Why is System.in declared as nullInputStream() instead of null?

Question

In the `System` class, `in`, `out`, and `err` are static fields. These fields are declared for example: ``` public final static InputStream in = nullInputStream(); ``` Why declare `nullInputStream()` instead of `null`?

Accepted Answer

The source code has the following comment:

```
/**
 * The following two methods exist because in, out, and err must be
 * initialized to null.  The compiler, however, cannot be permitted to
 * inline access to them, since they are later set to more sensible values
 * by initializeSystemClass().
 */
```

In short, since `System.in` is a `static final` variable, if it was set to `null`, the compiler would consider it as a constant, and would replace all the references to `System.in` in other classes with `null` (that's what inlining means). Which would obviously make everything non-functional. Some native code must be used to replace the value of this `System.in` final value (which normally should never change) once the system is initialized.

To resume: it's used to avoid a compiler optimization that should not be made in this very particular case, because System.in is a final field that can change, which is normally impossible.

Why is System.in declared as nullInputStream() instead of null?

Solution

Problem