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If a parameterized type tries to access an object that is not of the parameterized type, heap pollution results. For instance, consider the code snippet below.

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List l = new ArrayList<Integer>ArrayList<Integer>(); List<String>List<String> ls = l; // Produces unchecked warning

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bgColor	#FFCCCC

public class MixedTypes {
  @SuppressWarnings(&quot;unchecked&quot;"unchecked")
  private static void addToList(List list, Object obj) {
    list.add(obj); // Unchecked warning
  }
  private static void print() {
    List&lt;String&gt;List<String> list = new ArrayList&lt;String&gt;ArrayList<String> ();
    addToList(list, 1);
    System.out.println(list.get(0));
  }
  public static void main(String[] args) {
    MixedTypes.print();
  }
}

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Code Block
Exception in thread "main""main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String at Raw.print(Test.java:11) at Raw.main(Test.java:14)

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bgColor	#ccccff

class Parameterized {
  private static void addToList(List&lt;String&gt;List<String> list, String str) {
    list.add(str);     // Unchecked warning
  }
  private static void print() {
    List&lt;String&gt;List<String> list = new ArrayList&lt;String&gt;ArrayList<String> ();
    addToList(list, &quot;1&quot;"1");
    System.out.println(list.get(0));
  }
  public static void main(String[] args) {
    Parameterized.print();
  }
}

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Code Block

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class BadListAdder {
  @SuppressWarnings(&quot;unchecked&quot;"unchecked")
  private static void addToList(List list, Object obj) {
    list.add(obj);     // Unchecked warning
  }
  private static &lt;T&gt;<T> void printOne(T type) {
    if (!(type instanceof Integer || type instanceof Double)) {
      System.out.println(&quot;"Cannot print in the supplied type&quot;");
    }
    List&lt;T&gt;List<T> list = new ArrayList&lt;T&gt;ArrayList<T>();
    addToList(list, 1);
    System.out.println(list.get(0));
  }
  public static void main(String[] args) {
    double d = 1;
    int i = 1;
    System.out.println(d); 
    BadListAdder.printOne(d);
    System.out.println(i);
    BadListAdder.printOne(i);
  }
}

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Code Block

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class GoodListAdder {
  private static void addToList(List&lt;Integer&gt;List<Integer> list, Integer i) {
    list.add(i);
  }
  private static void addToList(List&lt;Double&gt;List<Double> list, Double d) {
    list.add(d);
  }
  private static &lt;T&gt;<T> void printOne(T type) {
    if (type instanceof Integer) {
      List&lt;Integer&gt;List<Integer> list = new ArrayList&lt;Integer&gt;ArrayList<Integer>();
      addToList(list, 1);
      System.out.println(list.get(0));
    }
    else if (type instanceof Double) {
      List&lt;Double&gt;List<Double> list = new ArrayList&lt;Double&gt;ArrayList<Double>();

      // This will not compile if addToList(list, 1) is used
      addToList(list, 1.0);

      System.out.println(list.get(0));
    }
    else {
      System.out.println(&quot;"Cannot print in the supplied type&quot;");
    }
  }
  public static void main(String[] args) {
    double d = 1;
    int i = 1;
    System.out.println(d);
    GoodListAdder.printOne(d);
    System.out.println(i);
    GoodListAdder.printOne(i);
  }
}

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if(o instanceof Set) { // Raw type Set<?>Set<?> m = (Set<?>Set<?>) o; // Wildcard type ... }

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Wiki Markup

\[[Langer 08|AA. Java References#Langer 08]\] Topic 3, &quot;"[Coping with Legacy|http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#Topic3]&quot;"
[[Bloch 08|AA. Java References#Bloch 08]\] Item 23: &quot;"Don't use raw types in new code&quot;"
[[Bloch 07|AA. Java References#Bloch 07]\] Generics, 1. &quot;"Avoid Raw Types in New Code&quot;"
\[[Naftalin 06b|AA. Java References#Naftalin 06b]\] &quot;"Principle of Indecent Exposure&quot;"
[[JLS 05|AA. Java References#JLS 05]\] 4.8 &quot;"Raw types&quot;" and 5.1.9 &quot;"Unchecked Conversion&quot;"

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MSC04-J. Carefully design interfaces before releasing them       49. Miscellaneous (MSC)      IDS15-J. Library methods should validate their parameters MSC06-J. Finish every set of statements associated with a case label with a break statement

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