Both of them increase the variable i by one. It's like saying i = i + 1. The difference is subtle. If you're using it in a loop like this, there's no difference:
for (int i = 0; i < 100; i++) {
}
for (int i = 0; i < 100; ++i) {
}
If you want to know the difference, look at this example:
int a = 0;
int b = a++; // b = 0; a = 1
a = 0;
b = ++a: // b = 1; a = 1
The idea is that ++a increments a and returns that value, while a++ returns a's value and then increments a.
The difference is that the post-increment operator i++ returns i as it was before incrementing, and the pre-increment operator ++i returns i as it is after incrementing. If you're asking about a typical for loop:
for (i = 0; i < 10; i++)
or
for (i = 0; i < 10; ++i)
They're exactly the same, since you're not using i++ or ++i as a part of a larger expression.
The way for loop is processed is as follows
1 First, initialization is performed (i=0)
2 the check is performed (i < n)
3 the code in the loop is executed.
4 the value is incremented
5 Repeat steps 2 - 4
This is the reason why, there is no difference between i++ and ++i in the for loop which has been used.
They both increment the number. ++i is equivalent to i = i + 1.
i++ and ++i are very similar but not exactly the same. Both increment the number, but ++i increments the number before the current expression is evaluted, whereas i++ increments the number after the expression is evaluated.
int i = 3;
int a = i++; // a = 3, i = 4
int b = ++a; // b = 4, a = 4
JLS§14.14.1, The basic for Statement, makes it clear that the ForUpdate expression(s) are evaluated and the value(s) are discarded. The effect is to make the two forms identical in the context of a for statement.
Both i++ and ++i are short-hand for i = i + 1.
In addition to changing the value of i, they also return the value of i, either before adding one (i++) or after adding one (++i).
In a loop the third component is a piece of code that is executed after each iteration.
for (int i=0; i<10; i++)
The value of that part is not used, so the above is just the same as
for(int i=0; i<10; i = i+1)
or
for(int i=0; i<10; ++i)
Where it makes a difference (between i++ and ++i )is in these cases
while(i++ < 10)
for (int i=0; i++ < 10; )
Here's a sample class:
public class Increment
{
public static void main(String [] args)
{
for (int i = 0; i < args.length; ++i)
{
System.out.println(args[i]);
}
}
}
If I disassemble this class using javap.exe I get this:
Compiled from "Increment.java"
public class Increment extends java.lang.Object{
public Increment();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iload_1
3: aload_0
4: arraylength
5: if_icmpge 23
8: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
11: aload_0
12: iload_1
13: aaload
14: invokevirtual #3; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
17: iinc 1, 1
20: goto 2
23: return
}
If I change the loop so it uses i++ and disassemble again I get this:
Compiled from "Increment.java"
public class Increment extends java.lang.Object{
public Increment();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iload_1
3: aload_0
4: arraylength
5: if_icmpge 23
8: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
11: aload_0
12: iload_1
13: aaload
14: invokevirtual #3; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
17: iinc 1, 1
20: goto 2
23: return
}
When I compare the two, TextPad tells me that the two are identical.
What this says is that from the point of view of the generated byte code there's no difference in a loop. In other contexts there is a difference between ++i and i++, but not for loops.
Source: Stackoverflow.com