A trick about non-nullable l type in kotln

最新推荐文章于 2022-01-29 18:29:22 发布

原创最新推荐文章于 2022-01-29 18:29:22 发布 · 341 阅读

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#not-null type #非null类型

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本文探讨了Kotlin中非空类型的一个鲜为人知的特性：尽管非空类型理论上不能被赋空值，但通过Java代码可以绕过这一限制，导致运行时NullPointerException。文章提供了示例代码，并建议在从Java代码赋值给非空类型变量前进行空值检查。

Note: 原创，非翻译。

What is “A trick about non-nullable type in Kotlin?”
The trick is that: "non-nullable type isn’t always being non-nullable at complie time."

As well known, a non-nullable type can’t be assigned with a null at compile time. If you force to run, your app will be failed at compile time. But Is there a way to around this constraint? Yes, there is.

Because you can compile Kotlin code with Java and Java doesn’t have a platform type called non-nullable. So that, you can pass a non-nullable type var as paramter into a Java method in which assign a null to it and return to the non-nullable var in Kotlin.

Having said that, your app still will throw an exception eventually but that will happen at runtime not at compile time. If so, the non-nullable type will lose it’s function(It’s better to detect a NullPointerExecption at compile time). Now, Let’s see how to concrete it:

        // Define a non-nullable type var.
        var test: String = ""
        
        // Assign a null to a non-nullable type var from Java code.
        test = NotificationUtil.test(test)
        
        public class NotificationUtil {
        
        public static String test(String test) {
           test = null;
           return test;
    }
}

What we should do for it ? To avoid a NullPointerExecption thrown at runtime?

Be careful when you assign a value to a non-nullable type var from Java code.
It’s better to check value whether it’s null from java code, before assigning it to a non-nullable type var.
The less java code you compiled with Kotlin, The less problems you will encounter.