【java】JavaがHashMapを巡る5つの方法

1.keyを単独で遍歴し、value(単一keyまたはvalueを取得する効率が高い)

for(String k:maps.keySet()){
            System.out.println(k+":"+maps.get(k));
        }
     
for(String value:maps.values()){
            System.out.println(value);
        }

2.Map.Entry<.......>汎用遍歴(効率的、推奨)

Map map = new HashMap();
for (Map.Entry entry : map.entrySet()) {
  System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

3.Iterator
Iterator key,value遍歴

Iterator iterator = maps.keySet().iterator();
        while(iterator.hasNext()){
            String key = iterator.next();
            System.out.println(key+":"+maps.get(key));
        }

Iterator iter= maps.values().iterator();
        while(iter.hasNext()){
            System.out.println(iter.next());
        }

Iterator Map.Entry<.......>汎用遍歴

Map map = new HashMap();
Iterator> entries = map.entrySet().iterator();
while (entries.hasNext()) {
  Map.Entry entry = entries.next();
  System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

4.java 8でforEachを使用して巡回

maps.forEach((k,v)-> System.out.println("Item :"+k+" Count:"+v));

5.キー探索値遍歴(非効率)

Map map = new HashMap();
for (Integer key : map.keySet()) {
  Integer value = map.get(key);
  System.out.println("Key = " + key + ", Value = " + value);
}

 
 

Mapitems=new HashMap<>();
        items.put("A",10);
        items.put("B",20);
        items.put("C",30);
        items.put("D",40);
        items.put("E",50);
        items.put("F",60);
 
        //1.  entrySet    
        Iterator iter1=items.entrySet().iterator();
        while(iter1.hasNext()){
            Map.Entry entry=(Map.Entry)iter1.next();
            System.out.println("Item :"+entry.getKey()+" Count:"+entry.getValue());
 
        }
 
        //2.  keySet    
        Iterator iter2=items.keySet().iterator();
        while(iter2.hasNext()){
            String key=(String) iter2.next();
            System.out.println("Item :"+key+" Count:"+items.get(key));
        }
 
        //3.  for    EntrySet    
        for(Map.Entryentry:items.entrySet()){
            System.out.println("Item :"+entry.getKey()+" Count:"+entry.getValue());
        }
 
        //4.  for    keySet    
        for (String key:items.keySet()){
            System.out.println("Item :"+key+" Count:"+items.get(key));
        }
 
        //5.java8   forEach    
        items.forEach((k,v)-> System.out.println("Item :"+k+" Count:"+v));