Question

我正在尝试从上一个视图中设置的前一个类访问整数ArrayList，但是当我转到新视图时它似乎没有正确保存arrayList我认为它删除了那里的任何东西？

是否有访问已经设置的“lightInfo”？

ConnectTest声明

public class ConnectTest extends MainActivity {
    TextView out;
    private static final int REQUEST_ENABLE_BT = 1;
    private BluetoothAdapter btAdapter = null;
    private BluetoothSocket btSocket = null;
    private OutputStream outStream = null;

    // Well known SPP UUID
    private static final UUID MY_UUID =
            UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

    // Insert your server's MAC address
    private static String address = "A4:5E:60:EB:3C:E8";
    ArrayList<Integer> temp = ((MainActivity) getApplicationContext()).lightInfo;

调用创建错误的temp：

 String finalMessage = "";
        for (int i = 0; i < temp.size(); i++) {
            finalMessage = finalMessage + temp.get(i) + "\n";
        }

        String message = finalMessage;

MainActivity声明

public class MainActivity extends AppCompatActivity {


    private final static int REQUEST_ENABLE_BT = 1;
    UUID myUUID = UUID.randomUUID();
    private static final int DISCOVER_DURATION = 300;
    private static final int REQUEST_BLU = 1;
    ArrayAdapter<String> mArrayAdapter;
    ArrayList<Double> currentBodyTemp;
    BluetoothAdapter mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
    boolean targetTimeZoneIsAhead = false;
    boolean targetTimeZoneIsBehind = false;

    int currentWakeTime = 420;
    int currentSleepTime = 1320;

    int targetWakeTime = 0;
    int targetSleepTime = 0;
    int daysLeft = 0;
    int timeDifference = 0;

    public ArrayList<Integer> lightInfo;

错误讯息：

10-27 12:59:56.141 15803-15803/com.teamfara.circadianrhythmmonitor4 D/AndroidRuntime: Shutting down VM
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime: FATAL EXCEPTION: main
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime: Process: com.teamfara.circadianrhythmmonitor4, PID: 15803
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime: java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.teamfara.circadianrhythmmonitor4/com.teamfara.circadianrhythmmonitor4.ConnectTest}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2964)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3218)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.access$1000(ActivityThread.java:198)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1676)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:145)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:6837)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:372)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:111)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at com.teamfara.circadianrhythmmonitor4.ConnectTest.<init>(ConnectTest.java:34)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at java.lang.reflect.Constructor.newInstance(Native Method)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at java.lang.Class.newInstance(Class.java:1684)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.Instrumentation.newActivity(Instrumentation.java:1080)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2954)
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3218) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.access$1000(ActivityThread.java:198) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1676) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:145) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:6837) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:372) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) 
10-27 12:59:56.151 15803-15803/com.teamfara.circadianrhythmmonitor4 E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)

谢谢你！

Answer 1

您不能简单地对任何对象进行类型转换并期望它能够正常工作。请使用正确的数据传递机制。试试这个：

Passing data through Intent and receiving it

Answer 2

您的应用程序崩溃了，因为您正在尝试访问活动全局范围内的getApplicationContext，而后者未经历其生命周期。由于您的ConnectTest扩展了MainActivity，因此您无需任何演员即可访问MainActivity的任何 非私有 成员。您只需标记为public或protected。如果两个类在同一个包中，那么即使是默认访问说明符也是如此。看看here

在其他视图中无法访问ArrayList？ android null指针异常

2 个答案: