输入的用户名不会出现在 App 标题后

Question

我正在开发一个简单的待办事项列表应用程序，在欢迎页面上输入您的姓名，当您单击转到您的任务（在任务页面上）时，您刚刚输入的用户名应该出现在顶部屏幕的（例如："App: Username"）但是当我尝试这个时，它在欢迎页面上显示为 "App: null"，在任务页面上显示为 "null: null"。谁能明白为什么？谢谢。

Welcome.java

package winfield.joe.assignment.two; 

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.RatingBar;
import android.widget.RatingBar.OnRatingBarChangeListener;
import android.widget.Toast;
import android.widget.ToggleButton;
import android.app.ProgressDialog;
import android.widget.CheckBox;

public class Welcome extends Activity {

    //global vars
    private CheckBox check;
    private ProgressDialog progress;
    private ToggleButton toggleButton;
    private RatingBar ratingBar;

    private String givenTitle;
    private String username;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_welcome);

        givenTitle = (String) this.getTitle();

        progress = new ProgressDialog(this);

        addListenerOnCheck();
        addListenerOnButton();
        addListenerOnRatingBar();

    }

    //on click CheckBox displays a toast message - CLICK!
    public void addListenerOnCheck() {

        check = (CheckBox) findViewById(R.id.check);
        check.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
               StringBuffer result = new StringBuffer();
               result.append("CLICK!");
               Toast.makeText(Welcome.this, result.toString(), Toast.LENGTH_SHORT).show();
            }

        });

    }

    //on click toggle button displays on/off
    public void addListenerOnButton() {

        toggleButton = (ToggleButton) findViewById(R.id.toggleButton);
        toggleButton.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
               StringBuffer result = new StringBuffer();
               result.append("Toggle = ").append(toggleButton.getText());          
               Toast.makeText(Welcome.this, result.toString(), Toast.LENGTH_SHORT).show();   
            }

        });

    }

    //Give a rating - Toast appears of star rating
    public void addListenerOnRatingBar() {

        ratingBar = (RatingBar) findViewById(R.id.ratingBar);
        //if rating value has changed, display the current rating on a toast
        ratingBar.setOnRatingBarChangeListener(new OnRatingBarChangeListener() {

            public void onRatingChanged(RatingBar ratingBar, float rating, boolean fromUser) {  
                Toast.makeText(Welcome.this, String.valueOf(ratingBar.getRating()), Toast.LENGTH_SHORT).show();
            }

        });

    }

    //Onclick - go to About
    public void aboutMe(View view) {

        Intent intent = new Intent(Welcome.this, About.class);

        startActivity(intent);

    }

    //Onclick - go to Tasks
    public void doStuff(View view) {

        this.setTitle(givenTitle + ": " + username);

        Intent intent = new Intent(Welcome.this, Tasks.class);
        startActivity(intent);

        //ProgressBarDialog appears saying its finding yours tasks and loads to 100 and changes activity
        progress.setMessage("Finding tasks...");
        progress.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        progress.setIndeterminate(true);
        progress.show();

        //Goes to 100
        final int totalProgressTime = 100;

        final Thread t = new Thread(){

           @Override
           public void run(){

              //Goes from 0 to 100 in jumps of 5 until the total amount of jumps reaches 100/total progress time
              int jumpTime = 0;
              while(jumpTime < totalProgressTime){
                 try {
                    Thread.sleep(200);
                    jumpTime += 5;
                    progress.setProgress(jumpTime);
                 } catch (InterruptedException e) {
                   // TODO Auto-generated catch block
                   e.printStackTrace();
                 }
              }

           }

       }; t.start();

    }

}

Tasks.java

package winfield.joe.assignment.two;

import android.app.AlertDialog;
import android.app.ListActivity;
import android.content.ContentValues;
import android.content.DialogInterface;
import android.content.Intent;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.*;
import winfield.joe.assignment.two.database.TaskContract;
import winfield.joe.assignment.two.database.TaskDBHelper;

public class Tasks extends ListActivity {

    //add required global variables
    private ListAdapter listAdapter;
    private TaskDBHelper helper;

    private String givenTitle;
    private String username;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_tasks);
        updateUI();

        this.setTitle(givenTitle + ": " + username);

    }

    //add in the menu (refers to menu.xml file)
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu,menu);
        return true;
    }

