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- Question
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
- 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.
Declarations and Access Control problems
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- 1. Which three form part of correct array declarations?
- public int a [ ]
- static int [ ] a
- public [ ] int a
- private int a [3]
- private int [3] a [ ]
- public final int [ ] a
Options- A. 1, 3, 4
- B. 2, 4, 5
- C. 1, 2, 6
- D. 2, 5, 6 Discuss
Correct Answer: 1, 2, 6
Explanation:
(1), (2) and (6) are valid array declarations.Option (3) is not a correct array declaration. The compiler complains with: illegal start of type. The brackets are in the wrong place. The following would work: public int[ ] a
Option (4) is not a correct array declaration. The compiler complains with: ']' expected. A closing bracket is expected in place of the 3. The following works: private int a []
Option (5) is not a correct array declaration. The compiler complains with 2 errors:
']' expected. A closing bracket is expected in place of the 3 and
<identifier> expected A variable name is expected after a[ ] .
- 2. Which three are valid method signatures in an interface?
- private int getArea();
- public float getVol(float x);
- public void main(String [] args);
- public static void main(String [] args);
- boolean setFlag(Boolean [] test);
Options- A. 1 and 2
- B. 2, 3 and 5
- C. 3, 4, and 5
- D. 2 and 4 Discuss
Correct Answer: 2, 3 and 5
Explanation:
(2), (3), and (5). These are all valid interface method signatures.(1), is incorrect because an interface method must be public; if it is not explicitly declared public it will be made public implicitly. (4) is incorrect because interface methods cannot be static.
- 3. Which two of the following are legal declarations for nonnested classes and interfaces?
- final abstract class Test {}
- public static interface Test {}
- final public class Test {}
- protected abstract class Test {}
- protected interface Test {}
- abstract public class Test {}
Options- A. 1 and 4
- B. 2 and 5
- C. 3 and 6
- D. 4 and 6 Discuss
Correct Answer: 3 and 6
Explanation:
(3), (6). Both are legal class declarations.(1) is wrong because a class cannot be abstract and final?there would be no way to use such a class. (2) is wrong because interfaces and classes cannot be marked as static. (4) and (5) are wrong because classes and interfaces cannot be marked as protected.
- 4. Which of the following is/are legal method declarations?
- protected abstract void m1();
- static final void m1(){}
- synchronized public final void m1() {}
- private native void m1();
Options- A. 1 and 3
- B. 2 and 4
- C. 1 only
- D. All of them are legal declarations. Discuss
Correct Answer: All of them are legal declarations.
Explanation:
All the given statements are legal declarations.- 5. What is the most restrictive access modifier that will allow members of one class to have access to members of another class in the same package?
Options- A. public
- B. abstract
- C. protected
- D. synchronized
- E. default access Discuss
Correct Answer: default access
Explanation:
default access is the "package oriented" access modifier.Option A and C are wrong because public and protected are less restrictive. Option B and D are wrong because abstract and synchronized are not access modifiers.
- 6. 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.- 7. 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.
- 8. 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} };
- 9. 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.
- 10. 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.
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- 1. Which three form part of correct array declarations?