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，您输入您的姓名，当您单击以转到任务时（在任务页面上），您刚输入的用户名应显示在顶部屏幕（例如：＆＃34;应用程序：用户名＆＃34;）但是当我尝试这个时它显示为＆＃34; App：null＆＃34;在欢迎页面和＆＃34; null：null＆＃34;在任务页面上。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();
}
}
activity_welcome.xml中的用户名
<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>
strings.xml中的相关字符串
<string name="app_name">ForYouToDo</string>
<string name="username">Username</string>
答案 0 :(得分：2)
转到Android - Pass Data Between Two Activities get NullPointerException
完整指南(How to pass data between two activity?)
并在您的Tasks Activity
this.setTitle(getResources().getString(R.string.app_name)+ ": " + username);