自加表达式在JAVA与C中的不同表现方式

今天看<>时突然发现一个关于 i++ 这种后缀表达式的小问题，与C/C++中的表现居然存在差别

public class Increment{
    public static void main(String[] args){
        int j = 0;
        int i = 0;
        for( i = 0; i < 100; i++ )
            j = j++;
        System.out.println(" j = " + j);
    }
}
javac Increment.java
java  Increment
得到的结果居然为 j = 0;
解析：The value of the expression j++ is the original value of j before it
was incremented. therefore, the preceding assignment first save the value of
j, the sets j to its value plus, and finally, resets j back to its original
value. In other words, the assigment is equivalent ot this sequence of statement:
    int tmp = j;
    j = j + 1;
    j = tmp;
The Lession is : Do not assign to the same variable more than once in a single
expression

but in C
//file t4.c
int main(void){
    int j = 0;
    int i = 0;
    for( i = 0; i < 100; i++)
        j = j++;
    printf("j= %d\n", j);
}
cc t4.c
./a.out
the output is:  j = 100;

what is the problem?