Chapter 3: Core Java APIs

Question 1

What operation is performed?

“string” + 10

Answer 1

concatenation

Question 2

What is the result of this expression?

10 + 3 + “” + 3 + 5

Answer 2

1335

Question 3

Define immutability

Answer 3

An object is immutable if it cannot be change after being created

Question 4

Which of the following are immutable?

int
String
Integer

Answer 4

String and Integer

Question 5

What strings go in the string pool?

Answer 5

String literal values that appear in the program

Question 6

How do you explicitly create a string that doesn’t go in the string pool?

Answer 6

new String(“string”)

Question 7

T or F: If s is a String, the following line would convert all characters in s to upper case

s.toUpperCase()

Answer 7

False. Strings are immutable. toUpperCase() returns a new string and that reference needs to be stored.

s = s.toUpperCase()

Question 8

String e = “example”;
String s = e.substring(1,4)

What is the value of s?

Answer 8

“xam”. index 1 is included (x) and index 4 is excluded (p)

Question 9

String s1 = “test”;
String s2 = “test”;

boolean b = s1 == s2;

What is the value of b?

Answer 9

True. Both strings are literals, so they point to the same object in the string pool

Question 10

String s1 = "test";
String s2 = new String("test");

boolean b = s1 == s2;

What is the value of b?

Answer 10

False. s1 is a literal and is stored in the string pool, s2 is not. Therefore, they are referencing different objects.

Question 11

String s1 = "test";
String s2 = new String("test");

boolean b = s1.equals(s2);

What is the value of b?

Answer 11

True. The contents of both strings are the same despite them being different objects.

Question 12

What can you use if you want a mutable representation of a string?

Answer 12

StringBuilder

Question 13

Why is StringBuilder more efficient in cases with lots of concatenation than String?

Answer 13

StringBuilder doesn’t store interim String values whereas every time String is changed, a new object is created

Question 14

T or F. This is a valid array declaration:

int arr[];

Answer 14

True. The brackets can come before or after the name of the array and there can be a space as well.

Question 15

T or F. .length can be used to find how many of an array’s slots are full.

Answer 15

False. .length is the total number of spaces allocated to the array

Question 16

T or F. When an array runs out of space, it’s size can be increased?

Answer 16

False. Arrays have a fixed size. A new array would need to be created.

Question 17

What happens when binarySearch is run on an unsorted array?

Answer 17

The result is unpredictable

Question 18

What happens when a binary search can’t find a value in an array?

Answer 18

It returns the negative of the index that value would have if it was in the array

Question 19

What is the correct syntax for varargs?

Answer 19

datatype… name

Question 20

T or F. This is a valid definition for a multidimensional array.

int [] arr [];

Answer 20

True

Question 21

T or F. The following is a valid array initialization.

int[][] arr = new int[][];

Answer 21

False. The size of the inner array needs to be specified.

new int[5][]

Question 22

How would you check if two different ArrayList objects are equal?

Answer 22

Use the equals method

Question 23

How would you check if two ArrayList reference variables refer to the same object?

Answer 23

Use == to compare the reference variables

Question 24

What is the return type of parseInt() ?

Answer 24

int

Question 25

What is the return type of valueOf(“9”) ?

Answer 25

Integer

Question 26

T or F. LocalDate is mutable

Answer 26

False

Question 27

What class would you use for an object that stores the date, the time, but not the time zone?

Answer 27

LocalDateTime

Question 28

What method of LocalDate gets the current date?

Answer 28

LocalDate.now()

Question 29

What method of LocalDate allows you to specify the date?

Answer 29

LocalDate.of()

Question 30

What is the Period class used for?

Answer 30

Specifying a time interval

Question 31

When creating a Period, can you chain multiple ‘of’ calls?

Answer 31

No. The last ‘of’ call overwrites the previous one.

Question 32

What method is used to create a custom DateTimeFormatter?

Answer 32

DateTimeFormatter.ofPattern()