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I am reading Javadoc for LinkedHashMap, where it is mentioned:

The putAll method generates one entry access for each mapping in the specified map, in the order that key-value mappings are provided by the specified map's entry set iterator.

My question is, what does it mean "one entry access for each mapping". It would be appreciated if anyone could help to provide an example to clarify this.

Jacob Schoen
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Lin Ma
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3 Answers3

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This paragraph applies to maps created with the special constructor that makes the iteration order based on last access order (vs. insertion order for a standard LinkedHashMap.

It simply says that if a key K is in the map and you call putAll(someOtherMap) where someOtherMap contains K too, this will be considered as an access to K and it will be moved to the end of the map (from an iteration order's perspective).

In other words, from an access perspective, putAll is equivalent to for (Entry e : entries) map.put(e); (in pseudo code).

Contrived example:

public static void main(String[] args) throws Exception {
    Map<String, String> m = new LinkedHashMap<> (16, 0.75f, true);

    m.put("a", "a");
    m.put("b", "b");
    System.out.println("m = " + m); // a, b
    m.put("a", "a");
    System.out.println("m = " + m); // b, a

    Map<String, String> m2 = new LinkedHashMap<>();
    m2.put("b", "b");

    m.putAll(m2);
    System.out.println("m = " + m); // a, b: putAll was considered as an access
                                    // and the order has changed
}
assylias
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The comment ("The putAll method generates one entry access for each mapping in the specified map, in the order that key-value mappings are provided by the specified map's entry set iterator.") in the API documentation should be viewed in the context. Here is the complete API document with the context:

A special constructor is provided to create a linked hash map whose order of iteration is the order in which its entries were last accessed, from least-recently accessed to most-recently (access-order). This kind of map is well-suited to building LRU caches. Invoking the put or get method results in an access to the corresponding entry (assuming it exists after the invocation completes). The putAll method generates one entry access for each mapping in the specified map, in the order that key-value mappings are provided by the specified map's entry set iterator. No other methods generate entry accesses...

This section describes what defines 'access' which affects the determination of "last accessed". In that context, it goes on to describe how put/get and putall are treated with respect to access of mappings. A put(k,v) and get(k) are treated as an access each. Similarly, putAll() is treated as one access for all mappings in the order as maintained by the entry set. You can imagine this as, for each putAll(), all the mappings' access counter will be bumped up by 1, in the order maintained by entry set.

Hope this is what you are looking for.

Anand
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LinkedHashMap preserve the order in which you put element in it. The putAll method is used to copy all of the mappings from the specified map to this map.

Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress.

"one entry access for each mapping" means the effect of putall call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map.

Achintya Jha
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