Kotlin Basics (7) Compound Symbols

1. Compound symbols

1. === !== (third class)

== 是比较值是否相等
=== 是比较对象的地址是否相等

2. !! (assert non-null operator)

!! 比如 var s = foo!! 如果foo不为null 就取出值，如果foo为null 就抛出异常

3. ? (whether the current object can be null)

4. ?. (safety caller)

code:

        if (foo != null) {
    
    
            return foo.bar()
        } else {
    
    
            return null
        }

5. ?: (similar to java judgment)

6. is

		val str = "helloWorld"
        System.out.println("========")
        println(str is String)//输出：true
        System.out.println("========")

Output information:

I/System.out: ========
    true
    ========

7、!is

		val x = 10
        System.out.println("========")
        println(x !is Int)//输出：false
        System.out.println("========")

Output information:

I/System.out: ========
    false
    ========

8. as (type-unsafe converter) , if the conversion is successful, the conversion will be successful, and if the conversion fails, an error will be reported

	private var view1: View? = null
    private var view2: View? = null
    private var view3: View? = null
    private var mListView1: ListView? = null
    private var mListView2: ListView? = null
    private var mListView3: ListView? = null
    ...
    	view1 = inflater.inflate(R.layout.pager_number, null)
        view2 = inflater.inflate(R.layout.pager_number, null)
        view3 = inflater.inflate(R.layout.pager_number, null)
        //as转换成功就转换成功
        mListView1 = view1!!.findViewById<View>(R.id.listview) as ListView
        mListView2 = view2!!.findViewById<View>(R.id.listview) as ListView
        mListView3 = view3!!.findViewById<View>(R.id.listview) as ListView

9. as? (type safety converter) , as? is safer than as, if the conversion is successful, the conversion is successful, and if the conversion fails, it will return null

1. The output is not null

		var name:String = "哈哈哈"
        Log.e("TAG", (name as? String).toString())

Output information:

2. Output null

E/TAG: 哈哈哈

		var name:String = "哈哈哈"
        Log.e("TAG", (name as? Int).toString())

Output information:

E/TAG: null