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- Question
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
- 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.
Declarations and Access Control problems
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- 1. 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.
- 2. 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.- 3. 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.
- 4. Which is valid in a class that extends class A?
class A { protected int method1(int a, int b) { return 0; } }
Options- A. public int method1(int a, int b) {return 0; }
- B. private int method1(int a, int b) { return 0; }
- C. public short method1(int a, int b) { return 0; }
- D. static protected int method1(int a, int b) { return 0; } Discuss
Correct Answer: public int method1(int a, int b) {return 0; }
Explanation:
Option A is correct - because the class that extends A is just simply overriding method1.Option B is wrong - because it can't override as there are less access privileges in the subclass method1.
Option C is wrong - because to override it, the return type needs to be an integer. The different return type means that the method is not overriding but the same argument list means that the method is not overloading. Conflict - compile time error.
Option D is wrong - because you can't override a method and make it a class method i.e. using static.
- 5. Which of the following code fragments inserted, will allow to compile?
public class Outer { public void someOuterMethod() { //Line 5 } public class Inner { } public static void main(String[] argv) { Outer ot = new Outer(); //Line 10 } }
Options- A. new Inner(); //At line 5
- B. new Inner(); //At line 10
- C. new ot.Inner(); //At line 10
- D. new Outer.Inner(); //At line 10 Discuss
Correct Answer: new Inner(); //At line 5
Explanation:
Option A compiles without problem.Option B gives error - non-static variable cannot be referenced from a static context.
Option C package ot does not exist.
Option D gives error - non-static variable cannot be referenced from a static context.
- 6. 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[ ] .
- 7. 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.
- 8. 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.- 9. 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.
- 10. 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} };
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- 1. Which two of the following are legal declarations for nonnested classes and interfaces?