Java HashMap MCQ Questions and Answers

Q. What is the purpose of the 'equals' method in the HashMap key-value pairs?

A. To compare the values of two keys
B. To determine if two keys are equal
C. To compare the hash codes of two keys
D. To establish the order of keys in the HashMap
Check Answer

Q. What is the default initial capacity of a HashMap in Java?

A. 0
B. 10
C. 16
D. It depends on the JVM
Check Answer

Q. What is the default load factor of a HashMap in Java?

A. 0.25
B. 0.5
C. 0.75
D. 1.0
Check Answer

Q. What is the time complexity of inserting a key-value pair into a HashMap?

A. O(1)
B. O(log n)
C. O(n)
D. O(n log n)
Check Answer

Q. What is the time complexity of retrieving a value from a HashMap using its key?

A. O(1)
B. O(log n)
C. O(n)
D. O(n log n)
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>(); 
     map.put("one", 1); map.put("two", 2); map.put("three", 3); 
     System.out.println(map.get("two"));

A. 1
B. 2
C. 3
D. null
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>();
      map.put("one", 1);
       map.put("two", 2); map.put("one", 3);
        System.out.println(map.get("one"));

A. 1
B. 2
C. 3
D. null
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>(); 
    map.put("one", 1); map.put("two", 2);
     map.remove("one"); 
    System.out.println(map.containsKey("one"));

A. true
B. false
C. null
D. An error occurs
Check Answer

Q. What is the purpose of the 'hashCode' method in the HashMap key-value pairs?

A. To compare the values of two keys
B. To determine if two keys are equal
C. To generate a unique identifier for each key
D. To establish the order of keys in the HashMap
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>(); 
     map.put("one", 1); 
     map.put("two", 2); 
     map.put("three", 3); 
     System.out.println(map.keySet());

A. [one, two, three]
B. [one, two]
C. [one, three]
D. [two, three]
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>();
     map.put("one", 1); 
     map.put("two", 2);
      map.put("three", 3);
       System.out.println(map.values());

A. [1, 2, 3]
B. [1, 2]
C. [1, 3]
D. [2, 3]
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>();
      map.put("one", 1); 
      map.put("two", 2); 
      map.put("three", 3);
       System.out.println(map.containsKey("four"));

A. true
B. false
C. null
D. An error occurs
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>(); 
    map.put("one", 1);
     map.put("two", 2);
      map.put("three", 3); 
      System.out.println(map.containsValue(2));

A. true
B. false
C. null
D. An error occurs
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>();
     map.put("one", 1); 
     map.put("two", 2); 
     map.put("three", 3); 
     System.out.println(map.size());

A. 1
B. 2
C. 3
D. 0
Check Answer

Q. What is the output of the following code snippet?

HashMap<String, Integer> map = new HashMap<>(); 
    map.put("one", 1); 
    map.put("two", 2); 
    map.put("three", 3); 
    map.clear(); 
    System.out.println(map.size());

A. 0
B. 1
C. 2
D. 3
Check Answer

Q. What is the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);
map.put("three", 3);

System.out.println(map.get("two"));

A.

2

B.

null

C.

Exception: Key not found

D.

1

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);
map.put("one", 11);

System.out.println(map.get("one"));

A.

1

B.

11

C.

null

D.

Exception: Duplicate key

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.remove("one");

System.out.println(map.containsKey("one"));

A.

true

B.

false

C.

null

D.

Exception: Key not found

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.keySet().remove("one");

System.out.println(map.get("one"));

A.

1

B.

null

C.

Exception: Key not found

D.

Exception: Invalid operation

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.clear();

System.out.println(map.isEmpty());

A.

true

B.

false

C.

null

D.

Exception: Map is not empty

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.putIfAbsent("one", 11);

System.out.println(map.get("one"));

A.

1

B.

11

C.

null

D.

Exception: Duplicate key

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.computeIfAbsent("three", k -> 3);

System.out.println(map.get("three"));

A.

3

B.

null

C.

Exception: Key not found

D.

