Chapter 3 : Core Java APIs Flashcards
What will be the output of the following source?
String name1 = "Fluffy";
String name2 = new String("Fluffy");
String name3 = "Fluffy";
boolean x = name1 == name2;
boolean y = name1 == name3;
System.out.println(x + " " + y);
a) true true
b) true false
c) false false
d) false true
d) false true
boolean x = name1 == name2; is false because the new String() creates a new object
boolean y = name1 == name3; is true because the literal “Fluffy” has one single immutable object in the String pool and therefore both references x and y point to it.
What will be the output of the following source?
int three = 3;
String four = “4”;
System.out.println(1 + 2 + three + four);
a) 1234
b) 64
c) 334
d) 10
b) 64
When doing String concatenation using the + operator, the statement is processed in pairs left-to-right
What will be the output of the following source?
String s = “1”;
s += “2”;
s += 3;
System.out.println(s);
a) 15
b) 123
c) 1223
d) 11223
b) 123
What method will give the expected result? Choose all that apply.
String string = “animals”;
System.out.println(string.____()); // Expected 7
a) size
b) charAt
c) indexOf
d) length
d) length
What method will give the expected result? Choose all that apply.
String string = “animals”;
System.out.println(string.____); // Expected s
a) indexOf(6)
b) indexOf(7)
c) charAt(6)
d) charAt(7)
c) charAt(6)
a) and b) are wrong because indexOf() returns an int
d) will throw an exception since it is out of bounds
What method will give the expected result? Choose all that apply.
String string = “animals”;
System.out.println(string.____); // Expected 4
a) indexOf(“al”)
b) indexOf(“al”, 4)
c) contains(“al”)
d) contains(“al”, 4)
a) indexOf(“al”) & b) indexOf(“al”, 4)
b) indexOf(“al”, 4) searches for the string “al” starting at position 4. Since “al” occurs starting at 4, it returns immediately.
c) and d) are wrong because contains() returns a boolean
What method will give the expected result? Choose all that apply.
String string = “animals”;
System.out.println(string.____); // Expected mals
a) substring(3)
b) substring(string.indexOf(‘m’)));
c) substring(3,6);
d) substring(3,7);
a) , b), d)
c) substring(3,6); would result in “mal”
What will be the output of the following source?
String string = “animals”;
System.out.println(string.substring(3,2));
a) i
b) mals
c) Does not compile
d) Exception thrown
d) Exception thrown
What method will give the expected result? Choose all that apply.
String string = “animals”;
System.out.println(string.____(“animals”)); // Expected true
a) equals
b) compare
c) equalsIgnoreCase
d) compareTo
a) equals & c) equalsIgnoreCase
b) compare, is not a valid method
d) compareTo is a method that does lexicographical comparison and it returns an integer. 0 means the strings are equals.
What will be the output of the following source?
String string = “animals”;
System.out.println(string.equals(null));
a) false
b) true
c) Exception is thrown
c) Does not compile
a) false
equals does not throw an exception when parameter is null
What will be the output of the following source? String string = “animals”; System.out.println(string.compareTo(null)); a) false b) -1 c) Exception is thrown c) Does not compile
c) Exception is thrown compareTo throws an exception when parameter is null
What method will give the expected result? Choose all that apply. String string = “animals”; System.out.println(string.____)); // Expected AnimAls a) toUpperCase(“a”) b) toUpperCase(‘a’) c) replace(“a”, “A”) d) replace(‘a’, ‘A’)
c) and d) a) and b) are wrong since those methods act on the whole string and do not take a String or char parameter
What will be the output of the following source? (Note that \t is a invisible tab) String string = “\tA\tBC\t”; System.out.print(string.trim()); System.out.println(“C”); a) \tA\tBC b) A\tBC c) ABC c) Does not compile
b) A\tBC
What will be the output of the following source? 4 StringBuilder a = new StringBuilder(“abc”); 5 StringBuilder b = a.append(“de”); 6 b = b.append(“f”).append(“g”); 7 System.out.println(“a=” + a); 8 System.out.println(“b=” + b); a) a=abc , b=abcdefg b) a=abc , b=fg c) a=abc , b=g d) a=abcdefg, b=abcdefg
d) a=abcdefg, b=abcdefg Both a and b are pointing to the same object. append() returns a reference to the object itself.
