How to implement a static method in Kotlin similar to Java or C#, this article summarizes several methods, namely: package-level function, companionObjects
, extension functions and object declarations. This requires everyone to choose according to different situations. ). These are good options, but they're not perfect, and we have better options in different situations. I have summarized several methods, namely: package-level functions, companion objects, extension functions and object declarations. This requires everyone to choose according to different situations.1. Package-level functions
The difference between Kotlin and Java and C# is that you can declare functions
directly in the package. The method is the same as in the category, so I won’t go into details here. It is indeed a very good choice. It is suitable for methods where functions do not need to include internal classes for data sharing.2. Companion object
Semantically speaking, companion functions are most similar to static methods in Java, so companion objects can completely implement static methods in Java All contents of the class. But using static methods in Java is sometimes no choice. In Kotlin, companion objects are just one of our options. Next I will introduce the companion objects. Have you ever noticed when using Java? When are the static variables
and methods in Java classes initialized? Is it when the static method is first called or when the corresponding Java class is loaded? The answer is when the Java class is loaded (that is, if you have called the instance variables and methods in the Java class, even if you have not used static variables and methods, the static variables have been initialized). Isn’t it appropriate to describe this phenomenon as accompanying?
fun main(args: Array<String>) { Books.getBestSellers() } class Books(var name: String, val page: Int) { fun getWordCount()=page*100 companion object ComBooks{ var bestSellers=arrayOf("Harry Potter\r\t","Lord of the Rings\r\t") fun getBestSellers() { bestSellers.forEach{v->println(v)} } } }
The companion class is declared using companion. It is loaded in the class where the companion object is located, and the companion object is initialized, just like Java static members. It can be anonymous or have the same name as the containing class. There are two types of calls:
Books.ComBooks.getBestSellsers()? or
Books.getBestSellsers()?. 3. Extension functions
public class Utils { public static boolean isEmpty(String string){ return string != null && string.length() == 0; } public static boolean isWeakEmpty(String string){ return isEmpty(string) && string.trim().length() == 0; } }
fun String.isEmpty() = this != null && this.length == 0; fun String.isWeakEmpty()= this.isEmpty() && this.trim().length == 0
4. Object declaration
Continue to consider the Utils class above. All methods (and sometimes variables) in this class are Static, this method does not need to be instantiated at all. In Java, we often declare this type as a static class. Is there a good solution for this situation in Kotlin? Is there a better solution than companion objects? Of course there is, object declaration is one.
object AppInfo{ var AppName = "Kotlin Message" var AppAuthor = "Riley Ge" fun toSimpleString() { println("AppName:$AppName,AppAuthor:$AppAuthor") } }
5. Summary
Having said so many methods, no one is worried that Kotlin does not have static methods now, right? Kotlin does not have it just because he Can do better. Moreover, Kotlin also gives you more choices. You can choose the appropriate method according to your actual situation to make your code efficient and beautiful.
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