Core Java APIs

Question 1

What does 3 + “c” result in?

Answer 1

“3c” (a String)

Question 2

Is String mutable or immutable?

Answer 2

immutable

Soooo you can concat it with + but there aren’t any String methods to change them.

HOWEVER, you can set it equal to something else to make it point to that instead. It won’t change the original object though, just create a new one

Question 3

string pool

Answer 3

An area in the JVM where string literals are stored. The JVM can optimize the use of string literals by allowing only one instance of a string in the pool.

Question 4

What is the difference between
String name = “Fluffy”;
and
String name = new String(“Fluffy”);

Answer 4

The first puts “Fluffy” in the string pool, and the second does not. The second one is less efficient, so do the first.

Question 5

mystring.length()

Answer 5

Returns the length of mystring

Question 6

mystring.charAt(i)

Answer 6

Returns the character at the i-th index of mystring

Question 7

mystring.indexOf(c)

Answer 7

Finds the first index of mystring that matches the char or String c

Question 8

mystring.indexOf(c, i)

Answer 8

Finds the first index of mystring that matches c starting from index i

Question 9

What does indexOf() return if it can’t find a match?

Answer 9

-1

Question 10

mystring.substring(x)

Answer 10

Returns the substring of mystring starting at index x until the end

Question 11

mystring.substring(x,y)

Answer 11

Returns the substring of mystring starting at index x and ending at index y (meaning don’t actually include the character at index y)

String s = “funtimes”;
return s.substring(3,7);

will return “time”

Question 12

mystring.toLowerCase()

Answer 12

Returns mystring as all lowercase

does not actually change the String

Question 13

mystring.toUpperCase()

Answer 13

Returns mystring as all uppercase

does not actually change the String

Question 14

mystring.equals(yourString)

Answer 14

Returns true if mystring and yourString contain exactly the same characters in the same order. Otherwise returns false.

Question 15

mystring.equalsIgnoreCase(yourString)

Answer 15

Like mystring.equals() but ignores case

Question 16

mystring.startsWith(str)

Answer 16

Returns true if mystring starts with str, otherwise returns false

Question 17

mystring.endsWith(str)

Answer 17

Returns true if mystring ends with str, otherwise returns false

Question 18

mystring.contains(str)

Answer 18

Returns true if mystring contains str, otherwise returns false

Question 19

mystring.replace(str1, str2)

Answer 19

Returns mystring with str1 replaced by str2

Note: Can use char or String parameters

Question 20

mystring.trim()

Answer 20

Returns mystring with white space removed from the beginning and end

Question 21

StringBuilder

Answer 21

Basically the mutable version of String!

Question 22

myStringBuilder.append(c)

Answer 22

Appends the character c to myStringBuilder

Question 23

StringBuilder sb1 = new StringBuilder();
StringBuilder sb2 = new StringBuilder("animal");
StringBuilder sb3 = new StringBuilder(10);

Answer 23

Create empty StringBuilder object

Create StringBuilder object with “animal” as it’s value

Create StringBuilder object with space for ten characters

Question 24

myStringBuilder.charAt()
myStringBuilder.indexOf()
myStringBuilder.length()
myStringBuilder.substring()

Answer 24

All the same as in the String class

Question 25

myStringBuilder.insert(n, str)

Answer 25

Inserts str at the n-th index of myStringBuilder

Question 26

myStringBuilder.delete(i, j)

Answer 26

Deletes characters from myStringBuilder starting at index i and stopping before index j

Question 27

myStringBuilder.deleteCharAt(i)

Answer 27

Deletes the character of myStringBuilder at the i-th index

Question 28

myStringBuilder.reverse()

Answer 28

Reverses myStringBuilder

Question 29

myStringBuilder.toString()

Answer 29

Converts myStringBuilder to a String

Question 30

How to declare an array of 3 ints

Answer 30

int[] nums = new int[3]

Question 31

How to delcare an array of 3 specific ints

Answer 31

int[] nums = new int[] {1, 2, 3} or shorter int[] nums = {1, 2, 3}

Question 32

4 ways to put the [] when declaring an array

Answer 32

int[] myArray int [] myArray int myArray[] int myArray []

Question 33

myArray.length

Answer 33

Returns the number of elements in myArray

Question 34

What must you import to use the Arrays class?

