In a java enhanced for loop, is it safe to assume the expression to be looped over will be evaluated only once?
Question
In Java, a for-each loop.
If I have a method that generates an array, called genArray()
.
In the following code, will the array each time be re-generated by calling genArray()
?
Or will Java call once the method and store a copy from the array?
for (String s : genArray())
{
//...
}
Thanks
Solution
About the enhanced for statement, the Java Language Specifications writes:
The enhanced for statement has the form:
EnhancedForStatement: for ( VariableModifiersopt Type Identifier: Expression) Statement
The Expression must either have type
Iterable
or else it must be of an array type (§10.1), or a compile-time error occurs.The scope of a local variable declared in the FormalParameter part of an enhanced
for
statement (§14.14) is the contained StatementThe meaning of the enhanced
for
statement is given by translation into a basicfor
statement.If the type of
Expression
is a subtype ofIterable
, then letI
be the type of the expression Expression.iterator()
. The enhancedfor
statement is equivalent to a basicfor
statement of the form:for (I #i = Expression.iterator(); #i.hasNext(); ) { VariableModifiersopt Type Identifier = #i.next(); Statement }
Where
#i
is a compiler-generated identifier that is distinct from any other identifiers (compiler-generated or otherwise) that are in scope (§6.3) at the point where the enhanced for statement occurs.Otherwise, the Expression necessarily has an array type,
T[]
. LetL1 ... Lm
be the (possibly empty) sequence of labels immediately preceding the enhancedfor
statement. Then the meaning of the enhanced for statement is given by the following basicfor
statement:T[] a = Expression; L1: L2: ... Lm: for (int i = 0; i < a.length; i++) { VariableModifiersopt Type Identifier = a[i]; Statement }
Where a and i are compiler-generated identifiers that are distinct from any other identifiers (compiler-generated or otherwise) that are in scope at the point where the enhanced for statement occurs.
So in your case, genArray()
doesn't return a subtype of Iterable
but an array type, so your enhanced for
statement is equivalent to the following basic for
statement:
String[] a = genArray();
...
for (int i = 0; i < a.length; i++) {
String s = a[i];
// ...
}
And genArray()
will thus be called only once (but the currently accepted answer is partially wrong).
OTHER TIPS
Java will call genArray() once, get the iterator object, and call that multiple times.
It should only get used once - it's the same as calling this:
String[] strings = genArray();
for (String s : strings) {
...