java - arrayList 似乎仅在先完成一个 Activity (而不是另一个 Activity )时才会创建

Question

我在为学校类(class)编写的这个应用程序中遇到问题。这是一个基本的锻炼跟踪应用程序。

我有一个 arrayList，但 Cardio.java Activity 只能保存到 arrayList(如果 Strength.java 已经先保存了某些内容)。不知道为什么，因为两者的代码是相同的。

我已经仔细检查过这两个 Activity 的代码是否相同，所以不确定为什么它似乎只在 strenght.java 首先保存到 arrayList 中。然后就可以正常工作了。

Cardio.java

public class Cardio extends AppCompatActivity {

    String cardio;
    String miles;
    String time;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_cardio);

        Button saveButton = (Button) findViewById(R.id.btnSave);
        saveButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DatabaseHelper dbUtil = new DatabaseHelper();

                Spinner cardioSpinner = findViewById(R.id.listCardioType);
                cardio = cardioSpinner.getSelectedItem().toString();
                EditText milesText = findViewById(R.id.txtMileageInput);
                miles = milesText.getText().toString();
                EditText timeText = findViewById(R.id.txtTimeInput);
                time = timeText.getText().toString();

                String record = cardio + ": " + miles + " miles, " + time + " mins.";

                Log.d("test", record);

                ArrayList strengthWorkouts = dbUtil.getArrayList("strength_workouts");
                Log.d("test", strengthWorkouts.toString());
                strengthWorkouts.add(record);
                dbUtil.saveArrayList(strengthWorkouts, "strength_workouts");

                Toast toast = Toast.makeText(Cardio.this, "Workout Saved", Toast.LENGTH_LONG);
                toast.setGravity(0, 0, -200);
                toast.show();
            }


        });

        Button shareButton = (Button) findViewById(R.id.btnShare);
        shareButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent myIntent = new Intent(Intent.ACTION_SEND);
                myIntent.setType("text/plain");
                String shareBody = "I just completed a workout! " + cardio + ": " + miles + " miles, " + time + " mins.";
                myIntent.putExtra(Intent.EXTRA_SUBJECT, shareBody);
                myIntent.putExtra(Intent.EXTRA_TEXT, shareBody);
                startActivity(Intent.createChooser(myIntent, "Share Using:"));

            }
        });

    }
}

<小时/>

强度.java

public class Strength extends AppCompatActivity {

    String exercise;
    String repCount;
    String weight;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_strength);

        Button saveButton = findViewById(R.id.btnSave);
        saveButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DatabaseHelper dbUtil = new DatabaseHelper();

                Spinner exerciseSpinner = findViewById(R.id.listWeightType);
                exercise = exerciseSpinner.getSelectedItem().toString();
                EditText repCountText = findViewById(R.id.txtRepInput);
                repCount = repCountText.getText().toString();
                EditText weightText = findViewById(R.id.txtWeightInput);
                weight = weightText.getText().toString();

                String record = exercise + ": " + repCount + " reps, " + weight + " lbs.";

                Log.d("test", record);

                ArrayList strengthWorkouts = dbUtil.getArrayList("strength_workouts");
                Log.d("test", strengthWorkouts.toString());
                strengthWorkouts.add(record);
                dbUtil.saveArrayList(strengthWorkouts, "strength_workouts");

                Toast toast = Toast.makeText(Strength.this, "Workout Saved", Toast.LENGTH_LONG);
                toast.setGravity(0, 0, -200);
                toast.show();

            }
        });

        Button shareButton = (Button) findViewById(R.id.btnShare);
        shareButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent myIntent = new Intent(Intent.ACTION_SEND);
                myIntent.setType("text/plain");
                String shareBody = "I just completed a workout! " + exercise + ": " + repCount + " reps, " + weight + " lbs.";
                myIntent.putExtra(Intent.EXTRA_SUBJECT, shareBody);
                myIntent.putExtra(Intent.EXTRA_TEXT, shareBody);
                startActivity(Intent.createChooser(myIntent, "Share Using:"));

            }
        });


    }
}

<小时/>

历史记录.java

public class History extends AppCompatActivity {

    private static final String TAG = "ListDataActivity";

    DatabaseHelper mDatabaseHelper;

    private ListView mListView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_history);

        mListView = findViewById(R.id.listView);
        DatabaseHelper dbUtils = new DatabaseHelper();
        ArrayList<String> strengthHistory = dbUtils.getArrayList("strength_workouts");


        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(
                this,
                android.R.layout.simple_list_item_1,
                strengthHistory );

        mListView.setAdapter(arrayAdapter);
    }
}

Answer 1

如果您的数据库中没有“strength_workouts”的数据，我认为您的 dbUtils.getArrayList("strength_workouts") 返回 null ArrayList。

如果您的 Activity 中列表为空，请尝试创建该列表:

ArrayList strengthWorkouts = dbUtil.getArrayList("strength_workouts");

if(strengthHistory == null) {
   strengthHistory = new ArrayList<String>();
}

strengthWorkouts.add(record);
dbUtil.saveArrayList(strengthWorkouts, "strength_workouts");