Android Kotlin学习（四）- NULL空安全_kotling 不能为null !!

Java出现最多的异常不知道是不是空指针异常，Kotlin的空安全避免了空指针异常导致程序崩溃

？和!!

1. “?"来做安全调用，如果对象为空，结果也为null

2. “!!”对象为null，回像JAVA一样报出空指针异常

    // test可以为null
    var test:String?=null;
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            binding = DataBindingUtil.setContentView(this,R.layout.activity_main);
            startTest();
        }
   
    private fun startTest() {
    	// 这样写编译会报错，因为test可能为null
   	//Log.i(TAG,"test = "+test.length);
   	// 这样写编译通过，会返回null
   	Log.i(TAG,"test = "+test?.length);
        //这样也会返回null
        Log.i(TAG,"test = "+test);
        //test null时，会抛出空指针异常
        Log.i(TAG,"test = "+test!!);
        test = "Hello";
        Log.i(TAG,"test1 = "+test);
    }
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打印日志如下：

2023-02-27 16:34:00.383 21084-21084/com.kotlin.demo I/MainActivity: test = null
2023-02-27 16:34:00.383 21084-21084/com.kotlin.demo D/AndroidRuntime: Shutting down VM
2023-02-27 16:34:00.388 21084-21084/com.kotlin.demo E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.kotlin.demo, PID: 21084
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.kotlin.demo/com.kotlin.demo.MainActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
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Elvis

果变量不为null的话，我们使用变量，否则，我们使用另外一个值

Log.i(TAG,"test = "+(test?.length?:-1));
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打印：

2023-02-27 17:01:13.186 21748-21748/com.kotlin.demo I/MainActivity: test = -1
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let

当一个变量可能为null时，可以通过let函数来处理，null时则不会被执行：

	Log.i(TAG,"test = "+(test?.length?:-1));
    test?.let {
         //此时test null，所以不会执行
        Log.i(TAG,"test == "+test?.length);
    }
    test = "Hello";
    test?.let {
        Log.i(TAG,"test === "+test?.length);
    }
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日志：

2023-02-27 17:27:29.143 22158-22158/com.kotlin.demo I/MainActivity: test = -1
2023-02-27 17:27:29.143 22158-22158/com.kotlin.demo I/MainActivity: test === 5
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2

lateinit

lateinit延时初始化，默认非null，使用时要保证不为null，否则报错：

	lateinit var test:String;
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this,R.layout.activity_main);
        startTest();
    }

    private fun startTest() {
        Log.i(TAG,"test = "+test);
        Log.i(TAG,"test = "+test!!);
        test = "Hello";
        Log.i(TAG,"test1 = "+test);
    }
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报错：

    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.kotlin.demo/com.kotlin.demo.MainActivity}: kotlin.UninitializedPropertyAccessException: lateinit property test has not been initialized
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
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