Variables and Data Types: Practice Questions (Computer Science A 1.2)

These are original Marlbridge practice questions, not past exam questions. All code is invented for these questions and was compiled and run before publication. The rubrics are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. No calculator is allowed on the exam, so work everything by hand. Code fragments are assumed to be inside a main method.

Question 1 (multiple choice · foundation)

Which of the following is a primitive data type in this course?

(A) String
(B) Scanner
(C) boolean
(D) Object

Answer and explanation

Answer: (C). boolean is one of the three primitive types in the course, with int and double.

(A) String is a class, so it is a reference type.
(B) Scanner is also a class, hence a reference type.
(D) Object is a class too; every class in Java is a subclass of it.

Question 2 (multiple choice · foundation)

A program stores the length of a rope in metres, which was measured as 3.75. Which declaration is appropriate?

(A) double length = 3.75;
(B) int length = 3.75;
(C) boolean length = 3.75;
(D) String length = 3.75;

Answer and explanation

Answer: (A). A measurement with a fractional part is a real number, so double is the appropriate type, and 3.75 is a double value.

(B) does not compile. A double cannot be stored in an int variable (possible lossy conversion from double to int).
(C) does not compile. A number is not a boolean, which holds only true or false.
(D) does not compile. A String value must be written in double quotes, and 3.75 is a number, not text.

Question 3 (multiple choice · core)

Which declaration will compile?

(A) int 3rdPlace = 3;
(B) double ratio = "2.5";
(C) boolean ready = true;
(D) int value = 8.0;

Answer and explanation

Answer: (C). true is a boolean value and ready is a legal name.

(A) fails because an identifier cannot begin with a digit.
(B) fails because "2.5" is a String, not a number. Quotes make it text.
(D) fails because 8.0 is a double, and storing it in an int could lose information. The compiler rejects it even though the fractional part is zero.

Question 4 (multiple choice · core)

What is printed by the following code segment?

int a = 6;
double b = a;
a = 10;
System.out.println(a + " " + b);

(A) 10 10.0
(B) 6 6.0
(C) 10 6
(D) 10 6.0

Answer and explanation

Answer: (D). b = a copies the value 6 into b as the double 6.0. Changing a afterwards to 10 does not change b, which has its own box. a is an int, so it prints as 10, and b is a double, so it prints as 6.0.

(A) assumes that b follows a. Variables are not linked.
(B) assumes that the assignment a = 10 had no effect on a.
(C) forgets that a double prints with a decimal point; b is 6.0, not 6.

Question 5 (multiple choice · core)

What value does avg hold after this declaration?

double avg = 7 / 2;

(A) 3.5
(B) 3.0
(C) 4.0
(D) Nothing: the statement does not compile

Answer and explanation

Answer: (B). Both operands of 7 / 2 are int values, so the division is integer division. Only the integer part of the quotient, 3, is kept. That int 3 is then stored in a double variable as 3.0.

(A) would need at least one operand to be a double, as in 7 / 2.0.
(C) rounds the 3.5 up. Integer division truncates; it does not round.
(D) is wrong because an int value can be stored in a double variable.

Question 6 (multiple choice · core)

What is printed by the code segment below?

int total = 5;
int Total = 9;
System.out.println(total);

(A) 5
(B) 9
(C) Nothing: it will not compile because the two names clash
(D) 14

Answer and explanation

Answer: (A). Java is case-sensitive, so total and Total are different variables. The statement prints the value of total, which is 5.

(B) prints the value of Total instead.
(C) assumes that the names are the same, but they differ in capitalisation.
(D) adds the two variables, but the code never does that.

Question 7 (choosing types · core)

A theatre program stores these items for each booking. For each, give the most appropriate type from int, double, boolean or String, and write a declaration with a sensible initial value.

(a) the number of tickets booked (b) the price of one ticket, in pounds and pence (such as 12.50) (c) whether the customer asked for a seat with extra leg room (d) the customer’s surname (e) the percentage discount, which can be a decimal such as 7.5

Worked solution

int tickets = 4;
double ticketPrice = 12.50;
boolean extraLegRoom = true;
String surname = "Okafor";
double discountPercent = 7.5;

(a) int: tickets are counted, so the value is a whole number.
(b) double: prices have a fractional part.
(c) boolean: the answer is yes or no.
(d) String: text. It is a reference type, so the value is written in double quotes.
(e) double: the value can have a fractional part such as 7.5.

Suggested Marlbridge rubric (5 marks): one mark for each of (a) to (e): correct type chosen and a declaration that compiles (valid name, matching value, quotes on the String).

Question 8 (finding declaration errors · core)

Each line below is a separate attempt at a declaration. Each contains one mistake. Say what is wrong and write a corrected line.

int 1stPrize = 500;
double price = "9.99";
boolean open = yes;
int seats = 40.0;
String city = Lahore;

Worked solution

Line	What is wrong	Corrected line
`int 1stPrize = 500;`	The name starts with a digit	`int firstPrize = 500;`
`double price = "9.99";`	`"9.99"` is a `String`, not a number	`double price = 9.99;`
`boolean open = yes;`	`yes` is not a `boolean` value; the only values are `true` and `false`	`boolean open = true;`
`int seats = 40.0;`	A `double` value cannot be stored in an `int`	`int seats = 40;`
`String city = Lahore;`	A `String` value needs double quotes; without them `Lahore` is read as a variable name	`String city = "Lahore";`

Suggested Marlbridge rubric (5 marks): one mark per line for naming the problem correctly and giving a corrected declaration that compiles. (Accept any valid alternative, such as double seats = 40.0;.)

Question 9 (tracing · stretch)

Consider the code segment:

int p = 4;
int q = p;
p = 9;
int r = q + p;
double s = r;
System.out.println(p + " " + q + " " + r + " " + s);

(a) Complete a trace table that shows the value of every variable after each statement.

(b) State the output.

Worked solution

(a)

After statement	p	q	r	s
`int p = 4;`	4	-	-	-
`int q = p;`	4	4	-	-
`p = 9;`	9	4	-	-
`int r = q + p;`	9	4	13	-
`double s = r;`	9	4	13	13.0

(b) The output is 9 4 13 13.0.

(c) When int q = p; ran, q received a copy of the value 4. A variable of a primitive type holds its own value, so later changing p does not affect q.

Suggested Marlbridge rubric (6 marks):

Mark	Awarded for
1	Rows for `p` and `q` correct (including `q` staying 4)
1	`p` becomes 9 on the third row
1	`r` is 13 (4 + 9)
1	`s` is 13.0 (a `double`)
1	Output `9 4 13 13.0` with `13.0` showing the decimal point
1	Explanation that `q` holds its own copy of the value

How did you do?

Questions 1 to 3: re-read “Storing information needs a plan” and the declaration tables in the study guide.
Questions 4, 6 and 9: trace with a table and remember that each primitive variable is a separate box.
Question 5: remember that division of two int values gives an int; Topic 1.3 covers this in full.
Questions 7 and 8: write five declarations of your own, deliberately break each one, and note the compiler message.

Then try the checklist.