Finalizing in JDK 16 - Pattern matching for instanceof

[Gavin Bierman]

In JDK 16 we are planning to finalize two JEPs:

  - Pattern matching for `instanceof`
  - Records

Whilst we don't have any major open issues for either of these features, I would
like us to close them out. So I thought it would be useful to quickly summarize
the features and the issues that have arisen over the preview periods so far. In
this email I will discuss pattern matching; a following email will cover the
Records feature.

Pattern matching
----------------

Adding conditional pattern matching to an expression form is the main technical
novelty of our design of this feature. There are several advantages that come
from this targeting of an expression form: First, we get to refactor a very
common programming pattern:

    if (e instanceof T) {
        T t = (T)e;         // grr...
        ...
    }

to

    if (e instanceof T t) {
                            // let the pattern matching do the work!
        ...
    }

A second, less obvious advantage is that we can combine the pattern matching
instanceof with other *expressions*. This enables us to compactly express things
with expressions that are unnecessarily complicated using statements. For
example, when implementing a class Point, we might write an equals method as
follows:

    public boolean equals(Object o) {
        if (!(o instanceof Point))
            return false;
        Point other = (Point) o;
        return x == other.x 
            && y == other.y;
    }

Using pattern matching with instanceof instead, we can combine this into a
single expression, eliminating the repetition and simplifying the control flow:

    public boolean equals(Object o) {
        return (o instanceof Point other)
            && x == other.x
            && y == other.y;
    }

The conditionality of pattern matching - if a value does not match a pattern,
then the pattern variable is not bound - means that we have to consider
carefully the scope of the pattern variable. We could do something simple and
say that the scope of the pattern variable is the containing statement and all
subsequent statements in the enclosing block. But this has unfortunate
'poisoning' consequences, e.g.

    if (a instanceof Point p) {
        ...
    } 
    if (b instanceof Point p) {         // ERROR - p is in scope
        ...
    }

In other words in the second statement the pattern variable is in a poisoned
state - it is in scope, but it should not be accessible as it may not be
instantiated with a value. Moreover, as it is in scope, we can't declare it
again. This means that a pattern variable is 'poisoned' after it is declared, so
the pattern-loving programmer will have to think of lots of distinct names for
their pattern variables. 

We have chosen another way: Java already uses flow analysis - both in checking
the access of local variables and blank final fields, and detecting unreachable
statements. We lean on this concept to introduce the new notion of flow scoping.
A pattern variable is only in scope where the compiler can deduce that the
pattern has matched and the variable will be bound. This analysis is flow
sensitive and works in a similar way to the existing analyses. Returning to our
example:

    if (a instanceof Point p) {
        // p is in scope   
        ... 
    } 
    // p not in scope here
    if (b instanceof Point p) {     // Sure!
            ...
    }

The motto is "a pattern variable is in scope where it has definitely matched".
This is intuitive, allows for the safe reuse of pattern variables, and Java
developers are already used to flow sensitive analyses. 

As pattern variables are treated in all other respects like normal variables
-- and this was an important design principle -- they can shadow fields.
However, their flow scoping nature means that some care must be taken to
determine whether a name refers to a pattern variable declaration shadowing a
field declaration or a field declaration. 

    // field p is in scope

    if (e instanceof Point p) {
        // p refers to the pattern variable
    } else {
        // p refers to the field
    }

We call this unfortunate interaction of flow scoping and shadowing the "Swiss
cheese property". To rule it out would require ad-hoc special cases or more
features, and our sense is that will not be that common, so we have decided to
keep the feature simple. We hope that IDEs will quickly come to help programmers
who have difficulty with flow scoping and shadowing.