Map.Entry m = (Map.Entry)i.next();

Why do I need to have (Map.Entry) in front of the i开发者_开发问答.next(); ? Why can't I have just Map.Entry m = i.next();?

Sorry. It is a HashMap.

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Because it's apparently not an Iterator<Map.Entry>. Maybe it's an Iterator<Object>, or an Iterator<Map.Entry<String, String>> or something different.

For example, the following Map

Map<String, Object> map = new HashMap<String, Object>();

gives the following Iterator back

Iterator<Map.Entry<String, Object>> iterator = map.entrySet().iterator();

which in turn gives the following Map.Entry back on next()

Map.Entry<String, Object> entry = iterator.next();

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because you are not using the generics correctly (it's a hassle I know to type out everything over and over and over but it's worth it) they will do that (the casting to Map.Entry) for you when it all comes together

HashMap<String, YourClass> map;
Iterator<Map.entry<String, YourClass>> it = map.entrySet().iterator();
while(it.hasNext()){
    Map.entry<String, YourClass> entry = it.next();//see no explicit cast
    //use entry
}

as a final hint copy paste (and a decent IDE) is your greatest friend with generics

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This is a bit of a wild guess because you haven't posted any code or stated which version of Java you are using, but I am guessing that:

• You are used to Java (≥1.5) code that uses generics, e.g. HashMap<K, V>
• You are now maintaining a codebase written for Java 1.4 or earlier that does not use generics.

e.g. you may know that a HashMap has String keys and Integer values, but this cannot be reflected in the type of the HashMap, so you must cast individual values.

With generics:

HashMap<String, Integer> map;

// ...

int sum = 0;
for (Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();
     it.hasNext;
     ) {
    Map.Entry<String, Integer> e = it.next();
    Integer value = e.getValue();
    sum += value.intValue();
}

Without generics:

HashMap map;

// ...

int sum = 0;
for (Iterator it = map.entrySet().iterator();
     it.hasNext;
     ) {
    Map.Entry e = (Map.Entry) it.next();
    Integer value = (Integer) e.getValue();
    sum += value.intValue();
}

If generics are unavailable (or you are working with a legacy library that gives you non-generic collections) you have no choice but to apply these casts.
BTW: Even if you do have generics, these casts are still applied under-the-hood by the compiler.

Note: leaving out foreach loops and auto-unboxing as these are also unavailable in Java 1.4 and earlier.

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Because i.next() returns object when you don't use generics and you have to cast it to be able to assign it.