Wrapper Classes: Practice Questions (Computer Science A 4.7)

These are original Marlbridge practice questions, not past exam questions. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. Every code fragment was compiled and run, and the outputs quoted come from those runs.

Question 1 (multiple choice · foundation)

Which line uses autoboxing?

(A) int n = Integer.parseInt("5");
(B) Integer k = 12;
(C) int m = k + 1; where k is an Integer
(D) String s = "12";

Answer and explanation

Answer: (B). An int value is assigned to an Integer variable, so the compiler boxes it.

(A) parseInt returns an int directly; no wrapper is involved.
(C) Here a wrapper is converted to a primitive, which is unboxing, not boxing.
(D) A String has no wrapper class involved.

Question 2 (multiple choice · core)

What is printed?

Integer x = 10;
Integer y = 3;
double r = x / y;
System.out.println(r);

(A) 3
(B) 3.0
(C) 3.3333333333333335
(D) The code does not compile

Answer and explanation

Answer: (B). x and y unbox to int, so x / y is integer division: 3. That int is then widened to the double 3.0, and println shows 3.0 (run and confirmed).

(A) The variable is a double, so the output has a decimal point.
(C) Needs a cast before dividing, as in (double) x / y.
(D) The code compiles; unboxing and widening are automatic.

Question 3 (multiple choice · core)

What is printed?

System.out.println(Integer.parseInt("42") + Integer.parseInt("8"));

(A) 428
(B) 50
(C) 42 8
(D) A NumberFormatException is thrown

Answer and explanation

Answer: (B). Both Strings convert to int, and + adds them: 50. For comparison, "42" + "8" joins two Strings into "428" (run and confirmed).

(A) is the result of joining Strings, not adding numbers.
(C) println receives one number and prints one value.
(D) Both Strings are valid whole numbers.

Question 4 (multiple choice · core)

Which declaration does not compile?

(A) double v = 7;
(B) Double w = 7.0;
(C) Double u = 7;
(D) Integer t = 7;

Answer and explanation

Answer: (C). The literal 7 is an int, and an int cannot be boxed into a Double. The compiler message was incompatible types: int cannot be converted to Double.

(A) int widens to double as an ordinary conversion.
(B) A double boxes to Double.
(D) An int boxes to Integer.

Question 5 (multiple choice · core)

What is printed?

Integer n = 5;
Integer m = n;
n++;
System.out.println(n + " " + m);

(A) 6 6
(B) 5 5
(C) 6 5
(D) 5 6

Answer and explanation

Answer: (C). Integer objects are immutable. n++ unboxes 5, adds 1 and boxes a new Integer 6 into n. m still refers to the original object holding 5.

(A) would be right only if the object could be changed in place.
(B) forgets that n++ changes n.
(D) swaps the values.

Question 6 (multiple choice · core)

What happens when this code runs?

Integer z = null;
int w = z;
System.out.println(w);

(A) It prints 0
(B) It prints null
(C) It does not compile
(D) It throws a NullPointerException

Answer and explanation

Answer: (D). Unboxing null means asking a missing object for its value, which throws NullPointerException (run and confirmed).

(A) Java does not substitute a default value.
(B) w is an int and cannot hold null.
(C) The code compiles, because null may be assigned to an Integer.

Question 7 (multiple choice · core)

Which call returns the double 12.5 from the String s, where s is "12.5"?

(A) Integer.parseInt(s)
(B) s.parseDouble()
(C) Double.parseDouble(s)
(D) Double(s)

Answer and explanation

Answer: (C). parseDouble is a class method called on Double.

(A) parseInt("12.5") throws a NumberFormatException.
(B) parseDouble is not an instance method of String.
(D) This is not a method call at all and does not compile.

Question 8 (multiple choice · stretch)

What is the output?

Integer k = 7;
Double q = 3.0;
System.out.println(k + 2.5);
System.out.println((int) (q * 2));

(A) 9.5 then 6
(B) 9 then 6
(C) 9.5 then 6.0
(D) 72.5 then 6

Answer and explanation

Answer: (A). k unboxes to 7, then 7 + 2.5 is the double 9.5. q unboxes to 3.0, q * 2 is 6.0 and the cast to int gives 6. The run printed 9.5 and 6.

(B) drops the decimals from the first line; 7 + 2.5 is a double sum.
(C) forgets that (int) removes the decimal part.
(D) treats the + as String joining; neither operand is a String.

Question 9 (constructed response · core)

Write sumStrings(String[] vals), which returns the sum of the whole numbers held as Strings in vals. For example {"12", "007", "-4"} gives 15. Assume every String is a valid whole number.

Answer and explanation

public static int sumStrings(String[] vals)
{
    int sum = 0;
    for (String s : vals)
    {
        sum += Integer.parseInt(s);
    }
    return sum;
}

For the example the sum is 12 + 7 + (-4) = 15 (run and confirmed).

Suggested Marlbridge rubric (4 points)

Point	Award for
1	Correct header returning `int` and taking `String[]`
2	Initialises an accumulator and traverses every element
3	Converts each String with `Integer.parseInt` (not `+` on Strings)
4	Adds to the accumulator and returns it

An indexed for loop is equally acceptable.

Question 10 (constructed response · stretch)

(a) Write meanOf(String[] vals), which returns the mean of the decimal numbers held as Strings in vals, or 0.0 if the array is empty.

(b) State the value returned for {"3.5", "4.25", "2"} and explain why Integer.parseInt could not be used.

Answer and explanation

(a)

public static double meanOf(String[] vals)
{
    if (vals.length == 0)
    {
        return 0.0;
    }
    double total = 0;
    for (String s : vals)
    {
        total += Double.parseDouble(s);
    }
    return total / vals.length;
}

(b) The total is 3.5 + 4.25 + 2.0 = 9.75 and 9.75 / 3 = 3.25 (run and confirmed). Integer.parseInt("3.5") would throw a NumberFormatException, because an int String may not contain a decimal point.

Suggested Marlbridge rubric (6 points)

Point	Award for
1	Returns `0.0` (and does not divide by zero) for an empty array
2	Accumulates in a `double`
3	Traverses every element
4	Uses `Double.parseDouble` on each String
5	Divides by the number of elements and returns a `double`; the sum must not be an `int`
6	Part (b): 3.25 and a correct reason (`parseInt` rejects a String with a decimal point)

How did you do?

Questions 1 to 8 are one mark each. If you slipped on boxing rules, re-read the boxing section of the Topic 4.7 study guide; then tick off the checklist.