Data Types

Question 1

What happens at compile time if a local variable remains uninitialized?

Answer 1

Compile time error.

Local variables must be explicitly initialized.

Question 2

What is the default value for a boolean instance variable if it is not explicitly initialized?

Answer 2

False.

Question 3

What are the primitive data types in Java

Answer 3

boolean
char
byte, short, int, long,
float and double.

Question 4

The ~ operator

Answer 4

Bitwise negation.

Also called the unary operator, it operates only on integral types.

Maybe think of it as the proofreading symbol for swapping two words.

Question 5

integral data type

Answer 5

a type that can contain integers.

They correspond to the primitive types:
byte, short, int and long

Question 6

What data integrity risk is present when converting an int to a float?

Answer 6

Loss of precision.

A value of float type is only precise out to nine digits.

Question 7

Integer.MIN_VALUE

Answer 7

A constant of the Integer class in the java.lang package representing the smallest value an integer variable can store.

-2^31 = -2147483648

Question 8

Integer.MAX_VALUE

Answer 8

A constant of the Integer class in the java.lang package representing the largest value an integer variable can store.

2^31 - 1 = 2147483647

(That number is a Mersenne prime)

Question 9

What are the two Boolean class constructors?

Answer 9

Boolean(String) and Boolean(boolean)

Question 10

What does the Boolean(String) constructor do?

Answer 10

It allocates a Boolean object representing the value true if the string argument is not null and is equal, ignoring case, to the string “true”.

Otherwise, it allocates a Boolean object representing the value false.

Question 11

What are the Boolean class static helper methods?

Answer 11

parseBoolean and valueOf

Question 12

T/F: Conversion from char to long does not need a cast

Answer 12

True.

Conversion from a smaller primitive to a larger does not need a cast.

Question 13

T/F: Conversion from int to float needs a cast.

Answer 13

False.

However, conversion from float to int does need a cast.

Question 14

T/F: A final variable that fits into a smaller type still needs a cast for it to be assigned.

Answer 14

False. The compiler already knows its value. This is called implicit narrowing.

Question 15

Does implicit narrowing apply to all primitive types?

Answer 15

No. It only applies to byte, short, int and char.

It does not apply to long, float, or double.

Question 16

Can a long, float, or double be implicitly narrowed using the final keyword?

Answer 16

No.

Question 17

Do wrapper classes have a no-args constructor?

Answer 17

No.

This means the statement new Integer() is invalid.

Question 18

What does this code do:

String foo = “145”;
Long.valueOf(foo).longValue();

Answer 18

Long.valueOf(foo) returns a Long object containing “145”

longValue() returns 145.

Question 19

Can chaining be used to assign a value to a variable at its time of declaration?

e.g. int a = b = c = 42;

Answer 19

No.

In this example, if c was not previously declared, this will not compile:

int a = b = c = 42;

Question 20

Is this code valid?

String s = 'a';

Answer 20

No.

‘a’ is a char.

“a” is a string.

Question 21

What class does every class directly or indirectly extend?

Answer 21

Object

Question 22

What happens to a number that falls exactly between two numbers when rounding?

Answer 22

It always rounds up.

Question 23

Is this a valid assignment:

Runnable r = new Thread( );

Answer 23

Yes.

Thread implements the interface Runnable.

Question 24

What is the legal range for a byte?

Answer 24

-128 to 127

Byte is an 8-bit signed integer.

Question 25

Is this a valid integer variable declaration:

Int x = 10;

Answer 25

No.

Int with capital ‘I’ is not valid.

The wrapper class that can hold an int is Integer.

Question 26

What does the equals method of a wrapper class first check?

Answer 26

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

Question 27

How do you make an object eligible for garbage collection?

Answer 27

By making sure there are no references pointing to it.

Question 28

What conditions must be met for primitive conversion be used?

Answer 28

When all of the following are met:

1. Expression is compile time constant of type byte, short, int or char.
2. Variable type is byte, short or char.
3. Value of expression can be represented in the type of variable.

Question 29

T/F: All local variables are initialized automatically and given default values if not explicitly initialized.

Answer 29

False.

Note: this is the case for all instance and static variables.

Question 30

T/F: It’s okay to leave a local variable not initialized if it is not used in the code.

Answer 30

True.

But why do it?

Question 31

What does equals( ) return?

Answer 31

It can be overridden, so you can’t tell unless you look at the code.

Question 32

What does hashCode( ) return?

Answer 32

It can be overridden, so you can’t tell unless you look at the code.

Question 33

Does the following code compile?

System.out.println(null + true);

Answer 33

No. The operator + cannot be applied to or boolean.