Answer 34

import java.util.* or import java.util.Arrays

Question 35

int[] nums = {6, 9, 1}; | Arrays.sort(nums);

Answer 35

Sorts nums in ascending order

Question 36

What are the rules of numbers and letters when sorting an array?

Answer 36

Numbers before letters, and uppercase before lowercase

Question 37

int[] numbers = {2,4,6,8}; System.out.println(Arrays.binarySearch(numbers, 2)); System.out.println(Arrays.binarySearch(numbers, 4)); System.out.println(Arrays.binarySearch(numbers, 1)); System.out.println(Arrays.binarySearch(numbers, 3)); System.out.println(Arrays.binarySearch(numbers, 9));

Answer 37

``` 0 1 -1 -2 -5 ``` Array must be sorted for this to work!

Question 38

How to import ArrayList?

Answer 38

import java.util.ArrayList;

Question 39

3 ways to declare an ArrayList (old way)

Answer 39

``` ArrayList list1 = new ArrayList(); ArrayList list2 = new ArrayList(10); ArrayList list3 = new ArrayList(list2); ```

Question 40

``` ArrayList list = new ArrayList(); list.add("word"); ```

Answer 40

Adds "word" to the ArrayList

Question 41

How to add an item at a specific index in an ArrayList called list

Answer 41

list.add(idx, item)

Question 42

How to remove an item from an ArrayList called list

Answer 42

list.remove(item) //returns true if item is in the list, and false if not or list.remove(index) //returns the item at that index

Question 43

How to set a specific index with an element in an ArrayList called list

Answer 43

E set(int index, E newElement) list.set(i, item) returns the element that got replaced. Keeps the ArrayList the same size

Question 44

ArrayList list = new ArrayList(); list. isEmpty(); list. size();

Answer 44

Checks if list is empty (returns a boolean) Then determines number of elements in list (returns an int)

Question 45

``` ArrayList list = new ArrayList(); list.clear() ```

Answer 45

Removes all elements of list No return

Question 46

``` ArrayList list = new ArrayList(); list.contains(obj); ```

Answer 46

Checks whether obj is in list. Returns a boolean

Question 47

``` ArrayList list1 = new ArrayList(); ArrayList list2 = new ArrayList(); ``` list1.equals(list2)

Answer 47

Checks if list1 and list2 have the same elements in the same order

Question 48

Converting String to a primitive (lets say int for this example)

Answer 48

Integer.parseInt("10");

Question 49

Converting String to a wrapper class (lets say Integer for this example)

Answer 49

Integer.valueOf("10");

Question 50

Autoboxing

Answer 50

The compiler automatically converts a primitive type into the matching wrapper class and vice versa.

Question 51

Wrapper Class

Answer 51

Integer for int, Boolean for boolean, etc.

Question 52

Convert from an ArrayList of Strings to an array of Strings ArrayList list = new ArrayList;

Answer 52

String[] stringArray = list.toArray(new String[0]); The "0" says to pick the same size as the list Without specifying "new String[0]" the array will be of type Object[]

Question 53

Convert from an array of Strings to a List of Strings String[] array = {"hawk", "robin"};

Answer 53

List list = Arrays.asList(array); Once it is a List, you cannot change its size

Question 54

How to sort an ArrayList ArrayList list = new ArrayList();

Answer 54

Collections.sort(list);

Question 55

What import statement must you use to work with date and time?

Answer 55

import java.time.*

Question 56

LocalDate

Answer 56

A class that deals with just the date, no time

Question 57

LocalTime

Answer 57

A class that deals with just the time, no date

Question 58

LocalDateTime

Answer 58

A class that deals with both date and time

Question 59

If the date is June 20th, 2016 at 12:30 pm, what is the output? System.out.println(LocalDate.now()); System.out.println(LocalTime.now()); System.out.println(LocalDateTime.now());

Answer 59

2016-06-20 12:30:00.000 2016-06-20T12:30:00.000

Question 60

Create a LocalDate object for June 20th, 2016

Answer 60

LocalDate date1 = LocalDate.of(2016, Month.JUNE, 20); or LocalDate date2 = LocalDate.of(2016, 6, 20);

