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MapExample.java
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package example.JavaCollectionExample;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
/**
* HashMap, LinkedHashMap, TreeMap
*
*
* ----------
* | Map | < - - - - - - - - -
* ---------- < \
* ＾ ＾ \ \
* extends | | \ implements \ implements
* / \ \ \
* / \implements \ \
* ---------- -------- ------------- ---------
* |SortedMap | |HashMap | |LinkedHashMap| |HashTable|
* ---------- -------- ------------- ---------
* ^
* | implements
* ----------
* |TreeMap |
* ----------
*
* Reference:
* - https://www.w3spoint.com/hashmap-linkedhashmap-treemap-hashtable-java
* - https://beginnersbook.com/2014/06/difference-between-hashmap-and-hashtable/
* - https://beginnersbook.com/java-collections-tutorials/
* - https://www.geeksforgeeks.org/iterate-map-java/?ref=leftbar-rightbar
*/
public class MapExample {
public static void main(String[] args) {
// example for HashMap.
HashMapExample hme = new HashMapExample();
hme.example();
hme.hashmapExample();
// hme.hashmapKeySetExample();
// hme.hashmapToIterator();
// example for LinkedHashMap.
LinkedHashMapExample lhme = new LinkedHashMapExample();
lhme.example();
// example for TreeMap.
// TODO: 增加 treeMap example
}
}
/**
* HashMap
* - 元素皆唯一
* - 沒保持當初建立的順序 (order)
* - 每個元素皆有 key, value
* - 允許 null key, null value.
*
* Method:
* - Map.entrySet()
* - Map.Entry<K, V>
*
* Reference:
* - https://beginnersbook.com/2013/12/hashmap-in-java-with-example/
*
*/
class HashMapExample {
void example() {
// * empty, remove, clear example for HashMap.
Map<Integer, String> map = generateData();
System.out.println("content: " + map);
System.out.println("# of element: " + map.size());
System.out.println("Is empty: " + map.isEmpty());
System.out.println("Contain 101: " + map.containsKey(100));
map.remove(102);
System.out.println("content: " + map);
System.out.println(map.get(100));
System.out.println(map.get(102));
map.clear();
System.out.println("content: " + map);
}
public Map<Integer, String> generateData() {
Map<Integer, String> map = new HashMap<Integer, String>();
map.put(100, "george");
map.put(101, "may");
map.put(102, "JJ");
return map;
}
public void hashmapExample() {
// * Map.entrySet() entries using for-each loop.
Map<Integer, String> map = generateData();
System.out.println("顯示hashmap資料: ");
System.out.println(map.entrySet());
for (Map.Entry<Integer, String> entry : map.entrySet()) {
System.out.println("Key: " + entry.getKey() + ", " + "Value: " + entry.getValue());
}
}
public void hashmapKeySetExample() {
// * Map using keySet() and values() methods
Map<Integer, String> map = generateData();
// using keySet() for iteration over keys.
for (Integer id : map.keySet()) {
System.out.println("key: " + id);
}
// using values() for iteration over values.
for (String name : map.values()) {
System.out.println("value: " + name);
}
}
public void hashmapToIterator() {
// * using iterators.
Map<Integer, String> map = generateData();
Iterator<Map.Entry<Integer, String>> itr = map.entrySet()
.iterator();
while(itr.hasNext()) {
Map.Entry<Integer, String> entry = itr.next();
System.out.println("key: " + entry.getKey() + "; value: " + entry.getValue());
}
}
public void useForEachForHashMap() {
// * using foreach method.
Map<Integer, String> map = generateData();
map.forEach( (k, v) -> {
System.out.println("key: " + k + "; value: " + v);
});
}
}
/**
* LinkedHashMap
* - 元素皆唯一
* - 可保持當初建立的順序 (order)
* - 每個元素皆有 key, value
* - 允許 null key, null value.
*
* Reference:
* - https://beginnersbook.com/2013/12/linkedhashmap-in-java/
*/
class LinkedHashMapExample {
void example() {
var map = new LinkedHashMap<String, String>();
map.put("kaohsiung", "高雄");
map.put("taipei", "台北");
map.put("tainan", "台南");
System.out.println(map.get("kaohsiung"));
//! 似乎沒有 map.getKey(), map.getValue()
}
}
/**
* TreeMap
* - 皆為唯一
* - 由大到小排序
* - 不允許空的key, 但允許空的value.
*
* Reference:
* - https://beginnersbook.com/2013/12/treemap-in-java-with-example/
*/
class TreeMapExample {
void example() {
//! 似乎 import 會有問題!
}
}