andyf - 8 months ago 23

Java Question

Why java

`+=`

`warning`

I tried eclipse & IntelliJ but both not show any warning.

Sample code:

`{`

long a = 20000000000000000L;

double b = 90.0;

a += b;

System.out.println(a); // 20000000000000088 NG

}

{

long a = 10000000000000000L;

double b = 90.0;

a += b;

System.out.println(a); // 10000000000000090 OK

}

{

long a = 20000000000000000L;

double b = 90.0;

a += (long) b;

System.out.println(a); // 20000000000000090 OK

}

Answer

According to the JLS, this compound assignment expression

```
a += b;
```

is equivalent to

```
a = (long) (a + b);
```

Since `b`

is a `double`

, binary numeric promotion occurs before the addition happens. Both operands are converted to `double`

values and the addition happens with floating point arithmetic.

The value `20000000000000090`

cannot be represented exactly as a `double`

and therefore you lose precision, getting `20000000000000088`

instead. It then gets cast back to a `long`

which has enough precision to represent `20000000000000088`

.

Your second snippet produces `10000000000000090`

which can be represented exactly as a `double`

.

Your third snippet is equivalent to

```
a = (long) (a + (long) b);
```

which uses integer arithmetic. `20000000000000090`

can be represented as a `long`

.

I don't know of any tools to warn you about this.

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