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- Question
Which interface does java.util.Hashtable implement?
Options- A. Java.util.Map
- B. Java.util.List
- C. Java.util.HashTable
- D. Java.util.Collection
- Correct Answer
- Java.util.Map
ExplanationHash table based implementation of the Map interface.
Objects and Collections problems
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- 1. 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:
LinkedHashMap is the collection class used for caching purposes. FIFO is another way to indicate caching behavior. To retrieve LinkedHashMap elements in cached order, use the values() method and iterate over the resultant collection.- 2. 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:
java.lang.StringBuffer is the only class in the list that uses the default methods provided by class Object.- 3. What is the widest valid returnType for methodA in line 3?
public class ReturnIt { returnType methodA(byte x, double y) /* Line 3 */ { return (long)x / y * 2; } }
Options- A. int
- B. byte
- C. long
- D. double Discuss
Correct Answer: double
Explanation:
However A, B and C are all wrong. Each of these would result in a narrowing conversion. Whereas we want a widening conversion, therefore the only correct answer is D. Don't be put off by the long cast, this applies only to the variable x and not the rest of the expression. It is the variable y (of type double) that forces the widening conversion to double.Java's widening conversions are:
- From a byte to a short, an int, a long, a float, or a double.
- From a short, an int, a long, a float, or a double.
- From a char to an int, a long, a float, or a double.
- From an int to a long, a float, or a double.
- From a long to a float, or a double.
- From a float to a double.
- 4. Which one creates an instance of an array?
Options- A. int[ ] ia = new int[15];
- B. float fa = new float[20];
- C. char[ ] ca = "Some String";
- D. int ia[ ] [ ] = { 4, 5, 6 }, { 1,2,3 }; Discuss
Correct Answer: int[ ] ia = new int[15];
Explanation:
Option A is correct. It uses correct array declaration and correct array construction.Option B is incorrect. It generates a compiler error: incompatible types because the array variable declaration is not correct. The array construction expects a reference type, but it is supplied with a primitive type in the declaration.
Option C is incorrect. It generates a compiler error: incompatible types because a string literal is not assignable to a character type variable.
Option D is wrong, it generates a compiler error <identifier> expected. The compiler thinks that you are trying to create two arrays because there are two array initialisers to the right of the equals, whereas your intention was to create a 3 x 3 two-dimensional array.
- 5. You want subclasses in any package to have access to members of a superclass. Which is the most restrictive access that accomplishes this objective?
Options- A. public
- B. private
- C. protected
- D. transient Discuss
Correct Answer: protected
Explanation:
Access modifiers dictate which classes, not which instances, may access features.Methods and variables are collectively known as members. Method and variable members are given access control in exactly the same way.
private makes a member accessible only from within its own class
protected makes a member accessible only to classes in the same package or subclass of the class
default access is very similar to protected (make sure you spot the difference) default access makes a member accessible only to classes in the same package.
public means that all other classes regardless of the package that they belong to, can access the member (assuming the class itself is visible)
final makes it impossible to extend a class, when applied to a method it prevents a method from being overridden in a subclass, when applied to a variable it makes it impossible to reinitialise a variable once it has been initialised
abstract declares a method that has not been implemented.
transient indicates that a variable is not part of the persistent state of an object.
volatile indicates that a thread must reconcile its working copy of the field with the master copy every time it accesses the variable.
After examining the above it should be obvious that the access modifier that provides the most restrictions for methods to be accessed from the subclasses of the class from another package is C - protected. A is also a contender but C is more restrictive, B would be the answer if the constraint was the "same package" instead of "any package" in other words the subclasses clause in the question eliminates default.
- 6. 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:
The char type is integral but unsigned. The range of a variable of type char is from 0 to 216-1 or 0 to 65535. Java characters are Unicode, which is a 16-bit encoding capable of representing a wide range of international characters. If the most significant nine bits of a char are 0, then the encoding is the same as seven-bit ASCII.- 7. 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 B is correct. A set is a collection that contains no duplicate elements. The iterator returns the elements in no particular order (unless this set is an instance of some class that provides a guarantee). A map cannot contain duplicate keys but it may contain duplicate values. List and Collection allow duplicate elements.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.
- 8. 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:
The iteration order of a Collection is the order in which an iterator moves through the elements of the Collection. The iteration order of a LinkedHashMap is determined by the order in which elements are inserted.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.
- 9. 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:
All of the collection classes allow you to grow or shrink the size of your collection. ArrayList provides an index to its elements. The newer collection classes tend not to have synchronized methods. Vector is an older implementation of ArrayList functionality and has synchronized methods; it is slower than ArrayList.- 10. 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.
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