Difference between declaring variables before or in loop?

Question

I have always wondered if, in general, declaring a throw-away variable before a loop, as opposed to repeatedly inside the loop, makes any (performance) difference? A (q

Answer 1

I had this very same question for a long time. So I tested an even simpler piece of code.

Conclusion: For such cases there is NO performance difference.

Outside loop case

int intermediateResult;
for(int i=0; i < 1000; i++){
    intermediateResult = i+2;
    System.out.println(intermediateResult);
}

Inside loop case

for(int i=0; i < 1000; i++){
    int intermediateResult = i+2;
    System.out.println(intermediateResult);
}

I checked the compiled file on IntelliJ's decompiler and for both cases, I got the same Test.class

for(int i = 0; i < 1000; ++i) {
    int intermediateResult = i + 2;
    System.out.println(intermediateResult);
}

I also disassembled code for both the case using the method given in this answer. I'll show only the parts relevant to the answer

Outside loop case

Code:
  stack=2, locals=3, args_size=1
     0: iconst_0
     1: istore_2
     2: iload_2
     3: sipush        1000
     6: if_icmpge     26
     9: iload_2
    10: iconst_2
    11: iadd
    12: istore_1
    13: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
    16: iload_1
    17: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
    20: iinc          2, 1
    23: goto          2
    26: return
LocalVariableTable:
        Start  Length  Slot  Name   Signature
           13      13     1 intermediateResult   I
            2      24     2     i   I
            0      27     0  args   [Ljava/lang/String;

Inside loop case

Code:
      stack=2, locals=3, args_size=1
         0: iconst_0
         1: istore_1
         2: iload_1
         3: sipush        1000
         6: if_icmpge     26
         9: iload_1
        10: iconst_2
        11: iadd
        12: istore_2
        13: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        16: iload_2
        17: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
        20: iinc          1, 1
        23: goto          2
        26: return
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           13       7     2 intermediateResult   I
            2      24     1     i   I
            0      27     0  args   [Ljava/lang/String;

If you pay close attention, only the Slot assigned to i and intermediateResult in LocalVariableTable is swapped as a product of their order of appearance. The same difference in slot is reflected in other lines of code.

• No extra operation is being performed
• intermediateResult is still a local variable in both cases, so there is no difference access time.

BONUS

Compilers do a ton of optimization, take a look at what happens in this case.

Zero work case

for(int i=0; i < 1000; i++){
    int intermediateResult = i;
    System.out.println(intermediateResult);
}

Zero work decompiled

for(int i = 0; i < 1000; ++i) {
    System.out.println(i);
}