What does '+=' mean again?

[Lisa Beglaw]

I can't remember.

[Mahesh x Bogadi]

a += b;
means
a = a+b;

[Barry Gaunt]

Originally posted by Mahesh Bogadi:
a += b;
means
a = a+b;

No it does not!

See

JLS 3 15.26.2

[Lisa Beglaw]

Thanks for the link Barry, but that website isn't very beginner friendly.

I finally managed to find it in my class notes...

number = number + value
number += value

Exactly like Mahesh said.

[Stuart Gray]

And be a bit careful because it is quite easy to write:

a=+2;

instead of:

a+=2;

The former will assign the value of 2 to a, whilst the latter will increment it by 2.

[Ryan McGuire]

Originally posted by Mahesh Bogadi:
a += b;
means
a = a+b;

Well... that's right most of the time.

With a=a+b, the 'a' expression gets evaluated twice, but with a+=b, it's evaluated only once.

For something like...

int i = 2;
i+=3;

the difference is moot.

However, for something like...

1.  int[] arr = new int[7];
2.  int index;
3.  
4.  for (index=0; index<arr.length; index++)
5.      arr[index] = index;
6. 
7.  index = 1;
8. 
9a. arr[index+=2] += 3;
    // ...or...
9b. arr[index+=2] = arr[index+=2] + 3;

lines 9a and 9b do two different things.

9a calculates the index into arr[] just once, and so is like...

index += 2;   // increment 1 to 3
arr[3] += 3;  // increment 3 to 6

But 9b is like...

index += 4;            // increment 1 to 5
arr[3] = arr[5] + 3;   // which would set arr[3] to 8

[Daniel .J.Hyslop]

hi lisa ,
I had a problem with a similar problem of identifying the difference between the two.Maybe someone could shine some light on what is exactly happening here.And what the underlying difference is?
declare and instatiate two variables:

int i = 1;
char c = 'c';

c+=i; //this compiles

c=c+i; //this does not
although both expressions say the same thing ie, add int i to char c and then initialise c to that value,one works and the other doesn`t.I can`t think that it has anything to do with casting types(although I could be wrong)so it must be something to do with the difference between the + operator and the += operator and the way they produce the outcome.Perhaps someone could shine some light on that ?

[Layne Lund]

Originally posted by Daniel .j.Hyslop:
hi lisa ,
I had a problem with a similar problem of identifying the difference between the two.Maybe someone could shine some light on what is exactly happening here.And what the underlying difference is?
declare and instatiate two variables:

int i = 1;
char c = 'c';

c+=i; //this compiles

c=c+i; //this does not
although both expressions say the same thing ie, add int i to char c and then initialise c to that value,one works and the other doesn`t.I can`t think that it has anything to do with casting types(although I could be wrong)so it must be something to do with the difference between the + operator and the += operator and the way they produce the outcome.Perhaps someone could shine some light on that ?

c += i;

contains an implicit class, so it is actually similar to

c = (char) (c + i);

So once again, the two statements given in the original post do NOT quite say the same thing.

Layne