Java заменить значение в map

Java HashMap replace()

The HashMap replace() method replaces the mapping and returns:

• the previous value associated with the specified key, if the optional parameter oldValue is not present
• true , if the optional parameter oldValue is present

Note: The method returns null , if either the specified key is mapped to a null value or the key is not present on the hashmap.

Example 1: Replace an Entry in HashMap

import java.util.HashMap; class Main < public static void main(String[] args) < // create an HashMap HashMaplanguages = new HashMap<>(); // add entries to HashMap languages.put(1, "Python"); languages.put(2, "English"); languages.put(3, "JavaScript"); System.out.println("HashMap: " + languages); // replace mapping for key 2 String value = languages.replace(2, "Java"); System.out.println("Replaced Value: " + value); System.out.println("Updated HashMap: " + languages); > >

HashMap: Replaced Value: English Updated HashMap:

In the above example, we have created a hashmap named languages . Here, we have used the replace() method to replace the entry for key 1 ( 1=English ) with the specified value Java .

Here, the replace() method does not have the optional oldValue parameter. Hence, it returns the old value ( English ).

Example 2: HashMap replace() with Old Value

import java.util.HashMap; class Main < public static void main(String[] args) < // create an HashMap HashMapcountries = new HashMap<>(); // insert items to the HashMap countries.put("Washington", "America"); countries.put("Ottawa", "Canada"); countries.put("Canberra", "Australia"); System.out.println("Countries:\n" + countries); // replace mapping countries.replace("Washington", "America", "USA"); // return true countries.replace("Canberra", "New Zealand", "Victoria"); // return false System.out.println("Countries after replace():\n" + countries); > >

Countries: Countries after replace():

In the above example, we have created a hashmap named countries . Notice the line,

countries.replace("Washington", "America", "USA");

Here, the replace() method includes the optional oldValue parameter ( America ). Hence, the mapping where key Washington maps to value America is replaced with new value USA .

countries.replace("Canberra", "New Zealand", "Victoria");

Here, in the hashmap, the key Canberra does not map to value New Zealand . Hence, the replace() method does not replace any value.

Note: We can use the Java HashMap clear() method to remove all the mappings from the hashmap.

HashMap put() Vs. replace()

The syntax of the put() and replace() method looks quite similar in HashMap .

// syntax of put() hashmap.put(key, value) // syntax of replace() hashmap.replace(key, value)

And, when the hashmap contains the mapping for the specified key, then both the methods replace the value associated with the specified key.

However, if the hashmap does not contain any mapping for the specified key, then

• the put() method inserts the new mapping for the specified key and value
• the replace() method returns null

Example 3: HashMap put() Vs. replace()

import java.util.HashMap; class Main < public static void main(String[] args) < // create an HashMap HashMaplanguages1 = new HashMap<>(); // insert entries to HashMap languages1.put(1, "Python"); languages1.put(2, "JavaScript"); // create another HashMap similar to languages1 HashMap languages2 = new HashMap<>(); // puts all entries from languages1 to languages2 languages2.putAll(languages1); System.out.println("HashMap: " + languages1); // use of put() languages2.put(3, "Java"); System.out.println("HashMap after put():\n" + languages2); // use of replace() languages1.replace(3, "Java"); System.out.println("HashMap after replace():\n" + languages1); > >

HashMap: HashMap after put(): HashMap after replace():

In the above example, we have created two hashmaps named languages1 and languages2 . We have used the HashMap putAll() method so that both hashmaps have the same mappings.

Here, the mapping for key 3 is not present in the hashmap. Hence,

• the put() method adds the new mapping (3 = Java) to HashMap
• the replace() method does not perform any operation

To learn more about adding entries, visit Java HashMap put().

Источник

Java заменить значение в map

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).

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