Scala属性定义中`= _`的作用及相关语法特性名称咨询

Great question! Let's break down what this syntax does and its official name clearly.

What does = _ do? #

When you declare a var (mutable variable) in Scala, the compiler enforces that every variable is initialized before it’s used. The = _ syntax tells the compiler to automatically assign the default value matching the variable’s type. Here are common examples of these default values:

• For reference types like String, the default is null
• For numeric types such as Int or Double, it’s 0 or 0.0 respectively
• For Boolean, it’s false
• For custom reference types, the default is also null

In your code var folderName: String = _, this means folderName starts out as null until you assign a concrete string value to it later in your code.

Official Name of This Syntax #

This feature is officially called Default Initialization in Scala’s documentation. You might also hear it referred to informally as Wildcard Initialization, since the underscore (_) acts as a placeholder for the default value of the declared type.

A quick caveat: This syntax only works with var variables. You can’t use = _ with val (immutable variables), because val requires an explicit, fixed initial value at the time of declaration.

内容的提问来源于stack exchange，提问作者Yuchen