How many strings are created? StringBuilder alpha = “”; for (char current=’a’; current <= ‘z’; current++) { alpha.append(current); } System.out.println(alpha); a) 1 b) 27 c) Does not compile d) Exception thrown
c) StringBuilder cannot be assigned a String literal. The following would work: StringBuilder alpha = new StringBuilder();
What will be the output of the following source? StringBuilder alpha = new StringBuilder(10); alpha.append(12345); System.out.println(alpha.length()); a) 5 b) 10 c) Does not compile d) Exception thrown
a) The constructor takes in 10 which means the object can hold up to 10 characters. However capacity != length
What will be the output of the following source? StringBuilder alpha = new StringBuilder().append(1).append(‘c’); alpha.append(“-“).append(true); System.out.println(alpha); a) 1c b) 1c-true c) Does not compile d) Exception thrown
b) 1c-true
What is the difference between StringBuilder and StringBuffer?
They do the same thing except StringBuffer is thread safe.
What will be the output of the following source? public class Junk { String name; public static void main(String[] args) { Junk a = new Junk(); Junk b = new Junk(); Junk c = a; System.out.print((a == c) + “ “); System.out.print((a == b) + “ “); System.out.print(a.equals(b)); } } a) false false false b) true false true c) true false false d) true true true
c) true false false a.equals(b) is false because, when equals() is not implemented, then the default behavior is to compare references.
What statements will compile? Choose all that apply. 1 int[] a; 2 int [] b; 3 int c[]; 4 int d []; 5 int[] e, f; 6 int g[], h; 7 int i[]; a) 1, 3 b) 1, 3, 5, 6 c) 1, 2, 5 d) 1, 2, 5, 7 e) All of them compile
e) All of them compile
How many arrays are declared? Choose all that apply. int[] a, b; int c[], d; a) 1 b) 2 c) 3 d) 4
c) 3 a, b, c are arrays d is just an int
Which of the following is true? Select all that apply. 3 String[] strings = { “apples” }; 4 Object[] objects = strings; 5 String[] moreStrings = (String[]) objects; 6 moreStrings[0] = new StringBuilder(); 7 objects[0] = new StringBuilder(); a) 5 does not compile b) 6 does not compile c) 6 throws exception d) 7 does not compile e) 7 throws exception
b) 6 does not compile & e) 7 throws an exception 7 throws a ArrayStoreException
What options will give the expected result? Choose all that apply. int[] numbers = new int[10]; for (int i = 0; i == 4; i++) { numbers[i] = i; } System.out.println(numbers.____); // Expected 5 a) size() b) length() c) length d) None of the above
d) None of the above The array is initialized to be of size 10, therefore lenght is 10 regardless of how many values are in the array. numbers.length will print 5
What is the output of the following? int[] numbers = new int[10]; for (int i = 0; i == numbers.length; i++) { numbers[i] = i; } System.out.println(numbers); a) [I@4d7e1886 b) 0123456789 c) Does not compile d) None of the above
a) [I@4d7e1886 The above [ means it is an array. The letter “I” means its an int. The string after the “@” is a hash value for the memory address
What is the output of the following? String[] strings = { “10”, “9”, “100” }; Arrays.sort(strings); for (String string : strings) System.out.print(string + “ “); a) 10 100 9 b) 9 10 100 c) Does not compile d) Exception thrown
a) 10 100 9 Strings are sorted in alphabetic order. “100” comes before “9”
What is the output of the following? int[] numbers = {2, 4, 6, 8}; System.out.print(Arrays.binarySearch(numbers, 2) + “ “); System.out.print(Arrays.binarySearch(numbers, 4) + “ “); System.out.print(Arrays.binarySearch(numbers, 1) + “ “); System.out.print(Arrays.binarySearch(numbers, 3) + “ “); System.out.print(Arrays.binarySearch(numbers, 9) + “ “); a) 0 1 0 1 4 b) 0 1 -1 -2 4 c) 0 1 0 1 5 d) 0 1 -1 -2 -5 e) None of the above
d) 0 1 -1 -2 -5 If binarySearch cannot find a value then it returns the index at which the missing value should be inserted in such a way that it will maintain the ascending sorted order. Then, it negates that values and substracts 1. For example, 9 : -[4] - 1 = -5
What is the output of the following? int[] numbers = {4, 2, 6, 8}; System.out.print(Arrays.binarySearch(numbers, 2) + “ “); System.out.print(Arrays.binarySearch(numbers, 4) + “ “); System.out.print(Arrays.binarySearch(numbers, 1) + “ “); System.out.print(Arrays.binarySearch(numbers, 3) + “ “); System.out.print(Arrays.binarySearch(numbers, 9) + “ “); a) 0 1 0 1 4 b) 0 1 -1 -2 4 c) 0 1 0 1 5 d) 0 1 -1 -2 -5 e) None of the above
e) None of the above The array is not sorted and therefore binarySearch is unpredictable.
