尝试在空对象引用上调用接口方法'android.content.SharedPreferences$Editor.putString(java.lang.String, java.lang.Stri

Question

我试图保存首选项，所以当用户输入名称并单击保存时，它会释放一个警告对话框，如果他们说是，它会将他们带到欢迎页面。然而，如果他们回到第一页，他们不需要再次输入名称，它只会显示(WELCOME, USER)。然而，它给了我一个错误，一个空异常错误。请帮助!此外，我需要的应用程序来保存用户名，所以当用户点击后退按钮，然后去到第一页尝试再次输入他们的名字，它只是把他们推到欢迎页面，因为名字已经类型之前我将只添加与问题相关的代码片段。

这是用户输入name的页面;

  import android.app.Activity;
    import android.app.AlertDialog.Builder;
    import android.content.Context;
    import android.content.DialogInterface;
    import android.content.Intent;
    import android.content.SharedPreferences;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.app.AlertDialog;
    import android.widget.EditText;
    public class userPreferences extends Activity {
    EditText editText;
    Button save;
    public static final String NAME="UserName";
    SharedPreferences sharedPreferences;
    SharedPreferences.Editor editor;
    String n;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.userpreferences);
        editText=(EditText)findViewById(R.id.editText);
        save=(Button)findViewById(R.id.button);
        sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
      save.setOnClickListener(new View.OnClickListener() {
          final AlertDialog.Builder alertDialog = new AlertDialog.Builder(userPreferences.this);
          @Override
          public void onClick(View arg0) {
                      alertDialog.setTitle("Alert Dialog");
              alertDialog.setMessage("Are you sure you want to save?");
              alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                          @Override
                          public void onClick(DialogInterface dialog, int which) {
                              n=editText.getText().toString();
                              sharedPreferences=getPreferences(MODE_PRIVATE);
                              editor=sharedPreferences.edit();
                              editor.putString(NAME,n);
                              editor.apply();
                              startActivity(new Intent(userPreferences.this,Welcome.class));
                          }
                      });
                      alertDialog.setNegativeButton("No", null);
                      alertDialog.show();

          }
      });
    }

}

这是欢迎页面;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
public class Welcome extends Activity {
TextView textView;
SharedPreferences sharedPreferences;
public static final String NAME="UserName";
SharedPreferences.Editor editor;
String n;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.welcome);
    textView=(TextView)findViewById(R.id.textView);
    textView.setText("Welcome," + editor.putString(NAME, n));
    sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
    editor.apply();
}
@Override
public void onBackPressed() {
    super.onBackPressed();
    startActivity(new Intent(Welcome.this, MainActivity.class));
    return;

}

逻辑读取错误

Attempt to invoke interface method 'android.content.SharedPreferences$Editor android.content.SharedPreferences$Editor.putString(java.lang.String, java.lang.String)' on a null object reference

请帮忙!

Answer 1

修改

sharedPreferences=getPreferences(MODE_PRIVATE);

sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
if(!sharedPreferences.getString(NAME,"Default value").equals("Default value")){
  startActivity(new Intent(userPreferences.this, Welcome.class));
  finish();
}

也在欢迎课中

sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
textView=(TextView)findViewById(R.id.textView);
textView.setText("Welcome," + sharedPreferences.getString(NAME,"Default value"));
editor = = sharedPreferences.edit();
editor.putString(NAME, sharedPreferences.getString(NAME,"Default value"));
editor.apply();

Answer 2

try:

// open your preferences using the name you originally used to create it
sharedPreferences = getSharedPreferences(NAME, MODE_PRIVATE);
// open for editing
editor = sharedPreferences.edit();
// you probably don't want to use the same name here as preferences name 
editor.putString(NAME,n);
editor.apply();

另外，您正在尝试使用编辑器。putString(名称、n);这不会从首选项中读取:

sharedPreferences = getSharedPreferences(NAME, MODE_PRIVATE);
String name = sharedPreferences.getString(NAME);
textView.setText("Welcome," + name);

为了保存这个用户名以备下次使用，您必须调用编辑器并在再次读取它之后保存它，在一个空对象上调用调用editor.apply()。这是您的onCreate()的完整工作样本:

@Override
protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.welcome);
     textView=(TextView)findViewById(R.id.textView);
     sharedPreferences = getSharedPreferences(NAME, MODE_PRIVATE);
     String name = sharedPreferences.getString(NAME);
     textView.setText("Welcome," + name);
     editor = sharedPreferences.edit();
     editor.putString(NAME, name);
     editor.apply();
}

Answer 3

问题在于，首先，您没有在Welcome类中初始化editor对象，因此它是空的。

但是，即使您初始化它，您也不会得到所需的行为，因为编辑器用于编辑SharedPreferences，而不能用于检索首选项。editor.putString(NAME, n)存储一个字符串并返回一个布尔值，它没有得到用户的名字。它也是空的，因为你没有初始化它。

您甚至不需要在Welcome类中使用Editor对象，相反，您需要使用sharedPreferences变量，如下所示:

TextView textView;
SharedPreferences sharedPreferences;
public static final String NAME = "UserName";
public static final String defaultName = "";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.welcome);
    textView = (TextView) findViewById(R.id.textView);
    sharedPreferences = getSharedPreferences(NAME, Context.MODE_PRIVATE);
    textView.setText("Welcome," + sharedPreferences.getString(NAME, defaultName));
}

编辑:如果你想跳过登录活动，如果用户已经输入了一个用户名，那么你会检查他们是否有，然后直接从你的登录活动启动该活动。

EditText editText;
Button save;
public static final String NAME="UserName";
public static final String default = "";
SharedPreferences sharedPreferences;
SharedPreferences.Editor editor;
String n;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.userpreferences);
    editText = (EditText) findViewById(R.id.editText);
    save = (Button) findViewById(R.id.button);
    sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
    if (sharedPreferences.getString(NAME, default).length() > 0) {
        // The user has already entered a username, so start the Welcome activity
        startActivity(new Intent(userPreferences.this, Welcome.class));
        finish();
    }
    // put the rest of your onCreate code here
}