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This article mainly explains "what is the HashMap traversal method". The explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the HashMap traversal method".

Ergodic mode Iterator entryIterator = map.entrySet () .iterator ()

While (entryIterator.hasNext ()) {

Map.Entry next = entryIterator.next ()

System.out.println ("key=" + next.getKey () + "value=" + next.getValue ())

}

Iterator iterator = map.keySet () .iterator ()

While (iterator.hasNext ()) {

String key = iterator.next ()

System.out.println ("key=" + key + "value=" + map.get (key))

}

Map.forEach ((key,value)-> {

System.out.println ("key=" + key + "value=" + value)

});

It is strongly recommended that you use the first EntrySet for traversal.

The first one can take out the key value at the same time, the second needs to get the value through key, which is less efficient, and the third needs more than JDK1.8, traversing the table through the outer layer and the red-black tree through the inner layer.

Notice

Using HashMap in a concurrent environment is prone to endless loops.

When the capacity of concurrent scenarios is expanded, a circular linked list is easy to appear when rehash () in the resize () method is called. In this way, when you get a key that does not exist, an endless loop occurs when the calculated index happens to be the subscript of the circular linked list.

Therefore, HashMap can only be used in a single thread, and try to preset the capacity and reduce the capacity expansion as much as possible.

HashMap is optimized in JDK1.8: when the length of the linked list written after the hash collision exceeds the threshold (the default is 8), the linked list will be converted to a red-black tree.

Suppose the hash conflict is very serious, and an array is followed by a long linked list, and the time complexity of re-entry is O (n).

If it is a red-black tree, the time complexity is O (logn).

The query efficiency is greatly improved.

Thank you for your reading, the above is the content of "what is the HashMap traversal method", after the study of this article, I believe you have a deeper understanding of what the HashMap traversal method is, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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