Exception: Invalid key

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.computeIfPresent("one", (k, v) -> v * 2);

System.out.println(map.get("one"));

A.

1

B.

2

C.

null

D.

Exception: Key not found

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.merge("one", 11, Integer::sum);

System.out.println(map.get("one"));

A.

12

B.

11

C.

1

D.

Exception: Duplicate key

Check Answer

Q. What will be the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

map.replaceAll((k, v) -> v * 2);

System.out.println(map.get("one"));
System.out.println(map.get("two"));

A.

2
4

B.

1
2

C.

null
null

D.

Exception: Invalid operation

Check Answer

Q. What does `getOrDefault` return when the key is missing?

A. null
B. The provided default value
C. Throws NullPointerException
D. The first entry in the map
Check Answer

Q. Which iteration order does HashMap guarantee?

A. Insertion order
B. Natural key order
C. No guaranteed order
D. Reverse insertion order
Check Answer

Q. How do you create a thread-safe HashMap alternative?

A. Collections.synchronizedMap(new HashMap<>())
B. new Hashtable<>()
C. new ConcurrentHashMap<>()
D. All of the above
Check Answer

Q. What happens when two keys have the same hashCode but are not equal?

A. One replaces the other
B. They go to the same bucket with equals used to distinguish
C. Map throws an exception
D. Map discards both entries
Check Answer

Q. Which method copies mappings from another Map?

A. put()
B. putAll()
C. merge()
D. clone()
Check Answer

Q. What is the default value returned by `remove(key)` when key is absent?

A. false
B. null
C. 0
D. Empty Optional
Check Answer

Q. Which Java version introduced tree bins to improve worst-case lookups?

A. Java 6
B. Java 7
C. Java 8
D. Java 11
Check Answer

Q. What does `clear()` do on a HashMap?

A. Removes all entries
B. Shrinks capacity to zero
C. Removes half the entries
D. Throws UnsupportedOperationException
Check Answer

Q. Which method checks for both key and expected value before removing?

A. remove(key)
B. remove(key, value)
C. compute()
D. replaceAll()
Check Answer

Q. What is the result of `putIfAbsent` when the key already exists?

A. Replaces the value
B. Leaves map unchanged and returns existing value
C. Removes the key
D. Throws IllegalStateException
Check Answer

Q. Which interface do HashMap keys and values generically implement?

A. Serializable
B. Cloneable
C. No interface is required; any Object works
D. Comparable
Check Answer

Q. What does `size()` return?

A. Current capacity
B. Number of entries
C. Load factor
D. Hash bucket count
Check Answer

Q. Which method replaces a value only if it is currently mapped?

A. replace(key, value)
B. put(key, value)
C. merge(key, value, fn)
D. computeIfAbsent(key, fn)
Check Answer

Q. How do you iterate over both keys and values at once?

A. for (String k : map.keySet())
B. for (Map.Entry e : map.entrySet())
C. map.iterator()
D. map.stream()
Check Answer

Q. Which call returns a Set of all keys?

A. map.values()
B. map.entrySet()
C. map.keySet()
D. map.keys()
Check Answer

Q. What does `containsValue` check?

A. Presence of a specific key
B. Presence of a specific value
C. Both key and value
D. Only null values
Check Answer

Q. What is returned by `clone()` on a HashMap?

A. Deep copy of keys/values
B. Shallow copy of the map structure
C. Empty map
D. Immutable copy
Check Answer

Q. Which statement about nulls is true for HashMap?

A. Allows one null key and multiple null values
B. Forbids null keys but allows null values
C. Forbids all nulls
D. Allows unlimited null keys
Check Answer

Q. Which operation can trigger a resize?

A. Adding entries beyond threshold
B. Calling size()
C. Calling keySet()
D. Calling containsKey()
Check Answer

Q. Which statement is true about fail-fast iterators of HashMap?

A. They never throw exceptions
B. They throw ConcurrentModificationException on structural change
C. They automatically synchronize modifications
D. They reorder entries to avoid conflicts
Check Answer