (-2x from both) x + 18 = 30 (-18 from both) x = 12 Test: 12 + 6 18 2 -------------- = --- = --- 12 + 9 + 6 27 3
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on number 2 question How did you got 2x +2
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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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