Question 61

Create a LocalTime object for 12:30:00 pm

Answer 61

LocalTime time = LocalTime.of(12, 30, 0); Can add one more parameter to specify nanoseconds, or one parameter less to not specify seconds

Question 62

What is wrong with this? LocalDate d = new LocalDate();

Answer 62

You can't create a LocalDate, LocalTime, or LocalDateTime object in this fashion. You gotta use of() or now() or whateva

Question 63

How to add 3 days, 3 weeks, 3 months, or 3 years to a LocalDate object called date

Answer 63

date = date.plusDays(3); date = date.plusWeeks(3); date = date.plusMonths(3); date = date.plusYears(3);

Question 64

How to go back 3 days, 3 hours, or 3 seconds to a LocalDateTime object called dateTime

Answer 64

dateTime = dateTime.minusDays(3); dateTime = dateTime.minusHours(3); dateTime = dateTime.minusSeconds(3);

Question 65

Period Object

Answer 65

An objet that can hold a certain period of time Period annually = Period.ofYears(1); // every 1 year Period quarterly = Period.ofMonths(3); // every 3 months Period everyThreeWeeks = Period.ofWeeks(3); // every 3 weeks Period everyOtherDay = Period.ofDays(2); // every 2 days Period everyYearAndAWeek = Period.of(1, 0, 7); // every year and 7 days

Question 66

How to add a Period called period to a LocalDate object called date

Answer 66

date = date.plus(period);

Question 67

Can you chain Period methods?

Answer 67

no! Period wrong = Period.ofYears(1).ofWeeks(1); is the same as Period wrong = Period.ofYears(1); wrong = Period.ofWeeks(1);

Question 68

What package is the class DateTimeFormatter in?

Answer 68

java.time.format

Question 69

DateTimeFormatter f = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT) Tell whether the following are legal and what they output 1) f.format(localDate); 2) f.format(localDateTime); 3) f.format(localTime);

Answer 69

1) Legal, shows whole object localDate 2) Legal, shows just the date of localDatetime 3) Throws runtime exception

Question 70

DateTimeFormatter f = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT) Tell whether the following are legal and what they output 1) f.format(localDate); 2) f.format(localDateTime); 3) f.format(localTime);

Answer 70

1) Throws runtime exception 2) Legal shows whole localDateTime object 3) Throws runtime exception

Question 71

DateTimeFormatter f = DateTimeFormatter.ofLocalizedTime(FormatStyle.SHORT) Tell whether the following are legal and what they output 1) f.format(localDate); 2) f.format(localDateTime); 3) f.format(localTime);

Answer 71

1) Throws runtime exception 2) Legal shows only time part 3) Legal shows whole localTime object

Question 72

What method do you use to create your own DateTimeFormatter pattern?

Answer 72

ofPattern()

Question 73

When specifying a DateTimeFormatter pattern, what do the following mean? 1) M 2) MM 3) MMM 4) MMMM 5) d 6) dd 7) yy 8) yyyy 9) h 10) hh 11) m 12) mm

Answer 73

1) Represents month as a single digit (or two if 10-12) 2) Represents month as two digits 3) Represents the three letter abbreviation of the month 4) Represents the entire month name 5) Represents day with its default amount of digits 6) Represents day as a two digit number 7) Represents year as a two digit number 8) Represents year as a four digit number 9) Represents hour as its default amount of digits 10) Represents hour as a two digit number 11) Represents minutes as default amount of digits 12) Represents minutes as two digits

Question 74

DateTimeFormatter f = DateTimeFormatter.ofPattern("MM dd yyyy"); LocalDate date = LocalDate.parse("01 02 2015", f); LocalTime time = LocalTime.parse("11:22");

Answer 74

Parses date using the formatter f Then parses time using the default formatter for time

Question 75

Difference between calling equals() on a String and calling equals() on a StringBuilder

Answer 75

``` Calling equals() on String objects will check whether the sequence of characters is the same. Calling equals() on StringBuilder objects will check whether they are pointing to the same object rather than looking at the values inside. ```

Question 76

Difference between calling equals() on an array and calling equals() on an ArrayList

Answer 76

Calling equals() on an array checks object reference equality Calling equals() on an ArrayList checks to see if the ArrayLists have the same elements in the same order

Question 77

Can you append a boolean value to a String?

Answer 77

yup