String concatenation with Null

Question

I have the following code ``` System.out.println("" + null); ``` and the output is `null`. How does Java do the trick in string concatenation?

Accepted Answer

Because Java converts the expression `"A String" + x` to something along the lines of `"A String" + String.valueOf(x)`

In actual fact I think it probably uses `StringBuilder`s, so that:

```
"A String " + x + " and another " + y
```

resolves to the more efficient

```
new StringBuilder("A String ")
    .append(x)
    .append(" and another ")
    .append(y).toString()
```

This uses the `append` methods on String builder (for each type), which handle `null` properly

Solution