a++被称为后缀。

a加1，返回原来的值。

++a被称为前缀。

对a加1，返回新值。

C#:

string[] items = {"a","b","c","d"};
int i = 0;
foreach (string item in items)
{
    Console.WriteLine(++i);
}
Console.WriteLine("");

i = 0;
foreach (string item in items)
{
    Console.WriteLine(i++);
}

输出:

1
2
3
4

0
1
2
3

Foreach和while循环取决于您使用的增量类型。对于下面这样的for循环，它没有什么区别，因为你没有使用i的返回值:

for (int i = 0; i < 5; i++) { Console.Write(i);}
Console.WriteLine("");
for (int i = 0; i < 5; ++i) { Console.Write(i); }

0, 1, 2, 3, 4 0, 1, 2, 3, 4

如果使用所计算的值，则增量类型变得显著:

int n = 0;
for (int i = 0; n < 5; n = i++) { }

我+ +;+ +我;两者都是相似的，因为它们不在表达式中使用。

class A {

     public static void main (String []args) {

     int j = 0 ;
     int k = 0 ;
     ++j;
     k++;
    System.out.println(k+" "+j);

}}

prints out :  1 1

下面是一个Java-Sample和字节码，后增量和前增量显示字节码没有区别:

public class PreOrPostIncrement {

    static int somethingToIncrement = 0;

    public static void main(String[] args) {
        final int rounds = 1000;
        postIncrement(rounds);
        preIncrement(rounds);
    }

    private static void postIncrement(final int rounds) {
        for (int i = 0; i < rounds; i++) {
            somethingToIncrement++;
        }
    }

    private static void preIncrement(final int rounds) {
        for (int i = 0; i < rounds; ++i) {
            ++somethingToIncrement;
        }
    }
}

现在对于字节码(javap -private -c PreOrPostIncrement):

public class PreOrPostIncrement extends java.lang.Object{
static int somethingToIncrement;

static {};
Code:
0:  iconst_0
1:  putstatic   #10; //Field somethingToIncrement:I
4:  return

public PreOrPostIncrement();
Code:
0:  aload_0
1:  invokespecial   #15; //Method java/lang/Object."<init>":()V
4:  return

public static void main(java.lang.String[]);
Code:
0:  sipush  1000
3:  istore_1
4:  sipush  1000
7:  invokestatic    #21; //Method postIncrement:(I)V
10: sipush  1000
13: invokestatic    #25; //Method preIncrement:(I)V
16: return

private static void postIncrement(int);
Code:
0:  iconst_0
1:  istore_1
2:  goto    16
5:  getstatic   #10; //Field somethingToIncrement:I
8:  iconst_1
9:  iadd
10: putstatic   #10; //Field somethingToIncrement:I
13: iinc    1, 1
16: iload_1
17: iload_0
18: if_icmplt   5
21: return

private static void preIncrement(int);
Code:
0:  iconst_0
1:  istore_1
2:  goto    16
5:  getstatic   #10; //Field somethingToIncrement:I
8:  iconst_1
9:  iadd
10: putstatic   #10; //Field somethingToIncrement:I
13: iinc    1, 1
16: iload_1
17: iload_0
18: if_icmplt   5
21: return

}

要理解FOR循环的作用

上图显示FOR可以转换为WHILE，因为它们最终具有完全相同的汇编代码(至少在gcc中)。所以我们可以把FOR分解成几部分，来理解它的功能。

for (i = 0; i < 5; ++i) {
  DoSomethingA();
  DoSomethingB();
}

等于WHILE版本

i = 0; //first argument (a statement) of for
while (i < 5 /*second argument (a condition) of for*/) {
  DoSomethingA();
  DoSomethingB();
  ++i; //third argument (another statement) of for
}

It means that you can use FOR as a simple version of WHILE: The first argument of FOR (int i) is executed, outside, before the loop. The third argument of FOR (i++ or ++i) is executed, inside, in the last line of the loop. TL:DR: no matter whether i++ or ++i, we know that when they are standalone, they make no difference but +1 on themselves. In school, they usually teach the i++ way, but there are also lots of people prefer the ++i way due to several reasons. NOTE: In the past, i++ has very little impact on the performance, as it does not only plus one by itself, but also keeps the original value in the register. But for now, it makes no difference as the compiler makes the plus one part the same.

我不知道其他语言，但在java++中，I是一个前缀增量，这意味着:将I增加1，然后在I所在的表达式中使用I的新值，i++是一个后缀增量，这意味着:在表达式中使用I的当前值，然后将其增加1。 例子:

public static void main(String [] args){

    int a = 3;
    int b = 5;
    System.out.println(++a);
    System.out.println(b++);
    System.out.println(b);

｝ 输出为:

4 5 6

正如这段代码所示(请参阅注释中拆解的MSIL)， c# 3编译器在for循环中不区分i++和++i。如果取i++或++i的值，肯定会有区别(这是在visual Studio 2008 / Release Build中编译的):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace PreOrPostIncrement
{
    class Program
    {
        static int SomethingToIncrement;

        static void Main(string[] args)
        {
            PreIncrement(1000);
            PostIncrement(1000);
            Console.WriteLine("SomethingToIncrement={0}", SomethingToIncrement);
        }

        static void PreIncrement(int count)
        {
            /*
            .method private hidebysig static void  PreIncrement(int32 count) cil managed
            {
              // Code size       25 (0x19)
              .maxstack  2
              .locals init ([0] int32 i)
              IL_0000:  ldc.i4.0
              IL_0001:  stloc.0
              IL_0002:  br.s       IL_0014
              IL_0004:  ldsfld     int32 PreOrPostIncrement.Program::SomethingToIncrement
              IL_0009:  ldc.i4.1
              IL_000a:  add
              IL_000b:  stsfld     int32 PreOrPostIncrement.Program::SomethingToIncrement
              IL_0010:  ldloc.0
              IL_0011:  ldc.i4.1
              IL_0012:  add
              IL_0013:  stloc.0
              IL_0014:  ldloc.0
              IL_0015:  ldarg.0
              IL_0016:  blt.s      IL_0004
              IL_0018:  ret
            } // end of method Program::PreIncrement             
             */
            for (int i = 0; i < count; ++i)
            {
                ++SomethingToIncrement;
            }
        }

        static void PostIncrement(int count)
        {
            /*
                .method private hidebysig static void  PostIncrement(int32 count) cil managed
                {
                  // Code size       25 (0x19)
                  .maxstack  2
                  .locals init ([0] int32 i)
                  IL_0000:  ldc.i4.0
                  IL_0001:  stloc.0
                  IL_0002:  br.s       IL_0014
                  IL_0004:  ldsfld     int32 PreOrPostIncrement.Program::SomethingToIncrement
                  IL_0009:  ldc.i4.1
                  IL_000a:  add
                  IL_000b:  stsfld     int32 PreOrPostIncrement.Program::SomethingToIncrement
                  IL_0010:  ldloc.0
                  IL_0011:  ldc.i4.1
                  IL_0012:  add
                  IL_0013:  stloc.0
                  IL_0014:  ldloc.0
                  IL_0015:  ldarg.0
                  IL_0016:  blt.s      IL_0004
                  IL_0018:  ret
                } // end of method Program::PostIncrement
             */
            for (int i = 0; i < count; i++)
            {
                SomethingToIncrement++;
            }
        }
    }
}