Chapter 19 Generics
Section 19.2 Motivations and Benefits
1. Which of the following statements is correct?
a. Generics can help detect type errors at compile time, thus make programs more robust.
b. Generics can make programs easy to read.
c. Generics can avoid cumbersome castings.
d. Generics can make programs run faster.
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2. Fill in the code in Comparable c = new Date();
a. <String>
b. <?>
c. <Date>
d. <E>
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6. Suppose ArrayList<Double>list = new ArrayList<>(). Which of the following statements are correct?
a. list.add(5.5); // 5.5 is automatically converted to new Double(5.5)
b. list.add(3.0); // 3.0 is automatically converted to new Double(3.0)
c. Double doubleObject = list.get(0); // No casting is needed
d. double d = list.get(1); // Automatically converted to double
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4. Suppose List list = new ArrayList(). Which of the following operations are correct?
a. list.add(“Red”);
b. list.add(new Integer(100));
c. list.add(new java.util.Date());
d. list.add(new ArrayList());
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5. Suppose List<String> list = new ArrayList<String>. Which of the following operations are correct?
a. list.add(“Red”);
b. list.add(new Integer(100));
c. list.add(new java.util.Date());
d. list.add(new ArrayList());
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Section 19.3 Declaring Generic Classes and Interfaces
7. To declare a class named A with a generic type, use
a. public class A<E> { … }
b. public class A<E, F> { … }
c. public class A(E) { … }
d. public class A(E, F) { … }
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8. To declare a class named A with two generic types, use
a. public class A<E> { … }
b. public class A<E, F> { … }
c. public class A(E) { … }
d. public class A(E, F) { … }
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9. To declare an interface named A with a generic type, use
a. public interface A<E> { … }
b. public interface A<E, F> { … }
c. public interface A(E) { … }
d. public interface A(E, F) { … }
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10. To declare an interface named A with two generic types, use
a. public interface A<E> { … }
b. public interface A<E, F> { … }
c. public interface A(E) { … }
d. public interface A(E, F) { … }
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11. To create a list to store integers, use
a. ArrayList<Object> list = new ArrayList<>();
b. ArrayList<Integer> list = new ArrayList<>();
c. ArrayList<int> list = new ArrayList<int>();
d. ArrayList<Number> list = new ArrayList<>();
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Section 19.4 Generic Methods
12. The method header is left blank in the following code. Fill in the header.
public class GenericMethodDemo {
public static void main(String[] args ) {
Integer[] integers = {1, 2, 3, 4, 5};
String[] strings = {“London”, “Paris”, “New York“, “Austin”};
print(integers);
print(strings);
}
{
for (int i = 0; i < list.length; i++)
System.out.print(list[i] + ” “);
System.out.println();
}
}
a. public static void print(Integer[] list)
b. public static void print(String[] list)
c. public static void print(int[] list)
d. public static void print(Object[] list)
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13. To create a generic type bounded by Number, use
a. <E extends Number>
b. <E extends Object>
c. <E>
d. <E extends Integer>
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Section 19.5 Case Study: Sorting an Array of Objects
3. Which of the following statements is correct?
a. Comparable<String> c = new String(“abc”);
b. Comparable<String> c = “abc”;
c. Comparable<Integer> c = 23;
d. Comparable<Double> c = 3.4;
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28. Fill in the most appropriate code in the blanks in the MyInt class?
public class MyInt implements {
int id;
public MyInt(int id) {
this.id = id;
}
public String toString() {
return String.valueOf(id);
}
public int compareTo( arg0) {
if (id > arg0.id)
return 1;
else if (id < arg0.id)
return –1;
else
return 0;
}
}
a. Comparable / Object
b. Comparable<MyInt> / MyInt
c. Comparable<MyInt> / Object
d. Comparable / MyInt
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Section 19.6 Raw Type and Backward Compatibility
14. Which of the following declarations use raw type?
a. ArrayList<Object> list = new ArrayList<>();
b. ArrayList<String> list = new ArrayList<>();
c. ArrayList<Integer> list = new ArrayList<>();
d. ArrayList list = new ArrayList();
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15. If you use the javac command to compile a program that contains raw type, what would the compiler do?
a. report syntax error
b. report warning and generate a class file
c. report warning without generating a class file
d. no error and generate a class file
e. report warning and generate a class file if no other errors in the program.
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16. If you use the javac –Xlint:unchecked command to compile a program that contains raw type, what would the
compiler do?
a. report compile error
b. report warning and generate a class file
c. report warning without generating a class file
d. no error and generate a class file
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Section 19.7 Wildcards
17. Is ArrayList<Integer> a subclass of ArrayList<Object>?
a. Yes
b. No
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23. Which of the following can be used to replace YYYYYYYY in the following code?
public class WildCardDemo3 {
public static void main(String[] args) {
GenericStack<String> stack1 = new GenericStack<>();
GenericStack<Object> stack2 = new GenericStack<>();
stack2.push(“Java”);
stack2.push(2);
stack1.push(“Sun”);
add(stack1, stack2);
WildCardDemo2.print(stack2);
}
public static <T> void add(GenericStack<T> stack1,
GenericStack<YYYYYYYY> stack2) {
while (!stack1.isEmpty())
stack2.push(stack1.pop());
}
}
a. ? super Object
b. ? super T
c. ? extends T
d. ? extends Object
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24. Which of the following can be used to replace YYYYYYYY in the following code?
public class WildCardDemo3 {
public static void main(String[] args) {
GenericStack<String> stack1 = new GenericStack<>();
GenericStack<Object> stack2 = new GenericStack<>();
stack2.push(“Java”);
stack2.push(2);
stack1.push(“Sun”);
add(stack1, stack2);
WildCardDemo2.print(stack2);
}
public static <T> void YYYYYYYY {
while (!stack1.isEmpty())
stack2.push(stack1.pop());
}
}
a. add(GenericStack<T> stack1, GenericStack<T> stack2)
b. add(GenericStack<? extends T> stack1, GenericStack<T> stack2)
c. add(GenericStack<T> stack1, GenericStack<? super T> stack2)
d. add(GenericStack<T> stack1, GenericStack<Object> stack2)
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18. Is ArrayList<Integer> a subclass of ArrayList<?>?
a. Yes
b. No
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19. Is ArrayList<Integer> a subclass of ArrayList<? extends Number>?
a. Yes
b. No
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20. Is ArrayList<Number> a subclass of ArrayList<? extends Number>?
a. Yes
b. No
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21. Is ArrayList<?> same as ArrayList<? extends Object>?
a. Yes
b. No
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22. Does <? super Number> represent a superclass of Number?
b. No
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Section 19.8 Erasure and Restrictions on Generics
25. ArrayList<String> and ArrayList<Integer> are two types. Does the JVM load two classes ArrayList<String> and
ArrayList<Integer>?
a. Yes
b. No
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26. Which of the following statements are true?
a. Generic type information is present at compile time.
b. Generic type information is not present at runtime.
c. You cannot create an instance using a generic class type parameter.
d. You cannot create an array using a generic class type parameter.
e. You cannot create an array using a generic class.
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27. If E is a generic type for a class, can E be referenced from a static method?
a. Yes
b. No