Java Generics

A Generic Quiz

Question #1

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List<String> list = new ArrayList<String>();

Java Generics

A Generic Quiz

Question #1

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List<String> list = new ArrayList<String>();

Java Generics

A Generic Quiz

Question #2

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List<String> list = new ArrayList<>();

Java Generics

A Generic Quiz

Question #2

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List<String> list = new ArrayList<>();

Java Generics

A Generic Quiz

Question #3

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List<Object> list = new ArrayList<String>();

Java Generics

A Generic Quiz

Question #3

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List<Object> list = new ArrayList<String>();

• Lists are not polymorphic (Object vs String)

Java Generics

A Generic Quiz

Question #4

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ArrayList<Object> list = new ArrayList<String>();

Java Generics

A Generic Quiz

Question #4

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ArrayList<Object> list = new ArrayList<String>();

• Lists are not polymorphic (Object vs String)
• Even if the list are of the same type

Java Generics

A Generic Quiz

Question #5

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ArrayList<String> list = new ArrayList<Object>();

Java Generics

A Generic Quiz

Question #5

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ArrayList<String> list = new ArrayList<Object>();

• Lists are not polymorphic (String vs Object)
• Even if the list are of the same type

Java Generics

A Generic Quiz

Question #6

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ArrayList list = new ArrayList<String>();

Java Generics

A Generic Quiz

Question #6

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ArrayList list = new ArrayList<String>();

• A typed list is assigned to an untyped list

Java Generics

A Generic Quiz

Question #7

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ArrayList<String> list = new ArrayList();

Java Generics

A Generic Quiz

Question #7

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ArrayList<String> list = new ArrayList();

• Valid, but it will generate a compiler warning

Java Generics

A Generic Quiz

Question #8

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ArrayList<String> stringList = null;
ArrayList<? extends String> extdStringList = stringList;

Java Generics

A Generic Quiz

Question #8

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ArrayList<String> stringList = null;
ArrayList<? extends String> extdStringList = stringList;

• Assigning String to something that extends String

Java Generics

A Generic Quiz

Question #9

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ArrayList<?> list = new ArrayList<Number>();

Java Generics

A Generic Quiz

Question #9

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ArrayList<?> list = new ArrayList<Number>();

• Assigning a typed list to a list of anything

Java Generics

A Generic Quiz

Question #10

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ArrayList<? extends Number> extendsList = null;
ArrayList<? super Number> superList = extendsList;

Java Generics

A Generic Quiz

Question #10

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ArrayList<? extends Number> extendsList = null;
ArrayList<? super Number> superList = extendsList;

Java Generics

A Generic Quiz

Question #11

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ArrayList<? super Number> superNumList = null;
ArrayList<? super Integer> superIntList = superNumList;

Java Generics

A Generic Quiz

Question #11

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ArrayList<? super Number> superNumList = null;
ArrayList<? super Integer> superIntList = superNumList;

• From a read perspective, the list of super-Integer contains Integers or it's super class, Number

Java Generics

A Generic Quiz

Question #12

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ArrayList<Object> list = new ArrayList<Object>();
list.add(“Hello”);

Java Generics

A Generic Quiz

Question #12

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ArrayList<Object> list = new ArrayList<Object>();
list.add(“Hello”);

• List of Object - we can add anything to that

Java Generics

A Generic Quiz

Question #13

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ArrayList<?> list = new ArrayList<String>();
list.add(“Hello”);

Java Generics

A Generic Quiz

Question #13

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ArrayList<?> list = new ArrayList<String>();
list.add(“Hello”);

• We do not know what the list contains therefore we cannot add anything to it
• Expecting capture<?>
• Effectively "read-only"

Java Generics

A Generic Quiz

Question #14

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ArrayList<? super Integer> list= new ArrayList<Integer>();
list.add(1);

Java Generics

A Generic Quiz

Question #14

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ArrayList<? super Integer> list= new ArrayList<Integer>();
list.add(1);

• Expects an Integer or a sub type

Java Generics

A Generic Quiz

Question #15

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ArrayList<? extends Number> list = new ArrayList<Integer>();
list.addAll(new ArrayList<Integer>());

Java Generics

A Generic Quiz

Question #15

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ArrayList<? extends Number> list = new ArrayList<Integer>();
list.addAll(new ArrayList<Integer>());

• addAll(Collection<? extends E> c) is expecting
  Collection<? extends capture<? extends Number>>)

Java Generics

A Generic Quiz

Question #16

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ArrayList<? super Integer> listA = new ArrayList<Number>();
ArrayList<? extends Integer> listB = new ArrayList<Integer>();
listB.addAll(listA);

Java Generics

A Generic Quiz

Question #16

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ArrayList<? super Integer> listA = new ArrayList<Number>();
ArrayList<? extends Integer> listB = new ArrayList<Integer>();
listB.addAll(listA);

• listB expects an Integer or sub class
• From a read perspective, all we know about listA
  is that it is an Object

Java Generics

A Generic Quiz

Question #17

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ArrayList<? super Integer> listA = new ArrayList<Number>();
ArrayList<? extends Integer> listB = new ArrayList<Integer>();
listA.addAll(listB);

Java Generics

A Generic Quiz

Question #17

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ArrayList<? super Integer> listA = new ArrayList<Number>();
ArrayList<? extends Integer> listB = new ArrayList<Integer>();
listA.addAll(listB);

• From a read perspective, listB is Integer
• Which is write compatible with listA

Java Generics

A Generic Quiz

Question #18

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ArrayList<Integer> intList = new ArrayList<Integer>();
intList.add(new Integer(1));
ArrayList<? extends Number> extendsNumList = intList;
Integer integer = extendsNumList.get(0);

Java Generics

A Generic Quiz

Question #18

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ArrayList<Integer> listA = new ArrayList<Integer>();

listA.add(new Integer(1));

ArrayList<? extends Number> listB = listA;
Integer integer = listB.get(0);

• From a read perspective, listB is a Number 

Java Generics

A Generic Quiz

Question #19

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ArrayList<String>[] twoDimArray = new ArrayList<String>[10];

Java Generics

A Generic Quiz

Question #19

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ArrayList<String>[] twoDimArray = new ArrayList<String>[10];

• Arrays of a generic types are not allowed
• Erasure of <String> at runtime

Java Generics

A Generic Quiz

Question #20

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Pair<String, String>[] array = new Pair[2];
array[0] = new Pair<String, String>(“1′′, “y”);
array[1] = new Pair(true, false);

Java Generics

A Generic Quiz

Question #20

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Pair<String, String>[] array = new Pair[2];
array[0] = new Pair<String, String>(“1′′, “y”);
array[1] = new Pair(true, false);

• The instantiation of new Pair[2] is not generic
• Pair<String,String>[]  is a compile time constraint
• But new Pair(true, false) is unchecked (warning)
• array cannot enforce anything beyond Pair

Java Generics

A Generic Quiz

Question #21

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Pair<?,?>[] array = new Pair<?,?>[2] ;
array[0] = new Pair<Integer,Integer>(0,0);
array[1] = new Pair<String,String>(“Hello”,”There”);

Java Generics

A Generic Quiz

Question #21

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Pair<?,?>[] array = new Pair<?,?>[2] ;
array[0] = new Pair<Integer,Integer>(0,0);
array[1] = new Pair<String,String>(“Hello”,”There”);

• Effectively the same as the previous example