SCJP Java Collections Framework 4

Question 1

What happens when you call `TreeMap.subMap(fromKey, toKey)` and then try to insert a key outside that range into the returned view?

Accepted Answer

An IllegalArgumentException is thrown

Answer

Inserting a key outside the range of a subMap view throws IllegalArgumentException because the view enforces its boundary constraints.

Question 2

Which statement is true about `LinkedHashMap`?

Accepted Answer

It maintains insertion order by default

Answer

LinkedHashMap maintains insertion order (or optionally access order) by using a doubly-linked list alongside the hash table.

Question 3

Given `List<String> list = Arrays.asList("a", "b", "c");`, what happens when you call `list.add("d")`?

Accepted Answer

An UnsupportedOperationException is thrown

Answer

Arrays.asList() returns a fixed-size list backed by the array; structural modifications like add() throw UnsupportedOperationException.

Question 4

Which collection class is best suited for implementing a LIFO (Last-In-First-Out) stack in modern Java?

Accepted Answer

java.util.ArrayDeque

Answer

ArrayDeque is the preferred stack implementation because it is faster than Stack (which is synchronized) and more memory-efficient than LinkedList.

Question 5

What is the contract between `equals()` and `hashCode()` that must be satisfied for objects used as HashMap keys?

Accepted Answer

If a.equals(b) is true, a.hashCode() must equal b.hashCode()

Answer

The contract requires that equal objects must have the same hash code; unequal objects may share hash codes (collisions are allowed).

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