具有不同类型的条件（三元）算子的局部变量型推断

Question

var test = "Hello World!";

In Java 10+, the above snippet compiles, and test is inferred to be a String at compile-time.

However, we can use the conditional (ternary) operator to return different types, such as:

var test = new Random().nextBoolean() ? "Hello World!" : 123;

If we were to print test.getClass() at runtime, it would output either:

• class java.lang.String
• class java.lang.Integer

This makes sense, but what would the type of test be at compile-time? Would it be Object , or something else?

Answer 1

Interestingly, Intellij tells me that the type of test is not Object , but rather this beast:

java.io.Serializable & Comparable<? extends java.io.Serializable & Comparable<?> & constant.Constable & constant.ConstantDesc> & constant.Constable & constant.ConstantDesc

The reason why this is the case is because the compiler infers the type of the variable to be the closest common superclasses between String and Integer , which are in-fact Serializable , Comparable , Constable , and ConstantDesc in Java 12.

There's an interesting article that elaborates on impossible types , which are what these types are known as (since they can only be inferred by the compiler): Representing the Impractical and Impossible with JDK 10 “var”

If we were to return objects that are entirely unrelated, such as Integer and ByteArrayOutputStream , then we would see that Java would infer the compile-time type of test to be Object :

var test = new Random().nextBoolean() ? 123 : new ByteArrayOutputStream();