The java.util.HashMap is i of the workhorses of JDK. Along alongside ArrayList, it is i of the most used classes from Java's collection framework. There is hardly a existent footing Java project, where I haven't seen the utilisation of HashMap. It is an implementation of hash tabular array information construction together with it's non a surprise that HashMap is so useful, as individual has rightly said, "if y'all could receive got only i information structure, driblet dead far a hash table". The hash tabular array information construction allows y'all to search for a value inward O(1) fourth dimension if y'all receive got key. In Java, at that spot are several implementations of hash tabular array information construction exists e.g. Hashtable, ConcurrentHashMap, LinkedHashMap etc but HashMap is your full general utilisation map. Though, if y'all receive got a particular demand y'all tin utilisation other hash tabular array implementations available inward JDK.
For example, if y'all desire to save the gild of mapping so y'all tin consider using LinkedHashMap. If y'all desire to driblet dead on mappings sorted so y'all tin utilisation TreeMap, which is a sorted map implementation.Similarly, if y'all demand a hash tabular array implementation which is thread-safe together with tin live on used inward a concurrent application without compromising the Scalability so consider using ConcurrentHashMap.
How does put() method of HashMap plant inward Java? (answer)
The put() method of HashMap plant inward the regulation of hashing. It is responsible for storing an object into backend array. The hashcode() method is used inward conjunction alongside a hash role to honour the right location for the object into the bucket. If a collision occurs so the entry object which contains both key together with value is added to a linked listing together with that linked listing is stored into the bucket location.
What is the requirement for an object to live on used as key or value inward HashMap? (answer)
The key or value object must implement equals() together with hashcode() method. The hash code is used when y'all insert the key object into the map piece equals are used when y'all endeavour to recollect a value from the map.
What volition move on if y'all endeavour to shop a key which is already nowadays inward HashMap? (answer)
If y'all shop an existing key inward the HashMap so it volition override the erstwhile value alongside the novel value together with put() volition render the erstwhile value. There volition non live on whatever exception or error.
Can y'all shop a naught key inward Java HashMap? (answer)
Yes, HashMap allows i naught key which is stored at the outset location of bucket array e.g. bucket[0] = value. The HashMap doesn't telephone band hashCode() on naught key because it volition throw NullPointerException, therefore when a user telephone band get() method alongside naught so the value of the outset index is returned.
Can y'all shop a naught value within HashMap inward Java? (answer)
Yes, HashMap besides allows naught value, y'all tin shop as many naught values as y'all desire as shown inward the hashmap illustration postal service inward this blog.
How does HashMap handgrip collisions inward Java? (answer)
The java.util.HashMap uses chaining to handgrip collisions, which agency novel entries, an object which contains both key together with values, are stored inward a linked listing along alongside existing value together with so that linked listing is stored inward the bucket location. In the worst case, where all key has the same hashcode, your hash tabular array volition live on turned into a linked listing together with searching a value volition receive got O(n) fourth dimension as opposed to O(1) time.
Which information construction HashMap represents? (answer)
The HashMap is an implementation of hash tabular array information construction which is idle for mapping i value to other e.g. id to cite as y'all tin search for value inward O(1) fourth dimension if y'all receive got the key.
Which information construction is used to implement HashMap inward Java? (answer)
Even though HashMap represents a hash table, it is internally implemented past times using an array together with linked listing information construction inward JDK. The array is used as bucket piece a linked list is used to shop all mappings which state inward the same bucket. From Java 8 onwards, the linked listing is dynamically replaced past times binary search tree, i time a number of elements inward the linked listing cross a for certain threshold to improve performance.
Can y'all shop a duplicate key inward HashMap? (answer)
No, y'all cannot insert duplicate keys inward HashMap, it doesn't allow duplicate keys. If y'all endeavour to insert an existing key alongside novel or same value so it volition override the erstwhile value but size of HashMap volition non alter i.e. it volition rest same. This is i of the argue when y'all acquire all keys from the HashMap past times calling keySet() it returns a Set, non a Collection because Set doesn't allow duplicates.
