条件演算子と親クラスへの型推論（Java編）

プログラミング言語 Javaにおける条件演算子(conditional operator) ?: と、子クラスから親クラスへの型推論（の一部）に関してメモ。

条件演算子の第2, 3項目に親クラスParentから派生した子クラスChild1, Child2を指定した場合、Javaでは部分式 (b ? c1 : c2) の静的型は共通の親クラスParentへと型推論される。*1

class Parent {}
class Child1 extends Parent {}
class Child2 extends Parent {}

// 恒等関数id（ジェネリックメソッド）
static <T> T id(T x) { return x; }

boolean b = /*...*/;
Child1 c1 = new Child1();
Child2 c2 = new Child2();
Parent r = id(b ? c1 : c2);  // ★OK: T=Parentに型推論される

Java Language Specification, Java SE 8 Editionより一部引用。

There are three kinds of conditional expressions, classified according to the second and third operand expressions: boolean conditional expressions, numeric conditional expressions, and reference conditional expressions. The classification rules are as follows:

If both the second and the third operand expressions are boolean expressions, the conditional expression is a boolean conditional expression. (snip)

If both the second and the third operand expressions are numeric expressions, the conditional expression is a numeric conditional expression. (snip)

Otherwise, the conditional expression is a reference conditional expression.

The process for determining the type of a conditional expression depends on the kind of conditional expression, as outlined in the following sections.
Chapter 15. Expressions, 15.25. Conditional Operator ?

A reference conditional expression is a poly expression if it appears in an assignment context or an invocation context (§5.2. §5.3). Otherwise, it is a standalone expression.
(snip)
The type of a standalone reference conditional expression is determined as follows:

(snip)

Otherwise, the second and third operands are of types S₁ and S₂ respectively. Let T₁ be the type that results from applying boxing conversion to S₁, and let T₂ be the type that results from applying boxing conversion to S₂. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T₁, T₂).

Chapter 15. Expressions, 15.25.3. Reference Conditional Expressions

メモ： Child1, Child2に対するboxing conversionは恒等変換のため*2、T₁=S₁=Child1, T₂=S₂=Child2となる。lub(Child1, Child2) はLeast Upper Bound型推論過程（§4.10.4）によりParentとなる。同型に対するcapture conversionも恒等変換のため*3、最終的にはParentへと型推論される。

U₁ = Child1, U₂ = Child2
ST(U₁) = EST(U₁) = { Object, Parent, Child1 }
ST(U₂) = EST(U₂) = { Object, Parent, Child2 }
EC = { Object, Parent }
MEC = { Parent }, G = {}
lub(U₁, U₂) = Parent

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条件演算子と親クラスへの型推論（Java編）