java hashmap syntax

1// Import the HashMap class
2import java.util.HashMap;
3
4	// First Example
5    // Create a HashMap object called capitalCities
6    HashMap<String, String> capitalCities = new HashMap<String, String>();
7
8    // Add keys and values (Country, City)
9    	capitalCities.put("England", "London");
10    	capitalCities.put("Germany", "Berlin");
11    	capitalCities.put("Norway", "Oslo");
12    	capitalCities.put("USA", "Washington DC");
13    	System.out.println(capitalCities);
14
15	//Second Example
16	// Create a HashMap object called stGrade
17	Map<String, Integer> stGrade = new HashMap<String, Integer>(); 
18
19	// Insert elements 
20       	stGrade.put("aaron", new Integer(90)); 
21       	stGrade.put("isaac", new Integer(100)); 
22       	stGrade.put("john", new Integer(35)); 
23       	stGrade.put("mohammad", new Integer(100)); 
24
25	// Get value
26	   	stGrade.get("mohammad"); 	// returns 100
27		stGrade.get("aaron"); 		// returns 90
28		stGrade.get("john");	 	// returns 35
29		stGrade.get("isaac"); 		// returns 100
30		

1import java.util.HashMap; // import the HashMap class
2
3// instantiate a HashMap instance
4HashMap<String, String> capitalCities = new HashMap<String, String>();
5
6// SCROLL DOWN FOR LIST OF ALL METHODS --------------------------------------
7// to use any methods below add the function name to the instance name
8// General Form: HashMapInstanceName.{functionName}({parameter list});
9// Specific Example Using the Put Method: 
10capitalCities.put("California", "Sacramento");
11
12// SCROLL DOWN FOR LIST OF ALL METHODS --------------------------------------
13
141. void	clear() 
15  	- Removes all of the mappings from this map.
16
172. Object clone() 
18    - Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
19
203. boolean	containsKey(Object key)
21	- Returns true if this map contains a mapping for the specified key.
22
234. boolean	containsValue(Object value)
24	- Returns true if this map maps one or more keys to the specified value.
25
265. V	get(Object key)
27	- Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
28
296. V	getOrDefault(Object key, V defaultValue)
30	- Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
31
327. boolean	isEmpty()
33	- Returns true if this map contains no key-value mappings.
34
358. Set<K>	keySet()
36	- Returns a Set view of the keys contained in this map.
37      
389. V	put(K key, V value)
39	- Associates the specified value with the specified key in this map.
40 
4110. V	remove(Object key)
42	- Removes the mapping for the specified key from this map if present.
43      
4411. boolean	remove(Object key, Object value)
45	- Removes the entry for the specified key only if it is currently mapped to the specified value.
46
4712. V	replace(K key, V value)
48	- Replaces the entry for the specified key only if it is currently mapped to some value.
49
5013. boolean	replace(K key, V oldValue, V newValue)
51	- Replaces the entry for the specified key only if currently mapped to the specified value.
52
5314. int	size()
54	- Returns the number of key-value mappings in this map.
55      
5615. Collection<V>	values()
57	- Returns a Collection view of the values contained in this map.

1//remember to first import java.util.*; first
2
3//you can swap out string or integer for other data types
4Map<String, Integer> d = new HashMap<>();
5

1//Hash map creation
2Map< String,Integer> hm =  
3                        new HashMap< String,Integer>(); 
4//inserting elements into hashmap
5       hm.put("a", new Integer(100)); 
6       hm.put("b", new Integer(200)); 
7       hm.put("c", new Integer(300)); 
8       hm.put("d", new Integer(400)); 

1- HasHMap can have null key, order is not guaranteed