Exercise: Declarations And Access Control

Questions for: Declarations And Access Control

Which one creates an instance of an array?
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 };
Answer: A

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.

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class A 
{  
    protected int method1(int a, int b) 
    {
        return 0; 
    } 
}
Which is valid in a class that extends class A?
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; }
Answer: A

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.

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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; 
    } 
}
A:
int
B:
byte
C:
long
D:
double
Answer: D

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.

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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?
A:
public
B:
private
C:
protected
D:
default access
Answer: D

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.

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Which three are valid method signatures in an interface?
  1. private int getArea();
  2. public float getVol(float x);
  3. public void main(String [] args);
  4. public static void main(String [] args);
  5. boolean setFlag(Boolean [] test);
A:
1 and 2
B:
2, 3 and 5
C:
3, 4, and 5
D:
2 and 4
Answer: B

(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.

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