Java vs C output

Question

This might seem simple but it's just stumbled me and my friends...

lets take the following piece of code- in java

//........

int a=10;
a= a-- + a--;
System.out.print("a="+a);
//........

in c

//........

int a=10;
a= a-- + a--;
printf("a= %d",a);
//.......

where in the former case you get output as 19 in C you get it as 18. the logic in c is understandable but in java?

in java if its like

int a=10;
a=a++;

in this case the output is 10.

So what's the logic?

Answer 1

a = a-- + a-- causes undefined behaviour in C. C does not define which decrement should be evaluated first.

a-- evaluates to the value of a, and after that it decrements a, so in Java a = a-- + a-- evaluates like this:

a = (10, decrement a) + (9, decrement a)

The second operand is 9 because first term caused a to be decremented.

In summary: With that expression, C does not define the evaluation order. Java defines it to be from left to right.