¿Cómo funciona HashMap en Java? ejemplo de código

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Ejemplo 1: métodos hashmap de Java

importjava.util.HashMap;// import the HashMap class// instantiate a HashMap instanceHashMap<String,String> capitalCities =newHashMap<String,String>();// SCROLL DOWN FOR LIST OF ALL METHODS --------------------------------------// to use any methods below add the function name to the instance name// General Form: HashMapInstanceName.functionName(parameter list);// Specific Example Using the Put Method: 
capitalCities.put("California","Sacramento");// SCROLL DOWN FOR LIST OF ALL METHODS --------------------------------------1.voidclear()-Removes all of the mappings from this map.2.Objectclone()-Returns a shallow copy of thisHashMap instance: the keys and values themselves are not cloned.3.booleancontainsKey(Object key)-Returnstrueifthis map contains a mapping for the specified key.4.booleancontainsValue(Object value)-Returnstrueifthis map maps one or more keys tothe specified value.5.Vget(Object key)-Returns the value towhich the specified key is mapped, or nullifthis map contains no mapping for the key.6.VgetOrDefault(Object key,V defaultValue)-Returns the value towhich the specified key is mapped, or defaultValue ifthis map contains no mapping for the key.7.booleanisEmpty()-Returnstrueifthis map contains no key-value mappings.8.Set<K>keySet()-Returns a Set view of the keys contained in this map.9.Vput(K key,V value)-Associates the specified value withthe specified key in this map.10.Vremove(Object key)-Removes the mapping for the specified key from this map if present.11.booleanremove(Object key,Object value)-Removes the entry for the specified key only if it is currently mapped tothe specified value.12.Vreplace(K key,V value)-Replaces the entry for the specified key only if it is currently mapped tosome value.13.booleanreplace(K key,V oldValue,V newValue)-Replaces the entry for the specified key only if currently mapped tothe specified value.14.intsize()-Returns the number of key-value mappings in this map.15.Collection<V>values()-Returns a Collection view of the values contained in this map.

Ejemplo 2: sintaxis java hashmap

// Import the HashMap classimportjava.util.HashMap;// First Example// Create a HashMap object called capitalCitiesHashMap<String,String> capitalCities =newHashMap<String,String>();// Add keys and values (Country, City)
    	capitalCities.put("England","London");
    	capitalCities.put("Germany","Berlin");
    	capitalCities.put("Norway","Oslo");
    	capitalCities.put("USA","Washington DC");System.out.println(capitalCities);//Second Example// Create a HashMap object called stGradeMap<String,Integer> stGrade =newHashMap<String,Integer>();// Insert elements 
       	stGrade.put("aaron",newInteger(90)); 
       	stGrade.put("isaac",newInteger(100)); 
       	stGrade.put("john",newInteger(35)); 
       	stGrade.put("mohammad",newInteger(100));// Get value
	   	stGrade.get("mohammad");// returns 100
		stGrade.get("aaron");// returns 90
		stGrade.get("john");// returns 35
		stGrade.get("isaac");// returns 100

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