Android Studio(锻炼应用程序)中的意图错误

Question

对于我的学校项目，我目前正在进行简单的锻炼应用程序。但是我陷入了倒计时计时器上，因为应用程序不断崩溃。首先，有一些受欢迎的屏幕；然后，您继续进行自己的锻炼（选择锻炼，休息和回合数）。它运行良好，甚至可以使用意图将所有这些变量进入下一阶级。这是按钮下一个代码：

    next.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Intent myIntent = new Intent(CustomWorkout.this, GetReady.class);
            myIntent.putExtra("all_exercises", Exercises); //arraylist of exercises
            myIntent.putExtra("total_rounds", total_rounds); //number of rounds
            myIntent.putExtra("total_rest", total_rest); //rest time
            startActivity(myIntent);
        }
    });

如您所见，我然后调用 GetReady（）类，该类是简单的5秒倒计时计时器，以准备自己进行首次锻炼。 GetReady 看起来像这样：

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.getready);
    getReady();
}

public void getReady(){
    timer = (TextView) findViewById(R.id.timer);
    cTimer= new CountDownTimer(5000, 1000) {
        public void onTick(long millisUntilFinished) {
            timer.setText("" + millisUntilFinished / 1000);
        }
        public void onFinish() {
            startActivity(new Intent(GetReady.this, Exercises.class));
        }
    }.start();
}

现在遇到了错误（以及应用程序崩溃）。在倒计时完成时，我调用 corest（） class（这是带有当前练习和按钮的名称的屏幕 ->单击时，您再次开始倒计时计时器）。代码：

Intent i = getIntent();
    all_exercises = (ArrayList<String>) i.getSerializableExtra("all_exercises"); //getting exercises from Intent set in first class
    rounds=(int)i.getSerializableExtra("total_rounds");
    exercise_name = (TextView) findViewById(R.id.exercise_name);
    exercise_name.setText(all_exercises.get(0));
    final Button done=(Button) findViewById(R.id.done);
    done.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Intent in = new Intent(Exercises.this, Countdown.class); //starting Countdown
            startActivity(in);
        }
    });

正常倒计时，然后发生崩溃。如果我在头等舱中设置该按钮应该调用练习类，而不是准备就绪，它不会崩溃（但它在倒计时类中崩溃）。

崩溃日志：

FATAL EXCEPTION: main
   Process: com.msimonic.worldofworkout, PID: 8968
   java.lang.RuntimeException: Unable to start activity ComponentInfo{com.msimonic.worldofworkout/com.msimonic.worldofworkout.Exercises}: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
   at android.app.ActivityThread.-wrap11(ActivityThread.java)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:148)
   at android.app.ActivityThread.main(ActivityThread.java:5417)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
   at com.msimonic.worldofworkout.Exercises.onCreate(Exercises.java:39)
   at android.app.Activity.performCreate(Activity.java:6237)
   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
   at android.app.ActivityThread.-wrap11(ActivityThread.java) 
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
   at android.os.Handler.dispatchMessage(Handler.java:102) 
   at android.os.Looper.loop(Looper.java:148) 
   at android.app.ActivityThread.main(ActivityThread.java:5417)

Answer 1

您必须在 getready 中传递数据，以便可以在练习中访问它们。为了少写代码，我将它们放入Bundle中，然后简单地转移此Bundle：

在OnClickListener中：

next.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
        Intent myIntent = new Intent(CustomWorkout.this, GetReady.class);
        Bundle b = new Bundle();
        b.putSerializable("all_exercises", Exercises); //arraylist of exercises
        b.putInt("total_rounds", total_rounds); //number of rounds
        b.putInt("total_rest", total_rest); //rest time
        myIntent.putExtra("myData", b);
        startActivity(myIntent);
    }
});

in getready.java ：

public void getReady(){
    timer = (TextView) findViewById(R.id.timer);
    cTimer= new CountDownTimer(5000, 1000) {
        public void onTick(long millisUntilFinished) {
            timer.setText("" + millisUntilFinished / 1000);
        }
        public void onFinish() {
            Bundle b = GetReady.this.getIntent().getBundleExtra("myData");
            Intent myNextIntent = new Intent(GetReady.this, Exercises.class);
            myNextIntent.putExtra("myDataFromGetReady", b); 
            startActivity(myNextIntent);
        }
    }.start();
}

in excercises.java ：

Intent i = getIntent();
Bundle b = i.getBundleExtra("myDataFromGetReady"); 
// now get the data from the Bundle: 
all_exercises = (ArrayList<String>) b.getSerializable("all_exercises"); 
rounds= b.getInt("total_rounds", -1);
exercise_name = (TextView) findViewById(R.id.exercise_name);
exercise_name.setText(all_exercises.get(0));
final Button done=(Button) findViewById(R.id.done);
// ... and so on ...