Chapter 4: Core APIs

Question 1

LocalDate belongs to the java.time package. True or False?

Answer 1

True

Question 2

Which package does DateTimeFormatter belong to?

Answer 2

java.time.format

Question 3

What does the put method do in a Java Map implementation?

Answer 3

The put method associates a specified key with a specified value in the map, replacing any existing value for that key.

Question 4

What does the put method return on Hashmap?

Answer 4

It returns the previous value associated with the key, or null if there was no mapping (or if null was previously mapped to the key).

Question 5

What happens if the key already exists in the Hashmap when calling put?

Answer 5

The old value is replaced with the new value, and the method returns the previous value.

Question 6

What is the role of putVal(hash(key), key, value, false, true) in the put method in Hashmap?

Answer 6

It is an internal method that handles the actual insertion logic, including hashing, bucket assignment, and collision resolution.

Question 7

Can put return null even if the key was previously mapped for hashmap?

Answer 7

Yes, if the previous mapping was explicitly set to null, the method will return null.

Question 8

Can a HashMap contain null keys and values?

Answer 8

Yes, HashMap allows one null key and multiple null values.

Question 9

Does Hashtable allow null keys or values?

Answer 9

No, Hashtable does not allow null keys or null values.

Question 10

How do you declare and initialize a String array in Java?

Answer 10

String[] arr = {“Java”, “17”, “Arrays”};
or
String[] arr = new String[]{“Java”, “17”, “Arrays”};

Question 11

Q: What is the default value of elements in a String[] array when using new String[5]?
Example: String[] arr = new String[5];

Answer 11

All elements are initialized to null.

Question 12

What is Function<’T, R’>?

Answer 12

✔ Function<T, R> is a functional interface in java.util.function.
✔ Represents a function that takes one argument of type T and returns a result of type R.

Question 13

Why is @FunctionalInterface used?

Answer 13

✔ Ensures the interface has exactly one abstract method (apply(T t)).
✔ Allows usage in lambda expressions and method references.

Question 14

How can Function<T, R> be composed?

Answer 14

✔ Supports function composition using andThen() and compose().

Function<Integer, Integer> multiplyBy2 = x -> x * 2;
Function<Integer, Integer> add3 = x -> x + 3;

Function<Integer, Integer> combined = multiplyBy2.andThen(add3);
System.out.println(combined.apply(5)); // (5 * 2) + 3 = 13

📌 andThen(f) applies this first, then f.
📌 compose(f) applies f first, then this.

Question 15

What does the java.time.Instant class represent in Java?

Answer 15

Instant represents a specific moment on the time-line in UTC, with nanosecond precision. It is the closest equivalent to java.util.Date in the java.time API.

Question 16

Is the Instant class mutable, and why?

Answer 16

No, Instant is immutable. This design ensures thread safety and avoids bugs from unintended modifications, making it safe for use in concurrent applications.

Question 17

How do you get the current time and add time to an Instant?

Answer 17

Use Instant.now() to get the current moment. Use methods like plusSeconds(), minusMillis(), etc., to return new Instant objects with adjusted times.

Question 18

What does the strip() method do in the String class?

Answer 18

It removes leading and trailing whitespace using Unicode-aware rules.

Question 19

How is strip() different from trim() in Java?

Answer 19

strip() uses Unicode whitespace rules, while trim() only removes characters ≤ U+0020 (ASCII whitespace).

Question 20

What is the output of “ \u2003Hello\u2003 “.strip()? (\u2003 = em space)

Answer 20

“Hello” — strip() removes Unicode spaces like \u2003.

Question 21

What does the isEmpty() method check in a String?

Answer 21

It returns true if the string has length 0, otherwise false.

Question 22

What is the result of “ “.isEmpty()?

Answer 22

false — because it contains a space character and length is not 0.

Question 23

How can you check if a string is empty or only contains whitespace?

Answer 23

Use s.strip().isEmpty() — this strips whitespace and then checks for emptiness.

Question 24

What does the isBlank() method return?

Answer 24

It returns true if the string is empty or contains only whitespace characters (including Unicode whitespace).

Question 25

What is the difference between isEmpty() and isBlank()?

Answer 25

isEmpty() checks for length 0 only, while isBlank() returns true for strings that are either empty or contain only whitespace.

Question 26

What is the result of " \t\n ".isBlank()?

Answer 26

true — it contains only whitespace characters (spaces, tabs, newlines).

Question 27

What is the default format of LocalDateTime.toString() in Java?

Answer 27

ISO 8601 format: ```"yyyy-MM-dd'T'HH:mm:ss"``` Example: "2025-04-10T15:30:00"

Question 28

How do you parse an ISO 8601 date-time string into a LocalDateTime?

Answer 28

LocalDateTime dt = LocalDateTime.parse("2025-04-10T15:30:00");

Question 29

What is ISO 8601 format used for in Java date-time APIs?

Answer 29

It’s the standard format for representing date and time in a machine-readable way, used by default in most of the Java Time API (LocalDate, LocalDateTime, Instant, etc.).

Question 30

What is the relationship between CharSequence and String?

Answer 30

String implements the CharSequence interface.

Question 31

Why would you use CharSequence instead of String in a method parameter?

Answer 31

To allow the method to accept any character sequence type like String, StringBuilder, or StringBuffer.

Question 32

Name three classes that implement the CharSequence interface.

Answer 32

String, StringBuilder, and StringBuffer.

Question 33

If String is a subtype of CharSequence, is ```List``` a subtype of ```List```?

Answer 33

No — Java generics are invariant, so ```List``` is not a subtype of ```List```.

Question 34

What does it mean that Java generics are invariant?

Answer 34

It means that ```Generic``` and ```Generic``` are not related types, even if A is a subtype of B.

Question 35

Can you override a method returning ```List``` with one returning ```List```?

Answer 35

No — because ```List``` is not a covariant return of ```List``` due to generic invariance.

Question 36

What must be true about the array before calling binarySearch(Object[] a, Object key)?

Answer 36

The array must be sorted in ascending order according to the natural ordering of its elements.

Question 37

What happens if the array passed to binarySearch() is not sorted?

Answer 37

The results are undefined — there is no guarantee of correct behavior.

Question 38

What does binarySearch() return if the key is not found in the array?

Answer 38

It returns (-(insertion point) - 1), where the insertion point is where the key would fit into the array.

Question 39

In System.out.println(a[(a = b)[3]]);, which version of a is used to access the array?

Answer 39

The original a is used because the array reference is evaluated before the assignment (a = b) happens.

Question 40

In compound expressions like a[(a = b)[i]], when does the assignment a = b take effect?

Answer 40

The assignment happens during index evaluation, but after the array reference a[...] is already resolved.

Question 41

How can reversing the assignment change the behavior? (e.g. (a = b)[a[3]])

Answer 41

Now a = b happens before the index access, so both the array and the index come from b, not the original a.

Question 42

Why does a text block like ```s3 = """Hello World """``` compare equal to a string like s4 = "Hello World\n"?

Answer 42

Because when a text block ends with the closing """ on its own line, Java automatically inserts a newline (\n) at the end of the string. So even though \n isn’t typed explicitly, it’s part of the string content.