What is the output of the following? public static void main(String…args) { System.out.print(args[0] + “ “); System.out.println(args.length); } a) Prints the first argument plus the length of args b) Prints the first argument c) Does not compile d) Exception thrown
a) Prints the first argument plus the length of args Varargs are treated like arrays
What statements will compile? Choose all that apply. 1 int[][] vars1; 2 int[] vars2[]; 3 int[] vars3[][]; 4 int[][] vars4 = {{1,4},{3},{3,6,9}}; a) 1, 4 b) 1, 3 c) 1, 2, 4 d) All statements compile
d) All statements compile
What method will give the expected result? Choose all that apply. ArrayList list = new ArrayList(); list.add(“hawk”); // [“hawk”] list.add(“owl”); // [“hawk”,”owl”] list.________; // [“pigeon”, “owl”] a) add(0, “pigeon”) b) add(1, “pigeon”) c) set(0, “pigeon”) d) set(1, “pigeon”)
c) set(0, “pigeon”)
What method will give the expected result? Choose all that apply. ArrayList list = new ArrayList(); list.add(“hawk”); // [“hawk”] list.add(“owl”); // [“hawk”,”owl”] list.________; // [“hawk”, “robin”, “owl”] a) add(0, “robin”) b) add(1, “robin”) c) set(0, “robin”) d) set(1, “robin”)
b) add(1, “robin”)
What method will give the expected result? Choose all that apply. ArrayList list = new ArrayList(); list.add(“hawk”); // [“hawk”] list.add(“owl”); // [“hawk”,”owl”] list.________; // [] a) removeAll() b) remove() c) empty() d) clear()
d) clear()
What method will give the expected result? Choose all that apply. ArrayList list = new ArrayList(); list.add(“hawk”); // [“hawk”] list.add(“owl”); // [“hawk”,”owl”] int x = list._____; // x = 2 a) length() b) size() c) count() d) max()
b) size()
What method will give the expected result? Choose all that apply. ArrayList list = new ArrayList(); list.add(“hawk”); // [“hawk”] list.add(“owl”); // [“hawk”,”owl”] list.________; // [“owl”] a) remove(0) b) set(0, null) c) clear(0) d) remove(“hawk”)
a) remove(0) and d) remove(“hawk”)
What is the output of the following? List one = new ArrayList<>(); List two = new ArrayList<>(); System.out.print(one.equals(two) + “ “); one.add(“a”); System.out.print(one.equals(two) + “ “); two.add(“a”); System.out.print(one.equals(two) + “ “); one.add(“b”); two.add(0, “b”); System.out.print(one.equals(two)); a) false false false false b) true false false false c) true false true false c) true false false true
c) true false true false
What method will give the expected result? Choose all that apply. int a = Integer._______; // a = 12 a) valueOf(12) b) parseInt(“12.0”) c) parseInt(“12”) d) valueOf(“12”)
a), c), d) The most efficient is c) because a) and d) use autoboxing
What method will give the expected result? Choose all that apply. double a = Float._______; // a = 12 a) valueOf(12) b) parseFloat(“12.0”) c) parseDouble(“12”) d) valueOf(“12”) e) Does not compile
a), b), d)
What is the output of the following? List heights = new ArrayList<>(); heights.add(null); int h = heights.get(0); System.out.println(h); a) Exception thrown b) Does not compile c) 0 d) null
a) Exception thrown int h = heights.get(0); The line above will result in autoboxing of the element returned from the get() method. But that object is null therefore a NullPointerException is thrown
What method will give the expected result? Choose all that apply. List< String > list = new ArrayList<>(); list.add(“hawk”); list.add(“robin”); String[] stringArray = list._______; a) toArray() b) array() c) toArray(new String[0]) d) asArray()
c) a) does not compile b) and d) are not valid methods
What method will give the expected result? Choose all that apply. String[] array = { “hawk”, “robin” }; List list = Arrays.asList(array); list.set(1,”test”); for (String b : array) System.out.print(b + “ “); list.remove(1); for (String b : array) System.out.print(b + “ “); a) hawk robin b) hawk test c) hawk test hawk d) Exception thrown
b) hawk test and d) Exception thrown When creating an ArrayList from an array, the array backs the ArrayList, which means: a) any modifications in the array are reflected in the ArrayList (and viceversa) b) the ArrayList size cannot be changed list.remove(1); The above threw an exception
What method can be used to get the current date, time, or both in Java 8?
