each loop, or an iterator?
Which is the most efficient way to traverse a collection?
List<Integer> a = new ArrayList<Integer>();
for (Integer integer : a) {
integer.toString();
}
or
List<Integer> a = new ArrayList<Integer>();
for (Iterator iterator = a.iterator(); iterator.hasNext();) {
Integer integer = (Integer) iterator.next();
integer.toString();
}
Please note, that this is not an exact duplicate of this, this, this, or this, although one of the answers to the last question comes close. The reason that this is not a dupe, is that most of these are comparing loops where you call get(i)
inside the loop, rather than using the iterator.
As suggested on Meta I will be posting my answer to this question.
If you are just wandering over the collection to read all of the values, then there is no difference between using an iterator or the new for loop syntax, as the new syntax just uses the iterator underwater.
If however, you mean by loop the old "c-style" loop:
for(int i=0; i<list.size(); i++) {
Object o = list.get(i);
}
Then the new for loop, or iterator, can be a lot more efficient, depending on the underlying data structure. The reason for this is that for some data structures, get(i)
is an O(n) operation, which makes the loop an O(n2) operation. A traditional linked list is an example of such a data structure. All iterators have as a fundamental requirement that next()
should be an O(1) operation, making the loop O(n).
To verify that the iterator is used underwater by the new for loop syntax, compare the generated bytecodes from the following two Java snippets. First the for loop:
List<Integer> a = new ArrayList<Integer>();
for (Integer integer : a)
{
integer.toString();
}
// Byte code
ALOAD 1
INVOKEINTERFACE java/util/List.iterator()Ljava/util/Iterator;
ASTORE 3
GOTO L2
L3
ALOAD 3
INVOKEINTERFACE java/util/Iterator.next()Ljava/lang/Object;
CHECKCAST java/lang/Integer
ASTORE 2
ALOAD 2
INVOKEVIRTUAL java/lang/Integer.toString()Ljava/lang/String;
POP
L2
ALOAD 3
INVOKEINTERFACE java/util/Iterator.hasNext()Z
IFNE L3
And second, the iterator:
List<Integer> a = new ArrayList<Integer>();
for (Iterator iterator = a.iterator(); iterator.hasNext();)
{
Integer integer = (Integer) iterator.next();
integer.toString();
}
// Bytecode:
ALOAD 1
INVOKEINTERFACE java/util/List.iterator()Ljava/util/Iterator;
ASTORE 2
GOTO L7
L8
ALOAD 2
INVOKEINTERFACE java/util/Iterator.next()Ljava/lang/Object;
CHECKCAST java/lang/Integer
ASTORE 3
ALOAD 3
INVOKEVIRTUAL java/lang/Integer.toString()Ljava/lang/String;
POP
L7
ALOAD 2
INVOKEINTERFACE java/util/Iterator.hasNext()Z
IFNE L8
As you can see, the generated byte code is effectively identical, so there is no performance penalty to using either form. Therefore, you should choose the form of loop that is most aesthetically appealing to you, for most people that will be the for-each loop, as that has less boilerplate code.
The difference isn't in performance, but in capability. When using a reference directly you have more power over explicitly using a type of iterator (eg List.iterator() vs. List.listIterator(), although in most cases they return the same implementation). You also have the ability to reference the Iterator in your loop. This allows you to do things like remove items from your collection without getting a ConcurrentModificationException.
eg
This is ok:
Set<Object> set = new HashSet<Object>();
// add some items to the set
Iterator<Object> setIterator = set.iterator();
while(setIterator.hasNext()){
Object o = setIterator.next();
if(o meets some condition){
setIterator.remove();
}
}
This is not, as it will throw a concurrent modification exception:
Set<Object> set = new HashSet<Object>();
// add some items to the set
for(Object o : set){
if(o meets some condition){
set.remove(o);
}
}
To expand on Paul's own answer, he has demonstrated that the bytecode is the same on that particular compiler (presumably Sun's javac?) but different compilers are not guaranteed to generate the same bytecode, right? To see what the actual difference is between the two, let's go straight to the source and check the Java Language Specification, specifically 14.14.2, "The enhanced for statement":
The enhanced for
statement is equivalent to a basic for
statement of the form:
for (I #i = Expression.iterator(); #i.hasNext(); ) {
VariableModifiers(opt) Type Identifier = #i.next();
Statement
}
In other words, it is required by the JLS that the two are equivalent. In theory that could mean marginal differences in bytecode, but in reality the enhanced for loop is required to:
.iterator()
method .hasNext()
.next()
So, in other words, for all practical purposes the bytecode will be identical, or nearly-identical. It's hard to envisage any compiler implementation which would result in any significant difference between the two.
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