APIs

Question 1

Does calling a method change a String?

ex.: “1234”.replace(‘1’, ‘9’);

Answer 1

No.

Strings are immutable. Calling methods does not change the object, it creates a new String object.

Question 2

Can you extend String class?

Answer 2

No.

Not because it is immutable, but because it is final.

Question 3

Can StringBuffer or StringBuilder be extended?

Answer 3

No.

They are final classes.

Question 4

Can Wrapper classes be extended?

Answer 4

No.

They are final classes.

Question 5

Does the String class have a method called reverse?

Answer 5

No.

But StringBuilder and StringBuffer do.

Question 6

How do you represent just a date with no time zone information?

Answer 6

java.time.LocalDate

Question 7

What are common classes of the java.time package?

Answer 7

LocalDate, LocalTime, LocalDateTime, Instant, Period, Duration are part of this package.

Question 8

How do you get time zone information?

Answer 8

Use ZonedDateTime

Question 9

How do you format Date objects?

Answer 9

java.time.format.DateTimeFormatter

LocalDate d1 = LocalDate.parse(“2015-01-01”, DateTimeFormatter.ISO_LOCAL_DATE);

Question 10

If using ArrayList API, what happens if the fromIndex and toIndex arguments of a subList ( ) method are equal?

Answer 10

The subList returns an empty list.

List foo - new ArrayList( );
foo.add("f");
foo.add("o");
foo.add("o");
List bar = new ArrayList(  foo.subList(1, 1) );

Question 11

T/F: Literal strings within the same class in the same package represent references to the same String object.

Answer 11

True.

Question 12

T/F: Literal strings within different classes in the same package represent references to the same String object.

Answer 12

True.

Question 13

When do Strings that are computed by constant expressions actually get computed?

Answer 13

At compile time.

Question 14

How are Strings computed by constant expressions treated?

Answer 14

When computed at compile time, they are treated as if they were literals.

Question 15

What does the java string intern( ) method do?

Answer 15

It returns the interned (canonical representation) of string.

Question 16

What is the result of explicitly interning a computed string?

Answer 16

It is the same string as any pre-existing literal string with the same contents.

Question 17

T/F: Strings computed at run time are the same as any pre-existing String object with the same contents.

Answer 17

False.

Strings computed at run time are newly created and therefore are distinct.

Question 18

What will the following code print?

String abc = "";
abc.concat("abc");
abc.concat("def");
System.out.print(abc);

Answer 18

It will print an empty string.

Question 19

For the String class, what does the following return?

String substring(int beginIndex)

Answer 19

This returns a new string that is a substring of this string.

Question 20

For the String class, what does the following return?

String substring(int beginIndex, int endIndex)

Answer 20

Returns a new string that is a substring of this string.

Question 21

For the String class, what does the following return?

int indexOf(int ch)

Answer 21

Returns the index within this string of the first occurrence of the specified character.

Question 22

For the String class, what does the following return?

int indexOf(int ch, int fromIndex)

Answer 22

Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

Question 23

For the String class, what does the following return?

int indexOf(String str, int fromIndex)

Answer 23

Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.

Question 24

For the String class, what does the following return?

int indexOf(String str)

Answer 24

Returns the index within this string of the first occurrence of the specified substring.
int indexOf(String str, int fromIndex)

Question 25

Can String be extended?

Answer 25

No.

It is final.

Question 26

Does StringBuilder extend String?

Answer 26

No, it extends Object.

Question 27

What does the following code return?

“hamburger”.substring(4, 8);

Answer 27

urge

Question 28

What does the following code return?

“smiles”.substring(1, 5)

Answer 28

mile

Question 29

What does the following code return?

“unhappy”.substring(2)

Answer 29

happy

Question 30

What does the following code return?

“emptiness”.substring(9)

Answer 30

”” (an empty string)

Question 31

What does the following code return?

“Harbison”.substring(3)

Answer 31

bison

Question 32

What will the third line print?

String s = “fancy-“;
s.append(“pants”);
System.out.println(s);

Answer 32

It won’t compile.

There is no append( ) method for String.

Question 33

Is the following valid syntax?

String String = “String”;

Answer 33

Oddly enough, yes.

There are no restrictions on naming variables the same as classes.