Application Program Interface (API) and Libraries: Practice Questions (Computer Science A 1.7)

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.

Library entries used below (as in the Java Quick Reference): static double sqrt(double x) returns the nonnegative square root of a double value; static double pow(double base, double exponent) returns the value of the first parameter raised to the power of the second; int length() returns the number of characters in a String; String substring(int from, int to) returns the substring beginning at index from and ending at index to - 1; String substring(int from) returns substring(from, length()); int indexOf(String str) returns the index of the first occurrence of str, or -1 if not found.

Question 1 (multiple choice · foundation)

Which statement best describes an application programming interface (API) specification?

(A) The compiler that translates Java code into a runnable program
(B) A collection of ready-made classes
(C) A variable that stores data about a class
(D) Documentation that informs the programmer how to use the classes in a library

Answer and explanation

Answer: (D). The specification is the documentation: it tells the programmer what each class and method is called, what it needs and what it does.

(A) describes a compiler, which checks code and prepares it to run.
(B) describes a library, the collection itself, not its documentation.
(C) describes an attribute.

Question 2 (multiple choice · foundation)

Classes in which of the following packages are available to every program by default, without an import?

(A) java.util
(B) java.lang
(C) Any package that contains a class you have used before
(D) Only packages whose names begin with java

Answer and explanation

Answer: (B). Classes in the java.lang package, such as String, Math and Integer, are available by default.

(A) java.util contains Scanner and ArrayList, which need an import.
(C) Using a class before does not change how it is found; the import rule applies each time.
(D) is wrong: java.util begins with java too but is not available by default.

Question 3 (multiple choice · core)

Which of the following is a behavior of the String class?

(A) The sequence of characters a String object holds
(B) The name of the variable that refers to the string
(C) A method that returns the number of characters in the string
(D) The package that the class belongs to

Answer and explanation

Answer: (C). length() is a method, and methods define behaviors: what objects can do or what can be done with them.

(A) is the attribute: the data related to the class.
(B) is just a variable name in your program; it is not part of the class.
(D) describes where the class is stored, not what it does.

Question 4 (multiple choice · core)

Using the entry static double sqrt(double x), which of the following statements compiles, assuming Math is the class that holds sqrt?

(A) int r = Math.sqrt(16);
(B) double r = Math.sqrt(16, 2);
(C) double r = Math.sqrt(16);
(D) double r = sqrt(16);

Answer and explanation

Answer: (C). sqrt takes one double parameter and returns a double. The int argument 16 is converted automatically, and the returned 4.0 can be stored in a double.

(A) stores a double in an int; the return type is double, so the compiler rejects it.
(B) supplies two arguments, but the parameter list has one.
(D) leaves out the class name. A class method is called through its class (Topic 1.10), so the compiler cannot find a method called sqrt here.

Question 5 (multiple choice · core)

What is displayed by the following statement?

System.out.println("computer".indexOf("pu") + "computer".indexOf("z"));

(A) 3
(B) 2
(C) 4
(D) -1

Answer and explanation

Answer: (B). The letters are c(0) o(1) m(2) p(3) u(4) t(5) e(6) r(7), so "pu" first occurs at index 3. There is no “z”, so indexOf("z") returns -1. The sum is 3 + (-1) = 2.

(A) forgets the second call.
(C) treats the not-found result as +1.
(D) forgets the first call.

Question 6 (multiple choice · core)

What is the value of "programming".substring(3, 7)?

(A) "ogra"
(B) "gramm"
(C) "rogr"
(D) "gram"

Answer and explanation

Answer: (D). The letters are p(0) r(1) o(2) g(3) r(4) a(5) m(6) m(7) i(8) n(9) g(10). The substring begins at index 3 and ends at index 6 (to - 1), giving g, r, a, m.

(A) starts one index too early (index 2).
(B) includes index 7, as if to were included.
(C) starts at index 1 instead of 3.

Question 7 (attributes and behaviors · core)

A programmer designs a class Thermostat. For each item, say whether it is an attribute or a behavior, and say how it would be represented in the code (a variable or a method).

(a) the current temperature (b) the target temperature (c) raising the target temperature by one degree (d) reporting whether the heating is on

Worked solution

Item	Attribute or behavior	Represented by
(a) current temperature	attribute	a variable that stores a number
(b) target temperature	attribute	a variable that stores a number
(c) raise the target by one degree	behavior	a method
(d) report whether the heating is on	behavior	a method (one that returns a `boolean`)

Attributes are the data related to the class, stored in variables; behaviors are what an instance can do, or what can be done with it, defined by methods.

Suggested Marlbridge rubric (4 marks): one mark for each item correctly classified with the matching representation.

Question 8 (reading entries · core)

Using the Math entries for pow and sqrt given above:

(a) Write one statement that stores 2^5 + the square root of 81 in a double variable total, and give the value.

(b) A student writes int p = Math.pow(2, 5);. Explain the compile-time error and give two different corrections.

Worked solution

(a) double total = Math.pow(2, 5) + Math.sqrt(81); Math.pow(2, 5) returns 32.0 and Math.sqrt(81) returns 9.0, so total is 41.0.

(b) pow returns a double, and a double cannot be stored in an int variable: the compiler reports a possible lossy conversion. Corrections: cast the result, int p = (int) Math.pow(2, 5); (32), or declare the variable as a double, double p = Math.pow(2, 5); (32.0).

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	`Math.pow(2, 5)` with arguments in the right order
1	`Math.sqrt(81)` called correctly
1	Value 41.0
1	Explains that `pow` returns a `double` that cannot be stored in an `int`
1	Gives a valid correction (a cast or a `double` variable)

Question 9 (packages and imports · stretch)

A program contains the line Scanner in = new Scanner(System.in); but no import line, and the compiler reports “cannot find symbol”.

(a) Explain why this error appears.

(b) For each of the classes String, Math, Scanner and ArrayList, say whether an import is needed, giving the package.

Worked solution

(a) Scanner is in the java.util package, which is not available by default. Without an import the compiler does not know where to find the class named Scanner.

(b) String (java.lang): no import. Math (java.lang): no import. Scanner (java.util): import needed. ArrayList (java.util): import needed.

(c) A library is a collection of classes. The instructions for using them are the API specification, the documentation.

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	Cause: `Scanner` is in a package that is not available by default
1	`String` and `Math`: no import, `java.lang`
1	`Scanner` and `ArrayList`: import needed, `java.util`
1	Library = collection of classes
1	API specification = documentation on how to use them

Question 10 (multiple choice · stretch)

Two entries appear in the documentation: String substring(int from, int to) and String substring(int from). The second returns substring(from, length()). What is the value of "classroom".substring(5)?

(A) "room"
(B) "oom"
(C) "class"
(D) "classroo"

Answer and explanation

Answer: (A). The letters are c(0) l(1) a(2) s(3) s(4) r(5) o(6) o(7) m(8), and the string has length 9. substring(5) is substring(5, 9), which runs from index 5 to index 8: r, o, o, m.

(B) starts at index 6.
(C) is substring(0, 5), the part before index 5, not the part from it.
(D) stops one character early and starts at index 0.

How did you do?

Questions 1 to 3: re-read “Why use code that someone else wrote?” and “Attributes and behaviors” in the study guide.
Questions 4, 8 and 9: reread “Reading API documentation” and “Using classes from a library”. Check types and the number of parameters before writing a call.
Questions 5, 6 and 10: write the indices under the string, then apply the entry.
Question 7: invent two more classes and sort their features.

Then try the checklist.