Why use Map.entrySet()

[Zhixiong Pan]

Hi ranchers,
In what cases, will Map.entrySet() be used? Thanks.

[Deepak Bala]

I have not personally used this myself. Each entry in the Map is represented by a Map.Entry, and the entry set can be used to iterate through the entries. Useful if you need to access each entry as a whole.

[Campbell Ritchie]

I think I have used it and John Meyers is correct. I am writing from memory, but it goes something like this:-

private Map<String, Foo> fooMap;
. . .
fooMap = new HashMap<String, Foo>();
. . .
fooMap.add("Campbell", fooCampbell);
fooMap.add("John Meyers", fooJMeyers);
fooMap.add("Zhixiong Pan", fooZhPan);
. . .
Set<Map.Entry<String, Foo>> fooMapSet = fooMap.entrySet();
iterator<Map.Entry<String, Foo>> iter = fooMapSet.iterator();
while(iter.hasNext())
{
   Foo myFoo = iter.next().getValue();
   myFoo.someMethodOrOther();
}//end while
. . .

It was the only way I could find of gaining access to the "V" entries in a HashMap in order without knowing the "K" values. Of course, you can get access to the "K" values using getKey() instead of getValue().

I think that is how it is supposed to work.

[Jim Yingst]

If you just need keys, use keySet(). If you just need values, use values(). If you're going to use keys and values in your subsequent code, then you're best off using entrySet().

I frequently see people do this without entrySet(), and it usually looks something like this:

for (Iterator it = map.keySet().iterator(); it.hasNext(); ) {
    Foo key = (Foo) it.next();
    Bar value = (Bar) map.get(key);
    // now do something with key and value
}

This works, but it's making the JVM do extra work for no good reason. Every time you call get() you're making the JVM spend time doing a hashcode lookup, or navigating a tree and evaluating a comparator. These operations may be fast, or not, but why do them if you don't have to? A Map.Entry gives you both key and value, together, in the most efficient manner possible.

for (Iterator it = map.entrySet().iterator(); it.hasNext(); ) {
    Map.Entry e = (Map.Entry) it.next();
    Foo key = (Foo) e.getKey();
    Bar value = (Bar) e.getValue();
    // now do something with key and value
}

Under JDK 5 and later it's a little nicer:

for (Map.Entry<Foo,Bar> e : map.entrySet()) {
    Foo key = e.getKey();
    Bar value = e.getValue();
    // now do something with key and value
}

[Henry Wong]

Originally posted by Jim Yingst:

I frequently see people do this without entrySet(), and it usually looks something like this:

for (Iterator it = map.keySet().iterator(); it.hasNext(); ) {
    Foo key = (Foo) it.next();
    Bar value = (Bar) map.get(key);
    // now do something with key and value
}

I do this a *lot*. I don't really know why -- maybe it has to do with muscle memory. It does seem to be a habit that I can't seem to break.

So, I guess my response to "Why use Map.entrySet()" is ... I don't, but I should...

Henry

[Bear Bibeault]

I must be ahead of the curve . I use it all the time!

[Mr. C Lamont Gilbert]

Whenever I am getting all the data out of the Map I use entrySet. I do use it a lot. Especially when copying the Map, or converting it into a different format. Most recently I created a database dump to xml and I had to get all the contents of a Map. I used entryset.