    //to get selected tasks from menu
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.action_add_task:
                //create new alert dialog which asks you if you want to make a new task entry
                AlertDialog.Builder builder = new AlertDialog.Builder(this);
                builder.setTitle("Add a new task");
                builder.setMessage("What do you need to do?");
                final EditText inputField = new EditText(this);
                builder.setView(inputField);
                //if 'add' then place it into the db, update it
                builder.setPositiveButton("Add", new DialogInterface.OnClickListener() {

                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        //Clicking add - stores data in the database
                        String task = inputField.getText().toString();
                        helper = new TaskDBHelper(Tasks.this);
                        SQLiteDatabase db = helper.getWritableDatabase();
                        ContentValues values = new ContentValues();
                        values.clear();
                        values.put(TaskContract.Columns.TASK,task);
                        db.insertWithOnConflict(TaskContract.TABLE,null,values,SQLiteDatabase.CONFLICT_IGNORE);
                        updateUI();
                    }
                });

                //quits out of alert dialog and goes back to the task list if 'cancel'
                builder.setNegativeButton("Cancel",null);

                builder.create().show();
                return true;

            default:
                return false;
        }
    }

    //Show database entries on the tasks page
    private void updateUI() {
        helper = new TaskDBHelper(Tasks.this);
        SQLiteDatabase sqlDB = helper.getReadableDatabase();
        Cursor cursor = sqlDB.query(TaskContract.TABLE, new String[]{
                TaskContract.Columns._ID, TaskContract.Columns.TASK
        },
                null, null, null, null, null);
        listAdapter = new SimpleCursorAdapter(
                this,
                R.layout.task_view,
                cursor,
                new String[]{TaskContract.Columns.TASK},
                new int[]{R.id.taskTextView},
                0
        );
        this.setListAdapter(listAdapter);
    }

    //Deletes tasks (clicking done next to them)
    public void onDoneButtonClick(View view) {
        View v = (View) view.getParent();
        TextView taskTextView = (TextView) v.findViewById(R.id.taskTextView);
        String task = taskTextView.getText().toString();
        //This query deletes the entry associated with the id/button pressed
        String sql = String.format("DELETE FROM %s WHERE %s = '%s'",
                        TaskContract.TABLE,
                        TaskContract.Columns.TASK,
                        task);

        helper = new TaskDBHelper(Tasks.this);
        SQLiteDatabase sqlDB = helper.getWritableDatabase();
        sqlDB.execSQL(sql);
        updateUI();
    }

    //onClick finished method assigned to the 'Finished' button
    public void finished(View view) {

            //Make alert dialog box appear and give it it's title
            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setTitle(getString(R.string.alert));

                // set dialog message
                builder.setCancelable(false)

                        //Positive response = return to list (tasks page)
                        .setPositiveButton(getString(R.string.positive),new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int id) {
                                //You return to the Tasks 
                                dialog.cancel();
                            }
                        })

                        //Neutral response = go to about (about page)
                        .setNeutralButton(getString(R.string.neutral),new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int id) {
                                //You go to About Activity  
                                Intent intent = new Intent(Tasks.this, About.class);
                                startActivity(intent);
                            }
                        })

                        //Negative response = exit app (homescreen)
                        .setNegativeButton(getString(R.string.negative),new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int id) {
                                // if this button is clicked, exit app to homescreen
                                Intent intent = new Intent(Intent.ACTION_MAIN);
                                intent.addCategory(Intent.CATEGORY_HOME);
                                startActivity(intent);
                            }
                        });

            //create it and show it
            AlertDialog alert = builder.create();
            alert.show();

        }

}

用户名 activity_welcome.xml

<EditText
    android:ems="10"
    android:hint="@+string/username"
    android:id="@+id/username"
    android:inputType="text"
    android:layout_height="wrap_content"
    android:layout_marginEnd="30dp"
    android:layout_marginStart="30dp"
    android:layout_marginTop="30dp"
    android:layout_width="match_parent">

    <requestFocus/>
</EditText>

来自strings.xml

的相关字符串

<string name="app_name">ForYouToDo</string>
<string name="username">Username</string>

Answer 1

转到Android - Pass Data Between Two Activities get NullPointerException

完整指南(How to pass data between two activity?)

并在你的 Tasks Activity

中做

this.setTitle(getResources().getString(R.string.app_name)+ ": " + username);

输入的用户名不会出现在 App 标题后

Entered Username won't appear after App title

java

android

android-intent

android-edittext