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The Java Interview Handbook: 300+ Interview Questions

Erasure

Explore Java generics erasure to understand why List<Integer> cannot be assigned to List<Number>. Learn how this design prevents type safety issues such as ClassCastException when adding incompatible objects to collections.

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1.

What is type erasure?

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Java
import java.util.*;
class Demonstration {
    public static void main( String args[] ) {
      // Raw type
      ArrayList al = new ArrayList();
      // ArrayList of Strings
      ArrayList<String> strs = new ArrayList<>();
      // ArrayList of ArrayList of Strings
      ArrayList<ArrayList<String>> lstOfStringLsts = new ArrayList<>();
      System.out.println("al.getClass().equals(strs.getClass()) = " + al.getClass().equals(strs.getClass()));
      System.out.println("al.getClass().equals(lstOfStringLsts.getClass()) = " + al.getClass().equals(lstOfStringLsts.getClass()));
    }
}
1.

Can you give examples of type erasure?

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1 / 4
Java
import java.util.*;
class Demonstration {
    public static void main( String args[] ) {
    }
    <T extends List<Integer>> void print(T lst) {
        for (Integer i : lst)
            System.out.println(i);
    }
    <T extends List<String>> void print(T lst) {
        for (String str : lst)
            System.out.println(str);
    }
    <T extends List> void print(T lst) {
        for (Object str : lst)
            System.out.println(str);
    }    
}
1.

What is the difference between C++ template library and Java generics?

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1 / 2
Java
import java.util.*;
class Demonstration {
    public static void main( String args[] ) {
        List<Integer> list = new ArrayList<>();
        someMethod(list);
    }
    /*
     * Uncomment the following method and comment out the
     * other method to see the compile error generated.
     *
     */
    /*static void someMethod(List<Number> nums){
      // ... method body
    }*/
    static void someMethod(List<? extends Number> nums){
      // ... method body
      System.out.println("Comment out this method and uncomment the already commented method to see the compiler error.");
    }
}

Consider the following snippet to understand why List<Integer> is ...