HashMap的遍历

用迭代器获取所有key 和 value

Map<String, String> map = new HashMap<String, String>();
map.put("a", "aaa");
map.put("b", "bbb");
map.put("c", "ccc");

//获取所有key值
Set<String> keys = map.keySet();
Iterator<String> iterator1 = keys.iterator();
while(iterator1.hasNext()){
	System.out.print(iterator1.next() + ", ");
}
System.out.println();

//获取所有value值
Collection<String> values = map.values();
Iterator<Integer> iterator2 = values.iterator();
while(iterator2.hasNext()){
	System.out.print(iterator2.next() + ", ");
}
System.out.println();

//获取不重复的value
Set<String> diffValues = new HasSet<String>();
for(String str: values){
	diffValues.add(str);
}
Iterator<String> iterator3 = diffValues.iterator();
while(iterator3.hasNext()){
	System.out.print(iterator3.next() + ", ");
}

HashMap的四种遍历方法

⭐1.for each map.entrySet()

Map<String, String> map = new HashMap<String, String>();
for(Entry<String, String> entry: map.entrySet()){
	entry.getKey();
	entry.geValue();
}

2.显示调用map.entrySet()的集合迭代器

Map<String, String> map = new HashMap<String, String>();
Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
while(iterator.hasNext()){
	Map.Entry<String, String> entry = iterator.next();
	entry.getKey();
	entry.getValue();
}

⭐3.for each map.keySet()，再调用get获取

Map<String, String> map = new HashMap<String, String>();
for(String key: map.ketSet()){
	map.get(key);
}

4.for each map.entrySet(),用临时变量保持map.entrySet();

Map<String, String> map = new HashMap<String, String>();
Set<Entry<String, String>> entrySet = map.entrySet();
for(Entry<String, String> entry: entrySet){
	entry.getKey();
	entry.getValue();
}

ArrayList和LinkedList的几种循环遍历方式及性能对比分析
https://www.trinea.cn/android/arraylist-linkedlist-loop-performance/