Objects and Collections problems
- 1. Which class does not override the equals() and hashCode() methods, inheriting them directly from class Object?
Options- A. java.lang.String
- B. java.lang.Double
- C. java.lang.StringBuffer
- D. java.lang.Character Discuss
Correct Answer: java.lang.StringBuffer
Explanation:
- 2. Which collection class allows you to associate its elements with key values, and allows you to retrieve objects in FIFO (first-in, first-out) sequence?
Options- A. java.util.ArrayList
- B. java.util.LinkedHashMap
- C. java.util.HashMap
- D. java.util.TreeMap Discuss
Correct Answer: java.util.LinkedHashMap
Explanation:
- 3. Which interface does java.util.Hashtable implement?
Options- A. Java.util.Map
- B. Java.util.List
- C. Java.util.HashTable
- D. Java.util.Collection Discuss
Correct Answer: Java.util.Map
Explanation:
- 4. What is the numerical range of char?
Options- A. 0 to 32767
- B. 0 to 65535
- C. -256 to 255
- D. -32768 to 32767 Discuss
Correct Answer: 0 to 65535
Explanation:
- 5. You need to store elements in a collection that guarantees that no duplicates are stored and all elements can be accessed in natural order. Which interface provides that capability?
Options- A. java.util.Map
- B. java.util.Set
- C. java.util.List
- D. java.util.Collection Discuss
Correct Answer: java.util.Set
Explanation:
Option A is wrong. A map is an object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value. The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order (ascending key order); others, like the HashMap class, do not (does not guarantee that the order will remain constant over time).
Option C is wrong. A list is an ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list. Unlike sets, lists typically allow duplicate elements.
Option D is wrong. A collection is also known as a sequence. The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list. Unlike sets, lists typically allow duplicate elements.
- 6. Suppose that you would like to create an instance of a new Map that has an iteration order that is the same as the iteration order of an existing instance of a Map. Which concrete implementation of the Map interface should be used for the new instance?
Options- A. TreeMap
- B. HashMap
- C. LinkedHashMap
- D. The answer depends on the implementation of the existing instance. Discuss
Correct Answer: LinkedHashMap
Explanation:
When a new LinkedHashMap is created by passing a reference to an existing Collection to the constructor of a LinkedHashMap the Collection.addAll method will ultimately be invoked.
The addAll method uses an iterator to the existing Collection to iterate through the elements of the existing Collection and add each to the instance of the new LinkedHashMap.
Since the iteration order of the LinkedHashMap is determined by the order of insertion, the iteration order of the new LinkedHashMap must be the same as the interation order of the old Collection.
- 7. Which collection class allows you to grow or shrink its size and provides indexed access to its elements, but whose methods are not synchronized?
Options- A. java.util.HashSet
- B. java.util.LinkedHashSet
- C. java.util.List
- D. java.util.ArrayList Discuss
Correct Answer: java.util.ArrayList
Explanation:
- 8. What line of code should replace the missing statement to make this program compile?
/* Missing Statement? */ public class foo { public static void main(String[]args)throws Exception { java.io.PrintWriter out = new java.io.PrintWriter(); new java.io.OutputStreamWriter(System.out,true); out.println("Hello"); } }
Options- A. No statement required.
- B. import java.io.*;
- C. include java.io.*;
- D. import java.io.PrintWriter; Discuss
Correct Answer: No statement required.
Explanation:
The usual method for using/importing the java packages/classes is by using an import statement at the top of your code. However it is possible to explicitly import the specific class that you want to use as you use it which is shown in the code above. The disadvantage of this however is that every time you create a new object you will have to use the class path in the case "java.io" then the class name in the long run leading to a lot more typing.
- 9. Which interface provides the capability to store objects using a key-value pair?
Options- A. Java.util.Map
- B. Java.util.Set
- C. Java.util.List
- D. Java.util.Collection Discuss
Correct Answer: Java.util.Map
Explanation:
- 10. Which of the following are Java reserved words?
- run
- import
- default
- implement
Options- A. 1 and 2
- B. 2 and 3
- C. 3 and 4
- D. 2 and 4 Discuss
Correct Answer: 2 and 3
Explanation:
(3) - This is a Java keyword
(1) - Is incorrect because although it is a method of Thread/Runnable it is not a keyword
(4) - This is not a Java keyword the keyword is implements
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