CollectionsFramework

How to retrieve all keys from a Map?

• map.entries()
• map.keySet()
• map.getKeys()
• map.getAll()
• map.getKeysList()

Answer explanation

map.keySet() returns a Set of keys.

Set<Integer> s = new HashSet<>(); s.add(1); s.add(1); s.size(); // What is the size?

• 0
• 1
• 2
• Exception
• Depends on JVM

Answer explanation

Set doesn't allow duplicates. Only one element remains.

Stack is which type of structure?

• FIFO
• LIFO
• Tree
• Graph
• Heap

Answer explanation

Stack is Last-In-First-Out.

TreeSet stores elements in which order?

• Insertion order
• No order
• Natural order
• Reverse order
• Hash order

Answer explanation

TreeSet sorts elements in natural order.

What does ListIterator allow?

• Only forward iteration
• Only reverse
• Forward and backward
• No iteration
• Only last element

Answer explanation

ListIterator supports bi-directional traversal.

What does poll() return when queue is empty?

• Exception
• null
• 0
• -1
• head

Answer explanation

poll() returns null if queue is empty.

What does removeIf() method do?

• Sort list
• Remove last element
• Remove conditionally
• Add element
• Clear all

Answer explanation

removeIf() removes items matching predicate.

What does subList(1, 3) return for list [10, 20, 30, 40]?

• [10, 20]
• [20, 30]
• [30, 40]
• [10, 30]
• [20, 40]

Answer explanation

subList includes index 1 and 2 (end index excluded).

What happens if you call get() on a key not in the Map?

• Throws Exception
• Returns null
• Returns 0
• Creates key
• Undefined

Answer explanation

Map returns null if key does not exist.

What happens when we access an invalid index in a List?

• Returns null
• Throws NullPointerException
• Throws IndexOutOfBoundsException
• Skips it
• Creates space

Answer explanation

Accessing invalid index in a list throws IndexOutOfBoundsException.