java - 使用共享首选项在 EditText 中保存数字

Question

我是 Android 编程新手，我创建了一个简单的计算器。我希望用户能够暂停应用程序，而不会丢失在两个 EditText 框中输入的数字。我正在尝试使用共享首选项，但由于某种原因我的应用程序不断崩溃。我该如何解决这个问题？

package com.example.simplecalculator;

import android.os.Bundle;


import android.app.Activity;
import android.content.SharedPreferences;
import android.view.Menu;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import com.example.simplecalculator.*;


public class MainActivity extends Activity {

SharedPreferences mySharedPreferences;
SharedPreferences.Editor editor;
public static final String MYPREFERENCES = "MyPreferences_001";
float answer;
final EditText numberone = (EditText)findViewById(R.id.number1);
final EditText numbertwo = (EditText)findViewById(R.id.number2);

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

    mySharedPreferences = getSharedPreferences(MYPREFERENCES, 0);
    String number1 = mySharedPreferences.getString("number1", null);
    String number2 = mySharedPreferences.getString("number2", null);
    numberone.setText(number1); 
    numbertwo.setText(number2);



}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

public void Addition(View view) {

answer = Float.parseFloat(numberone.getText().toString()) + Float.parseFloat(numbertwo.getText().toString());;

TextView value = (TextView)findViewById(R.id.answer);
value.setText("Your sum is " + answer);


}

public void Subtraction(View view) {

answer = Float.parseFloat(numberone.getText().toString()) - Float.parseFloat(numbertwo.getText().toString());;

TextView value = (TextView)findViewById(R.id.answer);
value.setText("Your difference is " + answer);

}


public void Multiplication(View view) {
    answer = Float.parseFloat(numberone.getText().toString()) * Float.parseFloat(numbertwo.getText().toString());;

TextView value = (TextView)findViewById(R.id.answer);
value.setText("Your product is " + answer);

}

public void Division(View view) {

    answer = Float.parseFloat(numberone.getText().toString()) / Float.parseFloat(numbertwo.getText().toString());;

TextView value = (TextView)findViewById(R.id.answer);
value.setText("Your quotient is " + answer);

}

public void Power(View view) {
    answer = (float) Math.pow(Float.parseFloat(numberone.getText().toString()), Float.parseFloat(numbertwo.getText().toString()));

TextView value = (TextView)findViewById(R.id.answer);
value.setText("Your answer is " + answer);

}

public void Root(View view) {

    answer = (float) Math.sqrt(Float.parseFloat(numberone.getText().toString()));

TextView value = (TextView)findViewById(R.id.answer);
value.setText("Your answer is " + answer);

}


public void onPause(){

    mySharedPreferences = getSharedPreferences(MYPREFERENCES, Activity.MODE_PRIVATE);
    editor = mySharedPreferences.edit();
    editor.putString("number1", numberone.getText().toString());
    editor.putString("number2", numbertwo.getText().toString());
    editor.commit();

}

}

Answer 1

final EditText numberone = (EditText)findViewById(R.id.number1);
final EditText numbertwo = (EditText)findViewById(R.id.number2);  

将其保留在 onCreate() 内，在 setContentView() 之后

此外，当sharedPref中没有存储任何内容时，第一次会发生什么，

String number1 = mySharedPreferences.getString("number1", null);
    String number2 = mySharedPreferences.getString("number2", null);  

然后返回一些值而不是“null”。

如果在计算器应用程序中经常使用 SharedPref，我建议创建一个用于获取和放置 SharedPref 的函数..
类似于:

public static void saveDataToPreferences(String key,
            String value) {
        SharedPreferences prefs = context.getSharedPreferences("your package name",
                Context.MODE_PRIVATE);
        Editor editor = prefs.edit();
        editor.putString(key, value);
        editor.commit();
    }
    public static String getDataFromPreferences(String key) {

        SharedPreferences prefs = context.getSharedPreferences("your package name",
                Context.MODE_PRIVATE);
        return prefs.getString(key, Constants.BLANK);
    }

如果函数位于您的 Activity 内部，则可以全局声明该 Activity 的上下文。否则将其传递到参数中。