Can y'all shop the duplicate value inward Java HashMap? (answer)
Yes, y'all tin pose duplicate values inward HashMap of Java. It allows duplicate values, that's why when y'all recollect all values from the Hashmap past times calling values() method it returns a Collection together with non Set. Worth noting is that it doesn't render List because HashMap doesn't supply whatever ordering guarantee for key or value.
If y'all desire to explore, y'all tin besides see Java Fundamentals: Collections course from Pluralsight to larn to a greater extent than virtually telephone commutation properties of different collections inward Java e.g. List, Set, together with Map.
Is HashMap thread-safe inward Java? (answer)
No, HashMap is non thread-safe inward Java. You should non portion an HashMap alongside multiple threads if i or to a greater extent than thread is modifying the HashMap e.g. inserting or removing a map. Though, y'all tin easily portion a read-only HashMap.
What volition move on if y'all utilisation HashMap inward a multithreaded Java application? (answer)
If y'all utilisation HashMap inward a multithreaded surround inward such a way that multiple threads structurally modify the map e.g. add, take or modify mapping so the internal information construction of HashMap may acquire corrupt i.e. unopen to links may driblet dead missing, unopen to may indicate to wrong entries together with the map itself may driblet dead completely useless. Hence, it is advised non to utilisation HashMap inward the concurrent application, instead, y'all should utilisation a thread-safe map e.g. ConcurrentHashMap or Hashtable.
What are different ways to iterate over HashMap inward Java? (answer)
Here are unopen to of the ways to iterate over HashMap inward Java:
past times using keySet together with iterator
past times using entrySet together with iterator
past times using entrySet together with enhanced for loop
past times using keySet together with get() method
You tin come across this article for illustration of each of these way to traverse an HashMap inward Java.
How create y'all take a mapping piece iterating over HashMap inward Java? (answer)
Even though HashMap provides remove() method to take a key together with a key/value pair, y'all cannot utilisation them to take a mapping piece traversing an HashMap, instead, y'all demand to utilisation the Iterator's take method to take a mapping as shown inward the next example:
You tin come across that nosotros receive got used Iterator.remove() method to take the electrical current entry piece traversing the map.
In which gild mappings are stored inward HashMap? (answer)
Random gild because HashMap doesn't supply whatever ordering guarantee for keys, values, or entries. When y'all iterate over an HashMap, y'all may acquire the different gild every fourth dimension y'all iterate over it.
Can y'all form HashMap inward Java? (answer)
No, y'all cannot form a HashMap because dissimilar List it is non an ordered collection. Albeit, y'all tin form contents of HashMap past times keys, values or past times entries past times sorting together with so storing the final result into an ordered map e.g. LinkedHashMap or a sorted map e.g. TreeMap.
What is charge factor inward HashMap? (answer)
Influenza A virus subtype H5N1 charge factor is a number which controls the resizing of HashMap when a number of elements inward the HashMap cross the charge factor e.g. if the charge factor is 0.75 together with when becoming to a greater extent than than 75% amount so resizing trigger which involves array copy.
How does resizing happens inward HashMap? (answer)
The resizing happens when map becomes amount or when the size of map crosses the charge factor. For example, if the charge factor is 0.75 together with when driblet dead to a greater extent than than 75% amount so resizing trigger which involves array copy. First, the size of the bucket is doubled together with so erstwhile entries are copied into a novel bucket.
How many entries y'all tin shop inward HashMap? What is the maximum limit? (answer)
There is no maximum boundary for HashMap, y'all tin shop as many entries as y'all desire because when y'all run out of the bucket, entries volition live on added to a linked listing which tin back upwards an infinite number of entries, of course of educational activity until y'all exhaust all the retention y'all have.