Date - LocalDate.now() Time - LocalTime.now() Date + Time - LocalDateTime.now()
What is the output of the following? LocalDate d = new LocalDate(); d.set(10, 1, 2015); System.out.println(d); a) January 10th, 2015 b) February 10th, 2015 c) October 1st, 2015 d) Does not compile
d) Does not compile LocalDate cannot be created using a constructor
What method will result in the expected behavior? Choose all that may apply. LocalTime time = LocalTime._______(6, 15); // 6:15am a) setTime b) getTime c) of d) set
c) of
What method will result in the expected behavior? Choose all that may apply. LocalDate date = LocalDate.of(2014, Month.JANUARY, 20); date = date._____; // date is now February 20th, 2014 a) addMonth(1) b) plusMonths(1) c) set(Date.MONTH, Month.FEBRUARY) d) set(Month.FEBRUARY)
b) plusMonths(1)
What is the output of the following? LocalDate date = LocalDate.of(2020, Month.JANUARY, 20); date = date.plusMonths(1).plusYears(1).plusHours(1).plusMinutes(1); a) February 20th, 2021 b) February 20th, 2021 02:01am c) Does not compile d) Exception thrown
c) Does not compile LocalDate does not support time-related methods (only date-related methods). Use LocalTime or LocalDateTime to modify time fields
What option will result in the expected behavior? Choose all that may apply. LocalDate date = LocalDate.of(2014, Month.JANUARY, 20); date = date.plus(_______); // January 25th, 2015 a) 5 b) Day.DATE, 5 c) DAYS, 5 d) Period.ofDays(5)
d) Period.ofDays(5)
What is the output of the following? LocalDate date = LocalDate.of(2014, Month.JANUARY, 20); Period period = Period.ofYears(1).ofMonths(1).ofDays(1); date = date.plus(period); a) February 21st, 2015 b) January 21st, 2014 c) Does not compile d) Throws an exception
b) January 21st, 2014 ofXXX() methods do not chain. Only the last value was applied. To give a period comprised of years, days, months, etc. then use the method of(int x, int y, int z)
What method will result in the expected behavior? Choose all that may apply. LocalDate date = LocalDate.of(2014, Month.JANUARY, 20); date._____; System.out.println(date); // date January 20th, 2015 is printed a) addYear(1) b) addYears(1) c) plusYears(1) d) None of the above
d) None of the above LocalDate objects are immutable and therefore the plusYears() method returns a new object with the result of the operation (original object remains intact). date = date.plusYears(1); The above would work.
What option will result in the expected behavior? Choose all that may apply. LocalDate date = LocalDate.of(2014, Month.MARCH, 20); System.out.println(date._________); // date printed as “3/20/2014” a) format(SimpleDateFormat.SHORT) b) format(“MM/DD/YYYY”) c) format(DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT)) d) format(DateTimeFormatter.ISO_LOCAL_DATE)
c) format(DateTimeFormatter .ofLocalizedDate(FormatStyle.SHORT))
What class can be used to format LocalDate, LocalTime, LocalDateTime objects? a) DateTimeFormatter b) LocalDateTimeFormatter c) SimpleDateFormat d) SimpleDateFormatter
a) DateTimeFormatter and c) SimpleDateFormat c) SimpleDateFormat is deprecated
What import statement is needed for DateTimeFormatter? a) java.time.DateTimeFormatter; b) java.time.format.DateTimeFormatter; c) java.lang.time.DateTimeFormatter; d) java.lang.date.format.DateTimeFormatter;
b) java.time.format.DateTimeFormatter;
What is the output of the following? LocalTime time = LocalTime.of(11, 22, 34); DateTimeFormatter shortDateTime = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT); System.out.println(shortDateTime.format(time)); a) 11:22:34 b) 11:22 AM c) 11:22:34 AM d) Exception thrown
d) Exception thrown An UnsupportedTemporalTypeException is thrown because ofLocalizedDate() returns a DateTimeFormatter which is able to format dates but not times.
What option will result in the expected behavior? Choose all that may apply. LocalDate date = LocalDate.of(2015, Month.FEBRUARY, 4); DateTimeFormatter f = DateTimeFormatter._______; System.out.println(date.format(f)); // Feb-4-15 a) ofPattern(“MMM-d-YY”) b) ofPattern(“MMM-D-YY”) c) pattern(“mmm-d-YY”) d) pattern(“MMM-D-YY”)
a) ofPattern(“MMM-d-YY”)
What option will result in the expected behavior? Choose all that may apply. DateTimeFormatter f = DateTimeFormatter.ofPattern(“MM dd yyyy”); LocalDate date = LocalDate.________; System.out.println(date); // 2015-01-02 a) of(2015, Month.JANUARY, 2) b) of(f.parse(“01 02 2015”)) c) f.parse(“01 02 2015”) d) parse(“01 02 2015”, f)
a) of(2015, Months.JANUARY, 2) and c) parse(“01 02 2015”)
