每次开始新活动时都会覆盖 ArrayList 共享首选项

Question

我有一个字符串数组列表，MainActivity 读取字符串，然后将其转换为 Gson，然后应用程序继续执行下一个活动（history.class 活动），然后通过检索它打印出数组列表。我已经实现了一个返回到 MainActivity 的按钮，但是每次我返回到 MainActivity 时，arraylist 都会被覆盖，有没有办法克服这个问题？另外，我不想使用close()或finish()作为返回MainActivity的方式。

MainActivity.class

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final ArrayList<String> arrPackage;
        setContentView(R.layout.activity_main);
        final SharedPreferences sharedPreferences = getSharedPreferences("USER",MODE_PRIVATE);
        final EditText address = findViewById(R.id.address);
        final TextView result = findViewById(R.id.result);
        Button save = findViewById(R.id.button);

        arrPackage = new ArrayList<>();
        final Intent next = new Intent(this,history.class);
        save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(address.getText().toString().isEmpty()) {
                    Toast.makeText(MainActivity.this,"Plz Enter all the data",Toast.LENGTH_LONG).show();
                }else{
                    String addressData = address.getText().toString().trim();
                    arrPackage.add(addressData);
                    Gson gson = new Gson();
                    String json = gson.toJson(arrPackage);
                    SharedPreferences.Editor editor = sharedPreferences.edit();
                    editor.putString("Set",json );
                    editor.commit();
                }
                startActivity(next);
            }
        });
    }
}

History.class

public class history extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_history);
        Button back = findViewById(R.id.back_btn);
        TextView result = findViewById(R.id.tv_history);
        final Intent back_activity = new Intent(this,MainActivity.class);
        final SharedPreferences sharedPreferences = getSharedPreferences("USER",MODE_PRIVATE);

        Gson gson = new Gson();
        String json = sharedPreferences.getString("Set", "");
        if (json.isEmpty()) {
            Toast.makeText(history.this,"There is something error",Toast.LENGTH_LONG).show();
        } else {
            Type type = new TypeToken<List<String>>() {
            }.getType();
            List<String> arrPackageData = gson.fromJson(json, type);
            StringBuilder builder = new StringBuilder();

            for(String data:arrPackageData) {
                builder.append(data + "\n");
            }
            result.setText(builder.toString());

        }
        back.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(back_activity);
            }
        });
    }
}

Answer 1

在您的 Activity 条目内的 AndroidManifest.xml 中添加这一行：

android:launchMode="singleTask"

例如：

<activity
        android:name="your_package.MainActivity"
        android:label="@string/app_name"
        android:launchMode="singleTask">
</activity>

这样，您的 MainActivity 将成为单实例 Activity，当您为其调用 startActivity 时，已经创建的 Activity 将出现在前台。

Answer 2

arrPackage = new ArrayList<>();

这一行会覆盖你的arrayList。要保存您的arrayList，您需要先检查首选项列表中是否有数据保存为

Gson gson = new Gson();
    String json = sharedPreferences.getString("Set", "");

    if (json.isEmpty()) {
        // runs when you app dont have data in shared preference initalize your arrayList here
        arrPackage = new ArrayList<>();
    } else {
        // when your app already stored list then read from the preference.
        Type type = new TypeToken<List<String>>() {
        }.getType();
        arrPackage  = gson.fromJson(json, type);
    }