- Java – Remove all mappings from HashMap example
- Top Related Articles:
- About the Author
- Java hashmap remove all values
- Nested Class Summary
- Nested classes/interfaces inherited from class java.util.AbstractMap
- Nested classes/interfaces inherited from interface java.util.Map
- Constructor Summary
- Java HashMap Clear Delete All Mappings Example
- How to clear HashMap in Java?
- 1. Using the clear method
- 2. By assigning a new object
- What is the preferred way to clear the HashMap?
Java – Remove all mappings from HashMap example
In the last tutorial we shared how to remove a specific mapping from HashMap based on key. In this example we are going to see how to remove all the mappings from HashMap. We will be using clear() method of HashMap class to do this:
public void clear() : Removes all of the mappings from this map. The map will be empty after this call returns.
Complete Code:
import java.util.HashMap; public class RemoveAllExample < public static void main(String[] args) < // Creating a HashMap of int keys and String values HashMaphashmap = new HashMap(); // Adding Key and Value pairs to HashMap hashmap.put(11,"Value1"); hashmap.put(22,"Value2"); hashmap.put(33,"Value3"); hashmap.put(44,"Value4"); hashmap.put(55,"Value5"); // Displaying HashMap Elements System.out.println("HashMap Elements: " + hashmap); // Removing all Mapping hashmap.clear(); // Displaying HashMap Elements after remove System.out.println("After calling clear():"); System.out.println("---------------------"); System.out.println("HashMap Elements: " + hashmap); > >
HashMap Elements: After calling clear(): --------------------- HashMap Elements: <>
As you can see all the mappings of HashMap have been removed after calling clear() method and HashMap became empty after that.
Top Related Articles:
About the Author
I have 15 years of experience in the IT industry, working with renowned multinational corporations. Additionally, I have dedicated over a decade to teaching, allowing me to refine my skills in delivering information in a simple and easily understandable manner.
Java hashmap remove all values
Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the «capacity» of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important. An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets. As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur. If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable , this class may use comparison order among keys to help break ties. Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be «wrapped» using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:
Map m = Collections.synchronizedMap(new HashMap(. ));
The iterators returned by all of this class’s «collection view methods» are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs. This class is a member of the Java Collections Framework.
Nested Class Summary
Nested classes/interfaces inherited from class java.util.AbstractMap
Nested classes/interfaces inherited from interface java.util.Map
Constructor Summary
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
Class HashMap
Type Parameters: K — the type of keys maintained by this map V — the type of mapped values All Implemented Interfaces: Serializable , Cloneable , Map Direct Known Subclasses: LinkedHashMap , PrinterStateReasons
Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable , except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.
This implementation provides constant-time performance for the basic operations ( get and put ), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the «capacity» of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.
An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put ). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.
If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable , this class may use comparison order among keys to help break ties.
Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be «wrapped» using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:
Map m = Collections.synchronizedMap(new HashMap(. ));
The iterators returned by all of this class’s «collection view methods» are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework.
Java HashMap Clear Delete All Mappings Example
This example shows how to clear HashMap in Java or how to delete all key value mappings from the HashMap object using the clear method of the HashMap class.
How to clear HashMap in Java?
There are a couple of ways using which you can delete all mappings from the HashMap.
1. Using the clear method
The clear method of the HashMap class removes all key-value mappings from the HashMap object.
The clear method does not take any arguments and its return type is void. The HashMap object will be empty after this method call.
2. By assigning a new object
You can also assign a new empty HashMap object to the same reference.
What is the preferred way to clear the HashMap?
Let’s first have a look at the source code of the clear method.
The clear method iterates the internal table and makes each entry null. All the key value references are set to null so that they be can garbage collected if they are not referenced anywhere else.
When you assign a new HashMap object to the same reference, similar to the clear method, all the key value objects need to be garbage collected (if they are not referenced anywhere else) plus the original HashMap object. Additionally, a new map object needs to be created and assigned to the same reference.
The important difference is that when you use the clear method, the same HashMap object can be reused. But when you assign a new object to the same reference, a new object needs to be created which is a costly operation in terms of performance.
Plus, as you can see from the code above, when you use the clear method the HashMap capacity does not change. So if you are going to add a roughly similar number of mappings to the HashMap, the clear method has an advantage as the table does not need to increase the capacity.
Please let me know your views in the comments section below.