95

I want to have two loop variables of different types. Is there any way to make this work?

@Override
public T get(int index) throws IndexOutOfBoundsException {
    // syntax error on first 'int'
    for (Node<T> current = first, int currentIndex; current != null; 
            current = current.next, currentIndex++) {
        if (currentIndex == index) {
            return current.datum;
        }
    }
    throw new IndexOutOfBoundsException();
}
Ciro Santilli新疆棉花TRUMP BAN BAD
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Nick Heiner
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4 Answers4

109

The initialization of a for statement follows the rules for local variable declarations.

This would be legal (if silly):

for (int a = 0, b[] = { 1 }, c[][] = { { 1 }, { 2 } }; a < 10; a++) {
  // something
}

But trying to declare the distinct Node and int types as you want is not legal for local variable declarations.

You can limit the scope of additional variables within methods by using a block like this:

{
  int n = 0;
  for (Object o = new Object();/* expr */;/* expr */) {
    // do something
  }
}

This ensures that you don't accidentally reuse the variable elsewhere in the method.

McDowell
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    Anyone have any idea why the language designers implemented this seemingly unnecessary constraint? – Jeff Axelrod May 05 '11 at 15:44
  • @glenviewjeff - that would be best asked as a separate question. – McDowell May 05 '11 at 21:17
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    @JeffAxelrod, maybe for historical reasons because Java was modeled after C++... see this post: http://stackoverflow.com/questions/2687392/is-it-possible-to-declare-two-variables-of-different-types-in-a-for-loop – Christophe Roussy Jul 03 '13 at 09:58
20

You can't like this. Either you use multiple variables of the same type for(Object var1 = null, var2 = null; ...) or you extract the other variable and declare it before the for loop.

Colin Hebert
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10

Just move variable declarations (Node<T> current, int currentIndex) outside the loop and it should work. Something like this

int currentIndex;
Node<T> current;
for (current = first; current != null; current = current.next, currentIndex++) {

or maybe even

int currentIndex;
for (Node<T> current = first; current != null; current = current.next, currentIndex++) {
Nikita Rybak
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    Neither will compile: you have to initialize variables before use. – unbeli Aug 22 '10 at 19:06
  • @unbeli well, I wasn't exercising in manual code compilation :) I just wanted to give the idea. – Nikita Rybak Aug 22 '10 at 19:30
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    @unbeli: Just to clarify: currentIndex needs to be initialized. The first thing Nikita does to it is "currentIndex++", which naturally brings up the question, increment what? current is fine because the first use is to set it to first. – Jay Aug 27 '10 at 17:22
  • Usually to better a write an incrementation in a for loop, one should use ++var as the notation var++ required from the compiler to duplicate the var's content _before_ incrementing it to return it as the result of the expression although nobody ever wants that. Of course, compiler will optimize that out but it is like throwing trash on the road waiting for others to clean-up. – Chucky Sep 06 '13 at 17:13
5

Variables declared in the initialization block must be of same type

we can't initialize the different data types in the for loop as per their design. I'm just putting a small example.

for(int i=0, b=0, c=0, d=0....;/*condition to be applied */;/*increment or other logic*/){
      //Your Code goes here
}
Vishnu Prasanth G
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