我的应用有一个主要活动，只是由不同的片段组成。在一个片段中，在我从服务器获取JSON值并尝试在复选框上设置复选标记后，应用程序崩溃并且我收到了
java.lang.NullPointerException：尝试调用虚方法'void 空对象上的android.widget.CheckBox.setChecked（boolean）' 参考
错误我正在尝试从复选框值所在的不同片段设置复选框值。
主要活动：
if(v == findViewById(R.id.login)){ //between fragments: from login screen to home page
username_text = (EditText) findViewById(R.id.username_text);
password_text = (EditText) findViewById(R.id.password_text);
RequestQueue requestQueue = Volley.newRequestQueue(this);;
String signin = "http://acarjsystem.herokuapp.com/auth/";
StringRequest request;
uname = username_text.getText().toString();
pword = password_text.getText().toString();
final ProgressDialog loading = new ProgressDialog(MainActivity.this);
loading.setTitle("Loading");
loading.setMessage("Attempting to Log In");
loading.show();
////////////////////
request = new StringRequest(Request.Method.POST, signin, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
final Fragment nextFrag;
final CheckBox check_locateCar, check_carOff,check_getImg, check_tase, check_carOn;
try {
check_locateCar = (CheckBox)findViewById(R.id.check_locateCar);
check_carOff = (CheckBox)findViewById(R.id.check_carOff);
check_getImg = (CheckBox)findViewById(R.id.check_getImg);
check_tase = (CheckBox)findViewById(R.id.check_tase);
check_carOn = (CheckBox)findViewById(R.id.check_carOn);
JSONObject jsonObject = new JSONObject(response);
if(jsonObject.getString("user").equals("logined")){
Toast.makeText(getApplicationContext(),"SUCCESS "+jsonObject.getString("user"),Toast.LENGTH_SHORT).show();
//startActivity(new Intent(getApplicationContext(),Welcome.class));
if(!jsonObject.getString("car_plate").equals("0")){
loading.hide();
if(jsonObject.getString("car_stat").equalsIgnoreCase("true")){
//////////////HERE//////////////////////////////////
if(jsonObject.getString("loc_stat").equalsIgnoreCase("true")){
/*THE ERROR IS AROUND THIS LINE*/
check_locateCar.setChecked(true);
}else{
check_locateCar.setChecked(false);
}
if(jsonObject.getString("ignition").equalsIgnoreCase("true")){
check_carOff.setChecked(true);
}else{
check_carOff.setChecked(false);
}
if(jsonObject.getString("taser").equalsIgnoreCase("true")){
check_tase.setChecked(true);
}else{
check_tase.setChecked(false);
}
if(jsonObject.getString("photo_stat").equalsIgnoreCase("true")){
check_getImg.setChecked(true);
}else{
check_getImg.setChecked(false);
}
//check_carOn.setChecked(false);
nextFrag = new locate_updatedstatus();
////////////////////////////////////////////////////
}
else{
loading.hide();
nextFrag = new locate_before();
}
}
else{
loading.hide();
nextFrag = new locate_car_reg();
}
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.fragment_container, nextFrag);
transaction.addToBackStack(null);
transaction.commit();
}else {
Toast.makeText(getApplicationContext(), "Error" +jsonObject.getString("user"), Toast.LENGTH_SHORT).show();
loading.hide();
}
} catch (JSONException e) {
e.printStackTrace();
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
}){
@Override
protected Map<String, String> getParams() throws AuthFailureError {
HashMap<String,String> hashMap = new HashMap<String, String>();
hashMap.put("username",username_text.getText().toString());
hashMap.put("password",password_text.getText().toString());
return hashMap;
}
};
requestQueue.add(request);
///////////////////////////
//Toast.makeText(MainActivity.this,uname+"\n"+pword, Toast.LENGTH_LONG).show();
//newFragment = new locate_login();
}
fragment_locate_updatedstatus的XML文件：
<CheckBox
android:id="@+id/check_locateCar"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/updates"
android:layout_centerHorizontal="true"
android:layout_marginTop="20dp"
android:text="Locate Car using Gmaps" />
<CheckBox
android:id="@+id/check_carOff"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignStart="@+id/check_locateCar"
android:layout_below="@+id/check_locateCar"
android:layout_marginTop="10dp"
android:text="Turn car off" />
<CheckBox
android:id="@+id/check_getImg"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="10dp"
android:layout_below="@+id/check_carOff"
android:layout_alignStart="@+id/check_carOff"
android:text="Capture images" />
