Working with Constructors, Methods and Encapsulation

Question 1

True/false: An abstract class can have non abstract methods.

Answer 1

True

Question 2

True/false: When overriding a method from a super class, you don’t need to consider the access modifier.

Answer 2

False

- an overriding method can’t make the overridden method less accessible.

Question 3

True/false: You can have as many classes as you want in one java file.

Answer 3

True
- but only one class can be public

Question 4

Which of the following are true about the “default” constructor:

1. It is provided by the compiler only if the class and any of its super classes does not define any constructor.
2. It takes no arguments.
3. A default constructor is used to return a default value.
4. To define a default constructor, you must use the default keyword.
5. It is always public.

Answer 4

2
- A constructor does not return any value at all and The access type of a default constructor is same as the access type of the class.

Question 5

True/false: An overloaded method means a method with the same name and same number and type of arguments exists in the super class and sub class.

Answer 5

False.

- This is called an overridden method

Question 6

True/false: Static methods don’t have the “this” - this includes static void main()

Answer 6

True

Question 7

True/false: The following class declaration is valid, assuming two interfaces J and I exists:
abstract class MyIJ implements J , I { }

Answer 7

True

Question 8

Which of the following statements are true:

1. private keyword can never be applied to a class.
2. synchronized keyword can never be applied to a class.
3. synchronized keyword may be applied to a non-primitive variable.
4. final keyword can never be applied to a class.
5. A final variable can be shadowed in a subclass.

Answer 8

2, 5

• private, protected and public can be applied to a nested class.
• Synchronized can only be applied to a method or a block.
• final can be applied to class, variable and methods.

Question 9

Which of these statements are true:

1. All classes must explicitly define a constructor.
2. A constructor can be declared private.
3. A constructor can declare a return value.
4. A constructor must initialize all the member variables of a class.
5. A constructor can access the non-static members of a class.

Answer 9

2, 5

• private is used for implementing Singleton Classes (2).
• All non-final instance variables get default values if not explicitly initialized (4).
• A constructor is non-static, and so it can access directly both the static and non-static members of the class (5).

Question 10

Which of the following are valid:

1. public TestClass(int a, int b) { }
2. public void TestClass(int a) { }
3. public TestClass(String s);
4. private TestClass(String s, int a) { }
5. public TestClass(String s1, String s2) { };

Answer 10

1, 4, 5

• Constructors cannot have empty bodies (i.e. they cannot be abstract) (3)
• You can apply public, private, protected to a constructor. But not static, final, synchronized, native and abstract. (4)
• The compiler ignores the extra semi-colon (5).