Interfaces & Inner Classes

Question 1

An interface is not a class.

Answer 1

True.

Question 2

Interfaces contain method definitions.

Answer 2

False - they usually only contain method headings.

Question 3

Interfaces can contain constant definitions.

Answer 3

True.

Question 4

Interface can never give a definition to its own method.

Answer 4

False - default methods (as of Java 8) have definitions. But generally, no.

Question 5

Interfaces solve which Java restriction?

Answer 5

Java’s restriction on multiple inheritance.

Question 6

An interface and all of its method headings should be declared public.

Answer 6

True.

Question 7

An interface can give private/protected/package access to methods in specific circumstances.

Answer 7

False.

Question 8

When a class implements an interface, that class can decide to make specific methods private.

Answer 8

False - they must all remain public.

Question 9

A method may be written with a parameter of an interface type.

Answer 9

True - this parameter accepts any class that implements the interface.

Question 10

Method headings in interfaces must end in a semicolon.

Answer 10

True.

Question 11

Write a class header for class “C” that implements 3 interfaces (I1, I2, and I3).

Answer 11

public class C implements I1, I2, I3 {…}

Question 12

How many of the methods listed in the interface(s) definition(s) must be defined by the class implementing them?

Answer 12

All methods listed must be implemented.

Question 13

Abstract classes cannot implement interfaces.

Answer 13

False - they may implement 1+ interfaces.

Question 14

Abstract classes must define all methods listed in the interface(s) they implement.

Answer 14

False - abstract classes do not need to provide implementation - this is left for the concrete class to do.

Question 15

Interfaces can be derived from other interfaces.

Answer 15

True.

Question 16

Write the header for an Interface “I1” that extends another interface, “I2”.

Answer 16

public interface I1 extends I2 {…}

Question 17

Variables defined in an interface must be public, static, and final.

Answer 17

True.

Question 18

The public, static, and final modifiers must precede all variables defined in an interface.

Answer 18

False, they may be omitted since the compiler understands that they must already be public, static, and final.

Question 19

What are the 2 possible cases of inconsistent interfaces?

Answer 19

Constants with the same name, but different values, and methods with the same name, but with different return types.

Question 20

The Serializable interface is completely empty.

Answer 20

True.

Question 21

The Cloneable interface is completely empty.

Answer 21

True.

Question 22

Object.clone() does a bit-by-bit copy of the object’s data in storage. This is adequate behaviour for what types of Objects?

Answer 22

For immutable class types.

Question 23

What Exception is thrown when the Object’s clone() method is called on an instance that does not implement cloneable?

Answer 23

CloneNotSupportedException

Question 24

CloneNotSupportedException is a checked exception.

Answer 24

True.

Question 25

How do you implement Cloneable for a class with mutable class type attributes?

Answer 25

Invoke clone() of the base class, then reset the values of mutable instance variables by invoking their respective clone methods.

Question 26

Cloneable can be implemented for all classes with mutable attributes.

Answer 26

False - Cloneable can only be properly implemented if the interface is already implemented for the mutable attributes.

Question 27

Inner classes must be defined at the beginning of the outer class's definition.

Answer 27

False - its location is irrelevant.

Question 28

The name of an inner class may be reused for something outside the outer class definition.

Answer 28

True - it is local to the outer class.

Question 29

The outer class has access to the inner class's private methods/attributes.

Answer 29

True.

Question 30

The inner class has access to the outer class's private methods/attributes.

Answer 30

True.

Question 31

The inner class should always be marked private.

Answer 31

False - not if the inner class is used outside the outer class.

Question 32

It is legal to invoke a (non-static) method of the inner class from the outer class, on an object of the outer class.

Answer 32

False, an object of the inner class must be used as a a calling object.

Question 33

Within the definition of the inner/outer classes, the modifiers public and private are equivalent.

Answer 33

True.

Question 34

When a class with an inner class is compiled, 2 separate .class files are produced.

Answer 34

True.

Question 35

If the outer class name is C1, and the inner class name is C2, then what would be the names of the .class files following compilation?

Answer 35

C1.class and C1$C2.class.

Question 36

When must an inner class be static? (2)

Answer 36

If an object of the inner class is created within a static method of the outer class, and if the inner class must have static members.

Question 37

Can instance variables of the outer class be referenced by a static inner class?

Answer 37

No.

Question 38

Can methods of the outer class be referenced by a static inner class?

Answer 38

Only static methods of the outer class can be invoked.

Question 39

Can an outer class reference static methods/variables of its static inner class?

Answer 39

Yes, if their invocation is prefaced with the name of the inner class and a dot.

Question 40

An inner class marked public can be used outside the outer class.

Answer 40

True.

Question 41

An instance of a non static inner class can be created before creating an instance of the outer class.

Answer 41

False - It must be created using an object of the outer class.

Question 42

Create an instance of an inner class C2, if the outer class is C1 (use default constructors).

Answer 42

C1 c1 = new C1(); C1.C2 c2 = c1.new C2();

Question 43

An instance of the inner class can invoke methods of the outer class.

Answer 43

False - it can only invoke methods from the inner class.

Question 44

You can create instances of static inner classes.

Answer 44

True.

Question 45

Write code that would create an instance of a static inner class (C2), if the outer class is C1.

Answer 45

C1.C2 c2object = new C1.C2();

Question 46

The following is valid: innerObject.InnerNonstaticMethod();

Answer 46

True.

Question 47

The following is valid: innerStaticObject.InnerStaticMethod();

Answer 47

True.

Question 48

The following is valid: OuterClass.InnerStaticClass.InnerNonstaticMethod();

Answer 48

False.

Question 49

The following is valid: OuterClass.InnerStaticClass.InnerStaticMethod();

Answer 49

True.

Question 50

The following is valid: InnerStaticObject.OuterMethod();

Answer 50

False.

Question 51

Outer non-static methods can be called from a static inner class's methods.

Answer 51

False.

Question 52

Outer static methods can be called from a static inner class's methods.

Answer 52

True.

Question 53

If a method is invoked in the inner class, and there exists a method by that name in both the outer and inner classes, which is invoked?

Answer 53

The inner class method.

Question 54

If there exists a method in both the outer and inner classes with the same name, how would you invoke the outer class's method?

Answer 54

OuterClassName.this.methodName();

Question 55

Can you nest classes within already nested classes?

Answer 55

Yes.

Question 56

Create an instance of class C, if class C is nested in class B, and class B is nested in class A.

Answer 56

A a1 = new A(); A.B b1 = a1.new B(); A.B.C c1 = b1.new C();

Question 57

An outer class can be a derived class.

Answer 57

True.

Question 58

An inner class can be a derived class.

Answer 58

True.

Question 59

If class A is nested with class B, and class C extends class A, will class C automatically have B as an inner class?

Answer 59

Yes.

Question 60

If class A is nested with class B, and class C extends class A, can class C override the nested class B?

Answer 60

No.

Question 61

Instantiate an instance of an anonymous class that extends the Car class, whose only (additional) attribute is (int) numberOfWheels.

Answer 61

Car c1 = new Car() { private int numberOfWheels; };

Question 62

The type assigned to an anonymous class must already exist.

Answer 62

True.

Question 63

The type assigned to an anonymous class can be an interface.

Answer 63

True - in fact it most often is.