I believe that everyone must be unfamiliar with Hashmap, and it is also more proficient. He here, the blogger helps everyone review the source code.

First give you a picture

This is a HashMap very clear that HashMap is composed of key values.

Ask a question

problem

How is HashMap deposits and values?

answer Put KEY and VALUE in an object and place the object in an array.
Let's take a look at the bottom source code

Object is entry (node ​​implements map.entry)

So see the first picture.Each line can see as an Entry object as a whole as an arrayThen, the first traversal method came out.

Traversal

 for (Map.Entry entry:hashMap.entrySet()) {// hashmap is an instantiated object
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        }

Traverse

For the first picture, we look at the first line (Key) is a set collection (the bottom of the SET is MAP implementation)
We can traverse all KEY values.

Set<String> keys= hashMap.keySet();
        for (String key :keys) {
            System.out.println(hashMap.get(key));
        }

By the way, the second line of the first picture (Value) is a collection of Collection, we can pass
.VALUES Gets Value Collection