When declaring a byte in Java, you need less memory space than declaring an integer. So is it more efficient (e.g. by using less memory), to iterate through e.g. an array by using a byte, when e.g. the array length < 128?
And if it is more efficient, are there any examples when I would really recognize improvements as a user?
for(byte b = 0; b < array.length; b++) {}
instead of
for(int i = 0; i < array.length; i++) {}
No. Write the clearest, most intuitive code you can. The JVM will shine the most in the common use cases.
If you are Concerned about writing efficient code, look to your algorithms and read books on efficient java.
Well, let's look at what the compiler generates.
Source code
short[] array = new short[0];
for(int i = 0; i < array.length; i++) {}
for(byte b = 0; b < array.length; b++) {}
Byte code
0: iconst_0 array = new short[0];
1: newarray short
3: astore_1
-------------------------------------------------------
4: iconst_0 i = 0
5: istore_2
6: goto 12
9: iinc 2, 1 i++
12: iload_2 i < array.length
13: aload_1
14: arraylength
15: if_icmplt 9
-------------------------------------------------------
18: iconst_0 b = 0
19: istore_2
20: goto 28
23: iload_2 b++
24: iconst_1
25: iadd
26: i2b
27: istore_2
28: iload_2 b < array.length
29: aload_1
30: arraylength
31: if_icmplt 23
-------------------------------------------------------
34: return
i = 0 and b = 0 generates the exact same code.
i < array.length and b < array.length generates the exact same code.
However, the code for i++ and b++ is quite different. 1 bytecode instruction for an int, 5 bytecode instructions for a byte.
What JIT does with that is unknown, but if anything, using byte as the array iterator variable makes the code slower.
Though, as others have already said, you very likely will not be able to tell the difference, especially relative to what is going on inside the loop (which in reality wouldn't be blank).
Write the code that makes sense for the code logic.
