Can we use class as key in HashMap?

Answer to your question is yes, objects of custom classes can be used as a key in a HashMap.

Can one use an employee class as a key in a HashMap What steps will you take?

employee1 and employee2's generated hashcode will be different, and two Employee objects will be present in the HashMap. By making the Employee class implement the Object class's equals and hashcode methods, both the employee1 and employee2 hashcode and equals methods will return the same thing.

What classes should we prefer to use a key in HashMap?

Since the String class is immutable, you cannot modify the value of a String once it is created. Therefore, it is recommended to use a String variable to hold keys in hash a map.

Can we use int as key in HashMap?

HashMap doesn't handle primitives, just objects. Related SO question, but with int being the value, not the key.

What will happen if class is mutable and used as key in HashMap?

If key's hash code changes after the key-value pair (Entry) is stored in HashMap, the map will not be able to retrieve the Entry. Key's hashcode can change if the key object is mutable. Mutable keys in HahsMap can result in data loss.

How to make Your Custom Class or Object as key in HashMap| Core Java Interview Question | 2020

Can we use StringBuffer as key in HashMap?

No, since StringBuffer overrides neither equals nor hashCode , so it is not suitable as a HashMap key (recall that HashMap relies on those two methods to tell if a given key is present in the map). Beyond that, StringBuffer s are mutable, and you typically want Map keys to be immutable.

What happens if we do not override hashCode () and equals () in HashMap?

You must override hashCode in every class that overrides equals. Failure to do so will result in a violation of the general contract for Object. hashCode, which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.

Can HashMap have duplicate keys?

HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.

Why string is mostly used as a key in HashMap class?

As we discuss above String is immutable, its hashcode is cached at the time of creation and it doesn t need to be calculated again. This makes it a great candidate for key in a Map and it s processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

Can we use primitive data type in HashMap?

We can't use primitive types because of a restriction around the way generics were designed. A HashMap allows one null key and multiple null values. It doesn't preserve the order of the elements and doesn't guarantee the order will remain the same over time.

Is a class is used for a String as a key to store the value in object?

Which of these is a class which uses String as a key to store the value in object? Explanation: None.

Can we create our own HashMap class in Java?

If we wish to create a HashMap of our own class, we need to ensure that the hashcode() of the key of HashMap doesn't change as if it happens then it is impossible to get object value of the key from HashMap. On runtime, JVM processes hash code for each object and give it on interest.

What is difference between ConcurrentHashMap and HashMap?

HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously.

Can we store objects in HashMap?

Internal working. All instances of Entry class are stored in an array declard as 'transient Entry[] table' . For each key-value to be stored in HashMap, a hash value is calculated using the key's hash code. This hash value is used to calculate the index in the array for storing Entry object.

Can we use custom object as key in TreeMap?

As TreeMap sorting is based only on the keys, I'm using a custom object as key in a treemap. I have respected in my opinion the contract between equals and compareTo in this case, if the two objects are equal, te compareTo returns 0.

Why we need to override hashCode and equals method?

In order to use our own class objects as keys in collections like HashMap, Hashtable etc.. , we should override both methods ( hashCode() and equals() ) by having an awareness on internal working of collection. Otherwise, it leads to wrong results which we are not expected.

Can we add object as key in Map?

Yes, we can use any object as key in a Map in java but we need to override the equals() and hashCode() methods of that object class.

Why is the HashMap key preferred to be immutable?

Make HashMap key object immutable

Immutability allows you to get same hash code every time, for a key object. So it actually solves most of the problems in one go. Also, this class must honor the hashCode() and equals() methods contract.

Why are strings immutable in Java?

The String is immutable in Java because of the security, synchronization and concurrency, caching, and class loading. The reason of making string final is to destroy the immutability and to not allow others to extend it. The String objects are cached in the String pool, and it makes the String immutable.

Why HashMap is faster than Hashtable?

HashMap is not synchronized, therefore it's faster and uses less memory than Hashtable. Generally, unsynchronized objects are faster than synchronized ones in a single threaded application.

Can we put null as key in HashMap?

HashMap allows one null key and multiple null values whereas Hashtable doesn't allow any null key or value.

Why HashMap is faster than HashSet?

HashMap is faster than HashSet because the values are associated to a unique key. In HashSet , member object is used for calculating hashcode value which can be same for two objects so equals() method is used to check for equality.

Can we override hashCode without equals?

Please note that even though equal objects must have equal hash codes, the reverse is not true. It is perfectly valid to override hashCode() without overriding equals() as objects with equal hash codes need not be equal. That's all about why we need to override equals and hashcode methods in Java.

What will happen if two different objects have the same hashCode?

Whenever two different objects have the same hash code, we call this a collision. A collision is nothing critical, it just means that there is more than one object in a single bucket, so a HashMap lookup has to look again to find the right object.

What happens if hashCode returns constant?

Returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode(). The hash code for the null reference is zero. So even if the hashcode() is overridden, it should not effect it.