Why foreach could not be used with String?

Question

I am trying something like

String s = "test string";
for(Character c: s) {

}

The compiler reports error. I am wondering the reason why I could not use foreach with String?

`String` is not an array nor Iterable. You can iterate over `s.toCharArray()`. — shmosel, Nov 05 '18 at 03:10

score 4 · Accepted Answer · edited Aug 18 '22 at 14:13

4

The reason is set out in JLS 14.14.2:

EnhancedForStatement:
    for ( {VariableModifier} LocalVariableType VariableDeclaratorId : Expression ) Statement 
...

The type of the Expression must be a subtype of the raw type Iterable or an array type (§10.1), or a compile-time error occurs.

A String is not a subtype of Iterable or an array type. Therefore .... compilation error.

As @shmosel mentions, you can iterate over the char[] returned by s.toCharArray(). However, that will create a new array¹.

^{1 - ... unless your JVMs JIT compiler is smart enough to optimize that away. I don't think they currently can do that, and I wouldn't bet on the Java designers wanting to implement that optimization.}

edited Aug 18 '22 at 14:13

Federico klez Culloca

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answered Nov 05 '18 at 03:29

Stephen C

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It would be kind of cool if in some future release of Java, `String` was made to `implements Iterable`. – Dawood ibn Kareem Nov 05 '18 at 03:43
I'd like to see JIT optimization of `Character` to `char` first. – Stephen C Nov 05 '18 at 03:48

Why foreach could not be used with String?

1 Answers1