Kotlin函数默认值完全解析

简要介绍

Kotlin函数定义时,支持对参数指定默认值,这样就有效减少Java之前定义重载函数的数量. 简要对比如下:

1. Java函数定义,如果sayHelloTo函数中的lastName可以使用默认值,则需要定义如下两个函数.

public class Person {
    public void sayHelloTo(String firstName,String lastName){
        System.out.println("Hi,"+firstName+" "+lastName);
    }
    public void sayHelloTo(String firstName){
        sayHelloTo(firstName,"noLastName");
    }
}

2. Kotlin定义如下,只需要一个函数

class Person {
    fun sayHelloTo(firstName: String, lastName: String = "noLastName") {
        println("Hi,$firstName $lastName")
    }
}

Kotlin函数默认值详细规则

1. 可以采用指定全部参数名和不指定参数名的调用方式,推荐采用指定参数名的调用方式(不能指定部分函数参数名,其他的不指定) 正确样例: sayHelloTo("Jerry","Aha") sayHelloTo(firstName = "Jerry",lastName = "Aha") 错误样例:sayHelloTo(firstName = "Jerry","Aha")
2. 在指定参数名时,所有无默认值参数传入值即可 样例:sayHelloTo(firstName = "Jerry",lastName = "Aha")
3. 在不指定参数名时,则需要区分情况:

• 如果函数定义时,所有具有默认值的参数都在最后,则可以只传入无默认值参数值. 样例: 函数定义:fun sayHelloTo(firstName: String, lastName: String = "Aha") 正确样例:sayHelloTo("Jerry")
• 如果函数定义时,具有默认值的参数并不都是在最后面,则必须传入所有参数值. 样例: 函数定义:fun sayHelloTo(firstName: String, middleName: String = "",lastName: String) 正确样例:sayHelloTo("Jerry","","Aha") 错误样例:sayHelloTo("Jerry","Aha")

Demo代码

1. Person.kt

class Person(
    val firstName: String = "Jerry",
    val lastName: String = "Yin"
) {
    fun sayHello() {
        System.out.println("Hi,$firstName $lastName")
    }

    fun sayHelloTo(firstName: String = "Baby", lastName: String = "Aha") {
        System.out.println("Hi,$firstName $lastName")
    }
}

2. Person2.kt

class Person2(
    val firstName: String = "Jerry",
    val lastName: String
) {
    fun sayHello() {
        System.out.println("Hi,$firstName $lastName")
    }

    fun sayHelloTo(firstName: String = "Baby", lastName: String) {
        System.out.println("Hi,$firstName $lastName")
    }
}

3. 调用代码:

Person().sayHello()
Person("Jerry1").sayHello()
Person("Jerry2", "Yin2").sayHello()
Person(firstName = "Jerry3").sayHello()
Person(lastName = "Yin4").sayHello()

Person().sayHelloTo()
Person().sayHelloTo("Baby1")
Person().sayHelloTo("Baby2", "Aha2")
Person().sayHelloTo(firstName = "Baby3")
Person().sayHelloTo(lastName = "Aha4")

Person2(lastName = "Yin5").sayHello()
Person2("Jerry6", "Yin6").sayHello()
Person2("", "").sayHelloTo(lastName = "Yin8")
Person2("", "").sayHelloTo("Jerry9", "Yin9")

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