Java预增和后增运算符 [英] Java preincrement and postincrement operators

问题描述

public class Sample 
{
    public static void main(String[] args) throws Exception 
    {
        //part 1
        int i=1;
        i=i++;
        i=++i;
        i=i++;
        System.out.println(i);

        //part 2
        i=1;
        int a=i++;
        a=++i;
        a=i++;
        System.out.println(a+"\n"+i);
    }
}

输出
2
3
4

Output
2
3
4

昨天我的朋友问了这个问题.我对此有些困惑. 第1部分将 i 值打印为 2 .帖子增量在这里不起作用.但是在第2部分中，它可以工作.我可以理解第2部分，但对第1部分感到困惑.实际如何运作?有人能让我明白吗?

Yesterday my friend asked this question. I little bit confused about this. part 1 prints the i value as 2. Post increment is not working here. But in the part 2, it works. I can understand the part 2 but i have confused in part 1. How actually it works? Can anybody make me understand?

推荐答案

第一部分应打印i = 2.这是因为:

Part one should print i = 2. This is because:

public class Sample 
{
    public static void main(String[] args) throws Exception 
    {
        //part 1
        int i=1;
        // i++ means i is returned (which is i = 1), then incremented, 
        //  therefore i = 1 because i is incremented to 2, but then reset 
        //  to 1 (i's initial value)
        i=i++;
        // i is incremented, then returned, therefore i = 2
        i=++i;
        // again, first i is returned, then incremented, therefore i = 2 
        //  (see first statement)
        i=i++;
        System.out.println(i);

        //part 2
        i=1;
        // first i is returned then incremented, so i = 2, a = 1
        int a=i++;
        // i is incremented then returned, so i = 3 and a = 3
        a=++i;
        // i is first returned, then incremented, so a = 3 and i = 4
        a=i++;
        System.out.println(a+"\n"+i);
    }
}

也许最简单的理解方法是引入一些额外的变量.这是您拥有的:

Perhaps the simplest way to understand this is to introduce some extra variables. Here is what you have:

// i = i++
temp = i;
i = i + 1;
i = temp; // so i equals the initial value of i (not the incremented value)

// i = ++i;
i = i + 1;
temp = i;
i = temp; // which is i + 1

// i = i++
temp = i;
i = i + 1;
i = temp; // so i equals the previous value of i (not i + 1)

请注意设置temp变量时的顺序不同-增量是之前还是之后，具体取决于它是后增量( i++)或预递增(++i).

Notice the difference in order of when the temp variable is set--either before or after the increment depending on whether or not it's a post-increment (i++) or a pre-increment (++i).

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