Hashmap containskey null

Does the. Asked 8 years ago. Active 8 years ago. Viewed 19k times. Bharath Naidu Bharath Naidu 2 2 gold badges 6 6 silver badges 14 14 bronze badges. Could you post stack trace? In the case when the map is null, put it this way. If the map is null, what are you looking in to find a key? It is an unordered list, i. A key in a Hashmap can map to one value, where a key is an object that you use to retrieve values whenever needed.

There are a few points to remember about Hashmaps in Java: A Hashmap in Java allows one null key and multiple null values. Implementation of Hashmap is unsynchronized. Hashmap implementation allows null values, null keys, and all optional map operations Hashmaps cannot contain duplicate keys. A key in a hashmap can map to one value.

Each key-value pair is called an entry. Hashmap gives constant-time performance for basic operations, i. Top Business Gamification Companies For by toptechproviders. Join HackerNoon.

If the given key is mapped, this method returns the value. However, the insertion order is not retained in the Hashmap. Internally, for every element, a separate hash is generated and the elements are indexed based on this hash to make it more efficient.

Changing Elements: After adding the elements if we wish to change the element, it can be done by again adding the element with the put method. Since the elements in the map are indexed using the keys, the value of the key can be changed by simply inserting the updated value for the key for which we wish to change. Removing Element: In order to remove an element from the Map, we can use the remove method. This method takes the key value and removes the mapping for a key from this map if it is present in the map.

Then using the next method we print the entries of HashMap. HashMap import java. HashMap; import java. HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to small String that represents the same String. A shorter value helps in indexing and faster searches.

HashSet also uses HashMap internally. Few important features of HashMap are: HashMap is a part of java. HashMap extends an abstract class AbstractMap which also provides an incomplete implementation of Map interface. It also implements Cloneable and Serializable interface. K and V in the above definition represent Key and Value respectively. HashMap allows null key also but only once and multiple null values. 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. Constructs an empty HashMap with the specified initial capacity and the default load factor 0. Constructs an empty HashMap with the specified initial capacity and load factor.

Constructs a new HashMap with the same mappings as the specified Map. Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned. Attempts to compute a mapping for the specified key and its current mapped value or null if there is no current mapping.

If the specified key is not already associated with a value or is mapped to null , attempts to compute its value using the given mapping function and enters it into this map unless null. If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

Returns a Set view of the mappings contained in this map. Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

Returns a Set view of the keys contained in this map. If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. If the specified key is not already associated with a value or is mapped to null associates it with the given value and returns null , else returns the current value. Removes the entry for the specified key only if it is currently mapped to the specified value. Replaces the entry for the specified key only if it is currently mapped to some value.

Replaces the entry for the specified key only if currently mapped to the specified value. Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Returns a Collection view of the values contained in this map. Constructs an empty HashMap with the default initial capacity 16 and the default load factor 0.

Associates the specified value with the specified key in this map. Copies all of the mappings from the specified map to this map.

Removes all of the mappings from this map.