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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.io.*;
  6.  
  7. /* Name of the class has to be "Main" only if the class is public. */
  8. class Box<T> {
  9.     private T element;
  10.  
  11.     public T getElement() {
  12.         return element;
  13.     }
  14.  
  15.     public void setElement(T element) {
  16.         this.element = element;
  17.     }
  18. }
  19.  
  20. class Ideone
  21. {
  22. 	public static void main (String[] args) throws java.lang.Exception
  23. 	{
  24.         List<Box> l = new ArrayList<>(); //Just List of Box with no specific type
  25.         Box<String> box1 = new Box<>();
  26.         box1.setElement("aa");
  27.         Box<Integer> box2 = new Box<>();
  28.         box2.setElement(10);
  29.  
  30.         l.add(box1);
  31.         l.add(box2);
  32.  
  33.         //Case 1
  34.         Box<Integer> b1 = l.get(0);
  35.         System.out.println(b1.getElement()); //why no error
  36.  
  37.         //Case 2
  38.         Box<String> b2 = l.get(1);
  39.         System.out.println(b2.getElement()); //throws ClassCastException
  40.         }
  41. }
Runtime error #stdin #stdout #stderr 0.11s 320576KB
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stdin
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Standard input is empty
stdout
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aa
stderr
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Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
	at Ideone.main(Main.java:39)
https://ideone.com/IcoVrZ
language:
Java (HotSpot 12)
created:
8 years ago
visibility:
public

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