Btw, the size() method of HashMap render an int, which has a limit, i time a number of entries cross the limit, size() volition overflow together with if your plan relies on that so it volition break. This number has been addressed inward JDK 8 past times introducing a novel method called mappingCount() which returns a long value. So, y'all should utilisation mappingCount() for large maps. See Java SE 8 for Really Impatient to larn to a greater extent than virtually novel methods introduced inward existing interfaces inward JDK 8.
What is the divergence betwixt capacity together with size of HashMap inward Java? (answer)
The capacity denotes how many entries HashMap tin shop together with size denotes how many mappings or key/value yoke is currently present.
What volition move on if ii different keys of HashMap render same hashcode()? (answer)
If ii keys of HashMap render same hash code so they volition halt upwards inward the same bucket, therefore collision volition occur. They volition live on stored inward a linked listing together.
That's all virtually unopen to of the of import Java HashMap interview questions. I receive got tried to reply them as well, but if y'all disagree alongside whatever reply so experience gratis to comment. Since HashMap is a really of import flat inward Java together with as of import from Java interview indicate of view, it pays to empathize this flat together with its implementation inward deep.
Further Reading
Java Programming Interview Exposed
Java Fundamentals: Collections
From Collections to Streams inward Java 8 Using Lambda Expressions
Other Java Interview Questions listing y'all may desire to check
fifty Java Programs from Coding Interviews (list)
For example, if y'all desire to save the gild of mapping so y'all tin consider using LinkedHashMap. If y'all desire to driblet dead on mappings sorted so y'all tin utilisation TreeMap, which is a sorted map implementation.Similarly, if y'all demand a hash tabular array implementation which is thread-safe together with tin live on used inward a concurrent application without compromising the Scalability so consider using ConcurrentHashMap.
Java HashMap Interview Questions
Here is my listing of HashMap questions from Java Interviews. This listing includes questions based on the internal implementation of HashMap, the Map API, how y'all utilisation HashMap together with mutual best practices piece using HashMap inward a Java application.How does put() method of HashMap plant inward Java? (answer)
The put() method of HashMap plant inward the regulation of hashing. It is responsible for storing an object into backend array. The hashcode() method is used inward conjunction alongside a hash role to honour the right location for the object into the bucket. If a collision occurs so the entry object which contains both key together with value is added to a linked listing together with that linked listing is stored into the bucket location.
What is the requirement for an object to live on used as key or value inward HashMap? (answer)
The key or value object must implement equals() together with hashcode() method. The hash code is used when y'all insert the key object into the map piece equals are used when y'all endeavour to recollect a value from the map.
What volition move on if y'all endeavour to shop a key which is already nowadays inward HashMap? (answer)
If y'all shop an existing key inward the HashMap so it volition override the erstwhile value alongside the novel value together with put() volition render the erstwhile value. There volition non live on whatever exception or error.
Can y'all shop a naught key inward Java HashMap? (answer)
Yes, HashMap allows i naught key which is stored at the outset location of bucket array e.g. bucket[0] = value. The HashMap doesn't telephone band hashCode() on naught key because it volition throw NullPointerException, therefore when a user telephone band get() method alongside naught so the value of the outset index is returned.
Can y'all shop a naught value within HashMap inward Java? (answer)
Yes, HashMap besides allows naught value, y'all tin shop as many naught values as y'all desire as shown inward the hashmap illustration postal service inward this blog.
How does HashMap handgrip collisions inward Java? (answer)
The java.util.HashMap uses chaining to handgrip collisions, which agency novel entries, an object which contains both key together with values, are stored inward a linked listing along alongside existing value together with so that linked listing is stored inward the bucket location. In the worst case, where all key has the same hashcode, your hash tabular array volition live on turned into a linked listing together with searching a value volition receive got O(n) fourth dimension as opposed to O(1) time.
