Table of Contents

## How can I get map value without iterating?

You can use the toArray method of the Set and Collections returned by the keySet and values method respectively of the HashMap class. You can then get the first element of an array to get one key or value using the index as 0.

**How can I get one entry from a HashMap without iterating?**

Is there a elegant way of obtaining only one Entry from HashMap, without iterating, if key is not known. hashMapObject. get(zeroth_index);

**What is entrySet in Java?**

entrySet() method in Java is used to create a set out of the same elements contained in the hash map. It basically returns a set view of the hash map or we can create a new set and store the map elements into them. Syntax: hash_map.entrySet()

### How do I find the value of a key?

- import java. util. HashMap; import java. util. Map;
- // Program to get Map’s key from the value in Java.
- class Main. {
- public static K getKey(Map map, V value) {
- for (K key: map. keySet()) {
- if (value. equals(map. get(key))) { return key;
- } }
- return null; }

**How do I insert a HashMap?**

put() method of HashMap is used to insert a mapping into a map. This means we can insert a specific key and the value it is mapping to into a particular map. If an existing key is passed then the previous value gets replaced by the new value. If a new pair is passed, then the pair gets inserted as a whole.

**Why do we use HashMap?**

HashMap are efficient for locating a value based on a key and inserting and deleting values based on a key. The entries of a HashMap are not ordered. ArrayList and LinkedList are an implementation of the List interface.

## How does a HashMap work?

A HashMap is a map used to store mappings of key-value pairs. HashMap in Java works on hashing principles. It is a data structure which allows us to store object and retrieve it in constant time O(1) provided we know the key. In hashing, hash functions are used to link key and value in HashMap.

**What would happen if I only override hashCode?**

Only Override HashCode, Use the default Equals: Only the references to the same object will return true. In other words, those objects you expected to be equal will not be equal by calling the equals method. Only Override Equals, Use the default HashCode: There might be duplicates in the HashMap or HashSet.

**What happens if we don’t override hashCode method?**

If you don’t override hashcode() then the default implementation in Object class will be used by collections. This implementation gives different values for different objects, even if they are equal according to the equals() method.

### Can hashCode return negative value?

In Java, we hash objects with a hashCode() method that returns an integer (32 bit) representation of the object. hashCode() to index conversion. To use hashCode() results as an index, we must convert the hashCode() to a valid index. Modulus does not work since hashCode may be negative.

**What happens if Hashcode returns same value?**

When two key return same hashcode, they end up in the same bucket. Now, in order to find the correct value, you used keys. equals() method to compare with key stored in each Entry of linked list there.

**How do I override Hashcode?**

Overriding hashCode method in Java

- Take a prime hash e.g. 5, 7, 17 or 31 (prime number as hash, results in distinct hashcode for distinct object)
- Take another prime as multiplier different than hash is good.
- Compute hashcode for each member and add them into final hash.
- Return hash.

## Why is 31 used in Hashcode?

The value 31 was chosen because it is an odd prime. If it were even and the multiplication overflowed, information would be lost, as multiplication by 2 is equivalent to shifting. The advantage of using a prime is less clear, but it is traditional.

**What is the importance of hashCode () and equals () methods?**

The hashcode() method returns the same hash value when called on two objects, which are equal according to the equals() method. And if the objects are unequal, it usually returns different hash values.

**Is hashCode unique in Java?**

Hashcode is a unique code generated by the JVM at time of object creation. It can be used to perform some operation on hashing related algorithm like hashtable, hashmap etc. An object can also be searched with this unique code. Returns: It returns an integer value which represents hashCode value for this Method.

### Why multiply hashCode with prime number?

Prime numbers are chosen to best distribute data among hash buckets. If the distribution of inputs is random and evenly spread, then the choice of the hash code/modulus does not matter. It only has an impact when there is a certain pattern to the inputs.