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- Question
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
- Correct Answer
- protected
ExplanationAccess 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.
Declarations and Access Control problems
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- 1. Which two code fragments will compile?
- interface Base2 implements Base {}
- abstract class Class2 extends Base
{ public boolean m1(){ return true; }} - abstract class Class2 implements Base {}
- abstract class Class2 implements Base
{ public boolean m1(){ return (7 > 4); }} - abstract class Class2 implements Base
{ protected boolean m1(){ return (5 > 7) }}
interface Base { boolean m1 (); byte m2(short s); }
Options- A. 1 and 2
- B. 2 and 3
- C. 3 and 4
- D. 1 and 5 Discuss
Correct Answer: 3 and 4
Explanation:
(3) is correct because an abstract class doesn't have to implement any or all of its interface's methods. (4) is correct because the method is correctly implemented ((7 > 4) is a boolean).(1) is incorrect because interfaces don't implement anything. (2) is incorrect because classes don't extend interfaces. (5) is incorrect because interface methods are implicitly public, so the methods being implemented must be public.
- 2. Which cause a compiler error?
Options- A. int[ ] scores = {3, 5, 7};
- B. int [ ][ ] scores = {2,7,6}, {9,3,45};
- C. String cats[ ] = {"Fluffy", "Spot", "Zeus"};
- D. boolean results[ ] = new boolean [] {true, false, true};
- E. Integer results[ ] = {new Integer(3), new Integer(5), new Integer(8)}; Discuss
Correct Answer: int [ ][ ] scores = {2,7,6}, {9,3,45};
Explanation:
Option B generates a compiler error: <identifier> expected. The compiler thinks 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 one 3 x 3 two-dimensional array.To correct the problem and make option B compile you need to add an extra pair of curly brackets:
int [ ] [ ] scores = { {2,7,6}, {9,3,45} };
- 3. What is the prototype of the default constructor?
public class Test { }
Options- A. Test( )
- B. Test(void)
- C. public Test( )
- D. public Test(void) Discuss
Correct Answer: public Test( )
Explanation:
Option A and B are wrong because they use the default access modifier and the access modifier for the class is public (remember, the default constructor has the same access modifier as the class).Option D is wrong. The void makes the compiler think that this is a method specification - in fact if it were a method specification the compiler would spit it out.
- 4. You want a class to have access to members of another class in the same package. Which is the most restrictive access that accomplishes this objective?
Options- A. public
- B. private
- C. protected
- D. default access Discuss
Correct Answer: default access
Explanation:
The only two real contenders are C and D. Protected access Option C makes a member accessible only to classes in the same package or subclass of the class. While default access Option D makes a member accessible only to classes in the same package.- 5. Which two cause a compiler error?
- float[ ] f = new float(3);
- float f2[ ] = new float[ ];
- float[ ]f1 = new float[3];
- float f3[ ] = new float[3];
- float f5[ ] = {1.0f, 2.0f, 2.0f};
Options- A. 2, 4
- B. 3, 5
- C. 4, 5
- D. 1, 2 Discuss
Correct Answer: 1, 2
Explanation:
(1) causes two compiler errors ( '[' expected and illegal start of expression) because the wrong type of bracket is used, ( ) instead of [ ]. The following is the correct syntax: float[ ] f = new float[3];(2) causes a compiler error ( '{' expected ) because the array constructor does not specify the number of elements in the array. The following is the correct syntax: float f2[ ] = new float[3];
(3), (4), and (5) compile without error.
- 6. 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.
- 7. 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.
- 8. 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.- 9. 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.- 10. 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:
Hash table based implementation of the Map interface.
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- 1. Which two code fragments will compile?