Map.entrySet() returns a Set < map. Entry < K, V > >. Map. Entry is an interface in map. Its purpose is to represent a mapping item (with Key and Value), and Set < map. Entry < K, V > > represents the Set of a mapping item. There are corresponding getKey and getValue methods in Map.Entry, namely JavaBean, so that we can get Key and Value from an item.
Here are four ways to traverse the Map:
public static void main(String[] args) {
Map<String, String> map = new HashMap<String, String>();
map.put("1", "value1");
map.put("2", "value2");
map.put("3", "value3");
// The first type: commonly used, secondary value
System.out.println("traverse key and value:") through Map.keySet);
for (String key : map.keySet()) {
System.out.println("key= "+ key + " and value= " + map.get(key));
}
// Second
System.out.println("use iterator to traverse key and value:") through Map.entrySet);
Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, String> entry = it.next();
System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}
// Third: recommended, especially when the capacity is large
System.out.println("traverse key and value through Map.entrySet");
for (Map.Entry<String, String> entry : map.entrySet()) {
System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}
// Fourth
System.out.println("traverse all value s through Map.values(), but not key s");
for (String v : map.values()) {
System.out.println("value= " + v);
}
}
The following is the source code of HashMap:
First, the underlying implementation of HashMap uses an Entry array
Let's see where Entry is defined. Continue with the source code. We found its definition in line 674 of the source code of HashMap. It turns out that it is an internal class of HashMap and implements the Map.Entry interface. Some of the following are reproduced
In that case, let's take a look at how the Map.Entry interface is defined. It turns out that it is an internal interface of Map and defines some methods
When you see here, you probably understand why HashMap chooses the Entry array to store key values. Right, because the Map.Entry interface implemented by Entry defines getKey(),getValue(), setKey(),setValue() and other methods, which are equivalent to a javaBean, encapsulating key value pairs for later operations, In fact, we can also think of not only HashMap, such as LinkedHashMap,TreeMap and other containers inherited from map. The storage key value pairs should use Entry, but the form of organizing Entry is different. HashMap uses the form of array and linked list, LinkedHashMap uses the form of linked list, and TreeMap should use the form of binary tree. If you don't believe it, please refer to the source code
LinkedHashMap:
How to initialize the linked list:
TreeMap:
Let's see if his put method looks familiar (insertion operation of binary sort tree)
ok, after understanding the way in which key value pairs are stored at the bottom of various maps, let's take a look at how to traverse maps. Here's a demonstration with HashMap
Map provides some common methods, such as keySet(), entryset (). The return value of keySet() method is the Set of key values in map; The return value of entrySet() also returns a Set set. The type of this Set is Map.Entry.
so, it is easy to write the following traversal code
Original text goes to: https://blog.csdn.net/kyi_zhu123/article/details/52769469