Kotlin learn quick start (5) - Space Security

Introduction

kotlin, the object can be divided into two types, and may be a non-null object is an empty object

Default is non-null object code detecting if a null is found not to impart an empty object, an error will be marked red.

The object may be empty, if you call a method, error code detection is also marked red

var s: String = "hello" //不可为空
s = null//标红报错

var s: String? = "hello"
s=null //代码检测通过
println(s.length)//标红报错

Call may be empty object property or method

The advantage is that, if we have an object must be not empty, then we can call its methods without a null pointer error.

But if we need to call an empty property of the object, how to do it?

Above we know that if a null object calls the method, the code error detection monogram red, so, kotlin in provides several ways for us to call a property or method can be empty object

1. Empty judgment conditions if (xx! = Null)

var s: String? = "hello"
if(s!=null){
    println(s.length)
} 

2. Use safety calls?.

Use ?., if the object is empty, you will get null, but the program will not stop

Support for chained calls when somewhere object is empty, null is returned

var s: String? = "hello"
println(s?.length) //s为空，则返回null，否则返回s.length
println(s?.length+1) //这里代码检测会报错
println(s?.length?.plus(1)) //修改之后的，通过代码检测

3. Elvis operator?:

val date = Expression 1:? 2 expression

If the expression 1 is null, the expression returns 2 content returned

val l: Int = if (b != null) b.length else -1

//相当于上面的代码
//如果b为空，则返回l=-1，b不为空，l=b.length
val l = b?.length ?: -1

4. !!

Object is empty, compile (no error code hints), but run time throws null pointer exception

var s: String = null
pritnln(s!!.length) //运行时候回报错