Can we create a HashMap key Singleton?

It's now clear that you can't use the Singleton class

Singleton class

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one "single" instance. This is useful when exactly one object is needed to coordinate actions across the system.

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Singleton pattern - Wikipedia

in a HashMap as a key. Even If you override the HashCode Object as a key is stored as a reference in Map.

Can we create HashMap key immutable or mutable will be better?

Both hashcode and equals method are used in put and get method of HashMap. You need to make sure you can always get the value object from the map after you put it with the key object. No matter you change the key object or not. But Immutable object is good enough to achieve that.

Can you instantiate a singleton?

We can distinguish a Singleton class from the usual classes with respect to the process of instantiating the object of the class. To instantiate a normal class, we use a java constructor. On the other hand, to instantiate a singleton class, we use the getInstance() method.

Can we create object of Singleton class?

It falls under the category of the creational design pattern in Java. A Singleton class in Java allows only one instance to be created and provides global access to all other classes through this single object or instance.

Can we use custom key in HashMap?

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

How to use User defined Object as key in HashMap with an example

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.

Can we override key in HashMap?

Implementation: HashMap implements Map interface and HashSet implements Set interface. Duplicates: HashSet doesn't allow duplicate values. HashMap stores key, value pairs and it does not allow duplicate keys.

Can we inherit singleton class in Java?

You cannot inherit or instantiate a static class. Unlike static classes, Singleton classes can be inherited, can have base class, can be serialized and can implement interfaces. You can implement Dispose method in your Singleton class. So, static classes are much less flexible compared to Singleton classes.

Can we serialize singleton class in Java?

Suppose if you serialize the Singleton class, and then again de-serialize that object, it will create a new instance, hence deserialization will break the Singleton pattern. The below code is used to illustrate how the Singleton pattern breaks with deserialization. Implements Serializable interface for Singleton Class.

Are singletons thread safe?

Is singleton thread safe? A singleton class itself is not thread safe. Multiple threads can access the singleton same time and create multiple objects, violating the singleton concept. The singleton may also return a reference to a partially initialized object.

Is a static class a singleton?

A Singleton can implement interfaces, inherit from other classes and allow inheritance. While a static class cannot inherit their instance members. So Singleton is more flexible than static classes and can maintain state. A Singleton can be initialized lazily or asynchronously and loaded automatically by the .

What is the best way to create singleton class in Java?

Eager initialization:

In eager initialization, the instance of Singleton Class is created at the time of class loading, this is the easiest method to create a Singleton class. By making the constructor as private you are not allowing other class to create a new instance of the class you want to create the Singleton.

How do you create Singleton design pattern?

What if HashMap key is mutable?

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.

Why String is good key for HashMap?

String is as a key of the HashMap

When you pass the key to retrieve its value, the hash code is calculated again, and the value in the position represented by the hash code is fetched (if both hash codes are equal).

What is difference between ConcurrentHashMap and HashMap?

ConcurrentHashMap is thread safe. HashMap iterator is fail-fast and ArrayList throws ConcurrentModificationException if concurrent modification happens during iteration. ConcurrentHashMap is fail-safe and it will never throw ConcurrentModificationException during iteration. HashMap allows key and value to be null.

Can we create a clone of a singleton object?

We can clone singleton object by using Object class clone method.. to make it More Secure. But you can override clone method, throw CloneNotSupportedException inside that so that anyother object can not be created even by cloning.

Can singleton class be cloned?

clone method is protected and you cannot subclass a singleton class.

Can singleton implement Serializable?

Serialization:- Serialization can also cause breakage of singleton property of singleton classes. Serialization is used to convert an object of byte stream and save in a file or send over a network. Suppose you serialize an object of a singleton class.

Can we subclass singleton class?

I was asked if a singleton class can be subclassed in any way? This was my answer. Yes. A singleton class has a private constructor which is not accessible to any other class inside the same package or outside.

How many ways can you break a singleton class in Java?

There are mainly 3 concepts that can break the singleton property of a class.

Can we create immutable class in Java?

In Java, when we create an object of an immutable class, we cannot change its value. For example, String is an immutable class. Hence, we cannot change the content of a string once created. Besides, we can also create our own custom immutable classes.

Why HashMap is faster than hash table?

HashMap is faster than Hashtable due to the fact that Hashtable implicitly checks for synchronization on each method call even in a single thread environment. HashMap allows storing null values, while Hashtable doesn't. HashMap can be iterated by an Iterator which is considered as fail-fast .

Can we have duplicate keys in a hash?

it can have duplicate values but not keys.

Can HashMap have null key?

HashMap allows one null key and multiple null values whereas Hashtable doesn't allow any null key or value. HashMap is generally preferred over HashTable if thread synchronization is not needed.