Which information construction HashMap represents? (answer)
The HashMap is an implementation of hash tabular array information construction which is idle for mapping i value to other e.g. id to cite as y'all tin search for value inward O(1) fourth dimension if y'all receive got the key.
Which information construction is used to implement HashMap inward Java? (answer)
Even though HashMap represents a hash table, it is internally implemented past times using an array together with linked listing information construction inward JDK. The array is used as bucket piece a linked list is used to shop all mappings which state inward the same bucket. From Java 8 onwards, the linked listing is dynamically replaced past times binary search tree, i time a number of elements inward the linked listing cross a for certain threshold to improve performance.
Can y'all shop a duplicate key inward HashMap? (answer)
No, y'all cannot insert duplicate keys inward HashMap, it doesn't allow duplicate keys. If y'all endeavour to insert an existing key alongside novel or same value so it volition override the erstwhile value but size of HashMap volition non alter i.e. it volition rest same. This is i of the argue when y'all acquire all keys from the HashMap past times calling keySet() it returns a Set, non a Collection because Set doesn't allow duplicates.
Can y'all shop the duplicate value inward Java HashMap? (answer)
Yes, y'all tin pose duplicate values inward HashMap of Java. It allows duplicate values, that's why when y'all recollect all values from the Hashmap past times calling values() method it returns a Collection together with non Set. Worth noting is that it doesn't render List because HashMap doesn't supply whatever ordering guarantee for key or value.
If y'all desire to explore, y'all tin besides see Java Fundamentals: Collections course from Pluralsight to larn to a greater extent than virtually telephone commutation properties of different collections inward Java e.g. List, Set, together with Map.
Is HashMap thread-safe inward Java? (answer)
No, HashMap is non thread-safe inward Java. You should non portion an HashMap alongside multiple threads if i or to a greater extent than thread is modifying the HashMap e.g. inserting or removing a map. Though, y'all tin easily portion a read-only HashMap.
What volition move on if y'all utilisation HashMap inward a multithreaded Java application? (answer)
If y'all utilisation HashMap inward a multithreaded surround inward such a way that multiple threads structurally modify the map e.g. add, take or modify mapping so the internal information construction of HashMap may acquire corrupt i.e. unopen to links may driblet dead missing, unopen to may indicate to wrong entries together with the map itself may driblet dead completely useless. Hence, it is advised non to utilisation HashMap inward the concurrent application, instead, y'all should utilisation a thread-safe map e.g. ConcurrentHashMap or Hashtable.
What are different ways to iterate over HashMap inward Java? (answer)
Here are unopen to of the ways to iterate over HashMap inward Java:
past times using keySet together with iterator
past times using entrySet together with iterator
past times using entrySet together with enhanced for loop
past times using keySet together with get() method
You tin come across this article for illustration of each of these way to traverse an HashMap inward Java.
How create y'all take a mapping piece iterating over HashMap inward Java? (answer)
Even though HashMap provides remove() method to take a key together with a key/value pair, y'all cannot utilisation them to take a mapping piece traversing an HashMap, instead, y'all demand to utilisation the Iterator's take method to take a mapping as shown inward the next example:
Iterator itr = map.entrySet().iterator(); while(itr.hasNext()){ Map.Entry electrical current = itr.next(); if(current.getKey().equals("matching"){ itr.remove(); // this volition take the electrical current entry. } }
You tin come across that nosotros receive got used Iterator.remove() method to take the electrical current entry piece traversing the map.
In which gild mappings are stored inward HashMap? (answer)
Random gild because HashMap doesn't supply whatever ordering guarantee for keys, values, or entries. When y'all iterate over an HashMap, y'all may acquire the different gild every fourth dimension y'all iterate over it.
Can y'all form HashMap inward Java? (answer)
No, y'all cannot form a HashMap because dissimilar List it is non an ordered collection. Albeit, y'all tin form contents of HashMap past times keys, values or past times entries past times sorting together with so storing the final result into an ordered map e.g. LinkedHashMap or a sorted map e.g. TreeMap.
