Does it make a difference if I declare variables inside or outside a loop in Java?
Is this
for(int i = 0; i < 1000; i++) {
int temp = doSomething();
someMethod(temp);
}
equal to this (with respect to memory usage)?
int temp = 0;
for(int i = 0; i < 1000; i++) {
temp = doSomething();
someMethod(temp);
}
And what if the temporary variable is for example an ArrayList?
for(int i = 0; i < 1000; i++) {
ArrayList<Integer> array = new ArrayList<Integer>();
fillArray(array);
// do something with the array
}
EDIT: with javap -c
I got the following output
Variable outside the loop:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_0
3: istore_2
4: iload_2
5: sipush 1000
8: if_icmpge 25
11: invokestatic #2 // Method doSomething:()I
14: istore_1
15: iload_1
16: invokestatic #3 // Method someMethod:(I)V
19: iinc 2, 1
22: goto 4
25: return
Variable inside the loop:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iload_1
3: sipush 1000
6: if_icmpge 23
9: invokestatic #2 // Method doSomething:()I
12: istore_2
13: iload_2
14: invokestatic #3 // Method someMethod:(I)V
17: iinc 1, 1
20: goto 2
23: return
And for the interested, this code:
public class Test3 {
public static void main(String[] args) {
for(int i = 0; i< 1000; i++) {
someMethod(doSomething());
}
}
private static int doSomething() {
return 1;
}
private static void someMethod(int temp) {
temp++;
}
}
produces this:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iload_1
3: sipush 1000
6: if_icmpge 21
9: invokestatic #2 // Method doSomething:()I
12: invokestatic #3 // Method someMethod:(I)V
15: iinc 1, 1
18: goto 2
21: return
But the optimization happens at runtime then. Is there a way to look at the optimized code? (Sorry for the long EDIT)