Chapter 3: Core Java APIs Flashcards
What operation is performed?
“string” + 10
concatenation
What is the result of this expression?
10 + 3 + “” + 3 + 5
1335
Define immutability
An object is immutable if it cannot be change after being created
Which of the following are immutable?
int
String
Integer
String and Integer
What strings go in the string pool?
String literal values that appear in the program
How do you explicitly create a string that doesn’t go in the string pool?
new String(“string”)
T or F: If s is a String, the following line would convert all characters in s to upper case
s.toUpperCase()
False. Strings are immutable. toUpperCase() returns a new string and that reference needs to be stored.
s = s.toUpperCase()
String e = “example”;
String s = e.substring(1,4)
What is the value of s?
“xam”. index 1 is included (x) and index 4 is excluded (p)
String s1 = “test”;
String s2 = “test”;
boolean b = s1 == s2;
What is the value of b?
True. Both strings are literals, so they point to the same object in the string pool
String s1 = "test";
String s2 = new String("test");
boolean b = s1 == s2;
What is the value of b?
False. s1 is a literal and is stored in the string pool, s2 is not. Therefore, they are referencing different objects.
String s1 = "test";
String s2 = new String("test");
boolean b = s1.equals(s2);
What is the value of b?
True. The contents of both strings are the same despite them being different objects.
What can you use if you want a mutable representation of a string?
StringBuilder
Why is StringBuilder more efficient in cases with lots of concatenation than String?
StringBuilder doesn’t store interim String values whereas every time String is changed, a new object is created
T or F. This is a valid array declaration:
int arr[];
True. The brackets can come before or after the name of the array and there can be a space as well.
T or F. .length can be used to find how many of an array’s slots are full.
False. .length is the total number of spaces allocated to the array
T or F. When an array runs out of space, it’s size can be increased?
False. Arrays have a fixed size. A new array would need to be created.
What happens when binarySearch is run on an unsorted array?
The result is unpredictable
What happens when a binary search can’t find a value in an array?
It returns the negative of the index that value would have if it was in the array
What is the correct syntax for varargs?
datatype… name
T or F. This is a valid definition for a multidimensional array.
int [] arr [];
True
T or F. The following is a valid array initialization.
int[][] arr = new int[][];
False. The size of the inner array needs to be specified.
new int[5][]
How would you check if two different ArrayList objects are equal?
Use the equals method
How would you check if two ArrayList reference variables refer to the same object?
Use == to compare the reference variables
What is the return type of parseInt() ?
int
What is the return type of valueOf(“9”) ?
Integer
T or F. LocalDate is mutable
False
What class would you use for an object that stores the date, the time, but not the time zone?
LocalDateTime
What method of LocalDate gets the current date?
LocalDate.now()
What method of LocalDate allows you to specify the date?
LocalDate.of()
What is the Period class used for?
Specifying a time interval
When creating a Period, can you chain multiple ‘of’ calls?
No. The last ‘of’ call overwrites the previous one.
What method is used to create a custom DateTimeFormatter?
DateTimeFormatter.ofPattern()
