Data Types

Question 1

What are the two constructors of the Boolean wrapper class?

Answer 1

Boolean (boolean) and Boolean (String)

This second constructor is deprecated but still shows up on the exam.

Question 2

What are the two static helper methods for the Boolean wrapper class?

Answer 2

parseBoolean and valueOf

Question 3

What does the Boolean.parseBoolean(String) method do?

Answer 3

It returns a primitive type that is equal to the value true if, and only if, the string argument is not null and returns “true” (as a constant Boolean.TRUE), regardless of case.

Question 4

What type do the parseWrappername( ) methods (e.g. parseInteger ( ) or parseBoolean( ) ) return?

Answer 4

Primitive types, not objects.

Question 5

What do Boolean.valueOf(String) and its overloaded method Boolean.valueOf(boolean) return?

Answer 5

A reference to either Boolean.TRUE or Boolean.FALSE (constants of the wrapper class Boolean).

Question 6

What does the following evaluate to?

new Boolean(“true”) == Boolean.TRUE

Answer 6

It evaluates to false.

Both have a Boolean wrapper object containing the value true, but they are different objects.

Question 7

Do any wrapper classes have a no-args constructor?

Answer 7

No.

Question 8

What parameters does Long.valueOf( ) takes?

Answer 8

None.

This means that Long.valueOf(“123”) does not compile.

Question 9

Can variables be assigned as abstract?

Answer 9

No.

Question 10

If a class is not nested, what modifiers can it have that are legal?

Answer 10

final, abstract, and public.

Question 11

If a class is nested, what modifiers can it have?

Answer 11

Public, private, protected, abstract, and final.

Question 12

How can an object be made eligible for garbage collection?

Answer 12

By making sure there are no references pointing to it.

Question 13

Is the following code valid?

Integer i = new Integer(42);
Long ln = new Long(42);
i == ln;

Answer 13

No. ln and i are references to different classes of unrelated objects.

If the compiler can figure out that something can NEVER happen, it flags an error.

Question 14

What is the signature of the equals( ) method?

Answer 14

boolean equals(Object o);

This means it can take any Object as an argument.

Question 15

How does the equals( ) method of a wrapper class work?

Answer 15

It first checks if the two objects are of the same wrapper class. If not, it returns false.

Question 16

Is the following a valid declaration?

String s = ‘string’;

Answer 16

No, it must be in double quotes.

Question 17

How is polymorphism achieved?

Answer 17

By extension.

Question 18

Does polymorphism make code more efficient in terms of execution?

Answer 18

Not necessarily.

It may even cause a slight degradation due to dynamic binding at run time.

Question 19

Can a static method refer to a static field?

Answer 19

Yes.

Question 20

What happens here?

byte foo = 3;
int bar = (int) (foo/2);
System.out.println(bar);

Answer 20

It will print 1.

Whenever both the operands of a mathematical operator (such as / and *) are integral types except long (i.e. byte, char, short, and int), the result is always the integer value that remains after truncating the fractional value.

Question 21

What happens here?

short foo = 5;
int bar = foo/2;
System.out.println(bar);

Answer 21

It will print 2.

Whenever both the operands of a mathematical operator (such as / and *) are integral types except long (i.e. byte, char, short, and int), the result is always the integer value that remains after truncating the fractional value.

Question 22

What is the keyword order for importing a static method?

Answer 22

“import static …”

Question 23

Are access modifiers valid inside a method scope, as in the code below?

//In file A.java
public class A{
   int a;
   public void m1( ){
     private int b = 0;
     a = b;
   }

Answer 23

No.

Access modifiers (public/private/protected) are valid only inside the scope of a class, not of a method.