What is charge factor inward HashMap? (answer)
Influenza A virus subtype H5N1 charge factor is a number which controls the resizing of HashMap when a number of elements inward the HashMap cross the charge factor e.g. if the charge factor is 0.75 together with when becoming to a greater extent than than 75% amount so resizing trigger which involves array copy.
How does resizing happens inward HashMap? (answer)
The resizing happens when map becomes amount or when the size of map crosses the charge factor. For example, if the charge factor is 0.75 together with when driblet dead to a greater extent than than 75% amount so resizing trigger which involves array copy. First, the size of the bucket is doubled together with so erstwhile entries are copied into a novel bucket.
How many entries y'all tin shop inward HashMap? What is the maximum limit? (answer)
There is no maximum boundary for HashMap, y'all tin shop as many entries as y'all desire because when y'all run out of the bucket, entries volition live on added to a linked listing which tin back upwards an infinite number of entries, of course of educational activity until y'all exhaust all the retention y'all have.
Btw, the size() method of HashMap render an int, which has a limit, i time a number of entries cross the limit, size() volition overflow together with if your plan relies on that so it volition break. This number has been addressed inward JDK 8 past times introducing a novel method called mappingCount() which returns a long value. So, y'all should utilisation mappingCount() for large maps. See Java SE 8 for Really Impatient to larn to a greater extent than virtually novel methods introduced inward existing interfaces inward JDK 8.
What is the divergence betwixt capacity together with size of HashMap inward Java? (answer)
The capacity denotes how many entries HashMap tin shop together with size denotes how many mappings or key/value yoke is currently present.
What volition move on if ii different keys of HashMap render same hashcode()? (answer)
If ii keys of HashMap render same hash code so they volition halt upwards inward the same bucket, therefore collision volition occur. They volition live on stored inward a linked listing together.
That's all virtually unopen to of the of import Java HashMap interview questions. I receive got tried to reply them as well, but if y'all disagree alongside whatever reply so experience gratis to comment. Since HashMap is a really of import flat inward Java together with as of import from Java interview indicate of view, it pays to empathize this flat together with its implementation inward deep.
Further Reading
Java Programming Interview Exposed
Java Fundamentals: Collections
From Collections to Streams inward Java 8 Using Lambda Expressions
Other Java Interview Questions listing y'all may desire to check
fifty Java Programs from Coding Interviews (list)
nineteen Java Overloading together with Overriding Interview Questions (list)
fifteen Java NIO together with Networking Interview Questions alongside Answers (see here)
fifteen Java NIO together with Networking Interview Questions alongside Answers (see here)
21 Java ArrayList Interview Questions alongside Answers (list)
21 Java Final modifier Interview Questions (list)
21 Java Inheritance Interview Questions alongside answers (list)
10 Date, Time, together with Calendar based Interview Questions alongside answers (list)
v main() method interview questions (list)
fifteen SQL together with UNIX questions from Java Interviews (list)
22 array concept interview questions from Java (list)
fifteen Java Enum based Interview Questions (list)
These questions volition non solely assist y'all to empathize HashMap amend but besides encourage y'all to honour out to a greater extent than virtually HashMap, its Java implementation together with hash tabular array information construction inward general. If y'all receive got whatever other HashMap based Java questions, which was asked to y'all inward an interview, experience gratis to portion alongside us.
v main() method interview questions (list)
fifteen SQL together with UNIX questions from Java Interviews (list)
22 array concept interview questions from Java (list)
fifteen Java Enum based Interview Questions (list)
These questions volition non solely assist y'all to empathize HashMap amend but besides encourage y'all to honour out to a greater extent than virtually HashMap, its Java implementation together with hash tabular array information construction inward general. If y'all receive got whatever other HashMap based Java questions, which was asked to y'all inward an interview, experience gratis to portion alongside us.
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