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kotlinlang

`SignInFragment.newInstance() as? Fragment` is equal to the following java code:
```
Fragment result = null;
Object expression = SignInFragment.newInstance();
if (expression instanceof Fragment) {
 result = (Fragment) expression;
}
// result is null if expression is not fragment or expression is null
```
`SignInFragment.newInstance() as Fragment` is equal to this java code:
```
Fragment result = (Fragment) SignInFragment.newInstance(); // May throw ClassCastExcpetion if object is not fragment
requireNotNull(result);
```
`SignInFragment.newInstance() as Fragment?` is equal to this java code:
```
Fragment result = (Fragment) SignInFragment.newInstance();
// result may be null if newInstance returns null
```