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Code Block

bgColor	#FFCCCC

class BadListAdderListAdder {
  @SuppressWarnings("unchecked")
  private static void addToList(List list, Object obj) {
    list.add(obj);     // Unchecked warning
  }

  private static <T> void printOne(T type) {
    if (!(type instanceof Integer || type instanceof Double)) {
      System.out.println("Cannot print in the supplied type");
    }
    List<T> list = new 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);
    BadListAdderListAdder.printOne(d);
    System.out.println(i);
    BadListAdderListAdder.printOne(i);
  }
}

However, despite list being correctly parameterized, this method prints '1' and never '1.0' because the int value '1' is always added to list without being type checked. This code produces the following output:

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Code Block

bgColor	#ccccff

class GoodListAdderListAdder {
  private static <T> void addToList(List<T> list, T t) {
    list.add(t);     // No warning generated
  }

  private static <T> void printOne(T type) {
    if (type instanceof Integer) {
      List<Integer> list = new ArrayList<Integer>();
      addToList(list, 1);
      System.out.println(list.get(0));
    }
    else if (type instanceof Double) {
      List<Double> list = new 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("Cannot print in the supplied type");
    }
  }

  public static void main(String[] args) {
    double d = 1;
    int i = 1;
    System.out.println(d);
    GoodListAdderListAdder.printOne(d);
    System.out.println(i);
    GoodListAdderListAdder.printOne(i);
  }
}

This code compiles cleanly and produces the correct output:

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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="fa6461553f58d872-ba5ad7c7-44ad49d0-ba6a8966-56a59948b6f23125fe45e681"><ac:plain-text-body><![CDATA[	[[Bloch 2008	AA. Bibliography#Bloch 08]]	Item 23: "Don't use raw types in new code"	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="f76107c5b7847327-a2a5c0e4-4b954840-a51ba11e-8cfe6b0b49a149a561eaa538"><ac:plain-text-body><![CDATA[	[[Bloch 2007	AA. Bibliography#Bloch 07]]	Generics, 1. "Avoid Raw Types in New Code"	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="3f827cbc35c4cdb8-2f6ecc69-427e47aa-89ef99e5-770e4d6bd74b40d8815b294a"><ac:plain-text-body><![CDATA[	[[Bloch 2005	AA. Bibliography#Bloch 05]]	Puzzle 88: Raw Deal	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="36361e2c3d05224e-f1736bb1-41a74429-950b930a-062316886db6e0f4e35343f7"><ac:plain-text-body><![CDATA[	[[Darwin 2004	AA. Bibliography#Darwin 04]]	8.3 Avoid Casting by Using Generics	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="10ee5884c02abbba-65256c2e-4b374951-90ff9472-df3e3fc5c6b898c83496ea26"><ac:plain-text-body><![CDATA[	[[JavaGenerics 2004	AA. Bibliography#JavaGenerics 04]]		]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="bbfd7d1ad99f670c-d685c8ba-42744d1b-9bae8e3b-27a1ed289f97d6189a04095d"><ac:plain-text-body><![CDATA[	[[JLS 2005	AA. Bibliography#JLS 05]]	[Chapter 5 "Conversions and Promotions"	http://java.sun.com/docs/books/jls/third_edition/html/conversions.html]	]]></ac:plain-text-body></ac:structured-macro>
	§4.8 "Raw Types"
	§5.1.9 "Unchecked Conversion"
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="69bef24786d39258-a8ef8752-44a34288-87978366-d51720962b1e7889c4ed96c3"><ac:plain-text-body><![CDATA[	[[Langer 2008	AA. Bibliography#Langer 08]]	Topic 3, "[Coping with Legacy	http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#Topic3]"	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="29a5ca9782da9de3-9e561bfc-4aad4ca0-a1568e8d-a919cbdedb557d6c7d17ea1f"><ac:plain-text-body><![CDATA[	[[Naftalin 2006	AA. Bibliography#Naftalin 06]]	Chapter 8, "Effective Generics"	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="c25581f528a3adae-ab857391-40854fe1-a1e5b6ec-e293230eee8a7d3488de1090"><ac:plain-text-body><![CDATA[	[[Naftalin 2006b	AA. Bibliography#Naftalin 06b]]	"Principle of Indecent Exposure"	]]></ac:plain-text-body></ac:structured-macro>
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="0965d3f3ba2996b5-d06f71ba-41014c95-a70fa11e-062c2a8ad0db9a6d28d4460a"><ac:plain-text-body><![CDATA[	[[Schildt 2007	AA. Bibliography#Schildt 07]]	"Create a checked collection"	]]></ac:plain-text-body></ac:structured-macro>

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