<CheckBox
android:id="@+id/check_tase"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="10dp"
android:layout_below="@+id/check_getImg"
android:layout_alignStart="@+id/check_getImg"
android:text="Tase Culprit" />
<CheckBox
android:id="@+id/check_carOn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignStart="@+id/check_tase"
android:layout_below="@+id/check_tase"
android:layout_marginTop="10dp"
android:text="Turn system off" />
错误
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setChecked(boolean)' on a null object reference
答案 0 :(得分：0)
尝试将片段事务移动到`nextFrag'：
if(v == findViewById(R.id.login)){ //between fragments: from login screen to home page
username_text = (EditText) findViewById(R.id.username_text);
password_text = (EditText) findViewById(R.id.password_text);
RequestQueue requestQueue = Volley.newRequestQueue(this);;
String signin = "http://acarjsystem.herokuapp.com/auth/";
StringRequest request;
uname = username_text.getText().toString();
pword = password_text.getText().toString();
final ProgressDialog loading = new ProgressDialog(MainActivity.this);
loading.setTitle("Loading");
loading.setMessage("Attempting to Log In");
loading.show();
////////////////////
request = new StringRequest(Request.Method.POST, signin, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
final Fragment nextFrag;
final CheckBox check_locateCar, check_carOff,check_getImg, check_tase, check_carOn;
try {
JSONObject jsonObject = new JSONObject(response);
if(jsonObject.getString("user").equals("logined")){
Toast.makeText(getApplicationContext(),"SUCCESS "+jsonObject.getString("user"),Toast.LENGTH_SHORT).show();
//startActivity(new Intent(getApplicationContext(),Welcome.class));
if(!jsonObject.getString("car_plate").equals("0")){
loading.hide();
if(jsonObject.getString("car_stat").equalsIgnoreCase("true")){
//////////////HERE//////////////////////////////////
//check_carOn.setChecked(false);
nextFrag = new locate_updatedstatus();
////////////////////////////////////////////////////
}
else{
loading.hide();
nextFrag = new locate_before();
}
}
else{
loading.hide();
nextFrag = new locate_car_reg();
}
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.fragment_container, nextFrag);
transaction.addToBackStack(null);
transaction.commit();
check_locateCar = (CheckBox)findViewById(R.id.check_locateCar);
check_carOff = (CheckBox)findViewById(R.id.check_carOff);
check_getImg = (CheckBox)findViewById(R.id.check_getImg);
check_tase = (CheckBox)findViewById(R.id.check_tase);
check_carOn = (CheckBox)findViewById(R.id.check_carOn);
if(jsonObject.getString("loc_stat").equalsIgnoreCase("true")){
/*THE ERROR IS AROUND THIS LINE*/
check_locateCar.setChecked(true);
}else{
check_locateCar.setChecked(false);
}
if(jsonObject.getString("ignition").equalsIgnoreCase("true")){
check_carOff.setChecked(true);
}else{
check_carOff.setChecked(false);
}
if(jsonObject.getString("taser").equalsIgnoreCase("true")){
check_tase.setChecked(true);
}else{
check_tase.setChecked(false);
}
if(jsonObject.getString("photo_stat").equalsIgnoreCase("true")){
check_getImg.setChecked(true);
}else{
check_getImg.setChecked(false);
}
}else {
Toast.makeText(getApplicationContext(), "Error" +jsonObject.getString("user"), Toast.LENGTH_SHORT).show();
loading.hide();
}
} catch (JSONException e) {
e.printStackTrace();
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
}){
@Override
protected Map<String, String> getParams() throws AuthFailureError {
HashMap<String,String> hashMap = new HashMap<String, String>();
hashMap.put("username",username_text.getText().toString());
hashMap.put("password",password_text.getText().toString());
return hashMap;
}
};
requestQueue.add(request);
///////////////////////////
//Toast.makeText(MainActivity.this,uname+"\n"+pword, Toast.LENGTH_LONG).show();
//newFragment = new locate_login();
}
答案 1 :(得分：0)
我正在尝试从复选框值所在的不同片段设置复选框值。
片段需要通过代理通过主/父Activity进行通信。要做到这一点
onAttach覆盖中，将OnCheckBoxSelectedListener成员设置为Activity参数。
1）为所需的侦听器方法定义一个接口（在你的情况下是一个OnCheckBoxSelectedListener监听器）。
interface OnCheckBoxSelectedListener
{
public void SetChecked(bool checked);
}
2）让主Activity实现OnCheckBoxSelectedListener
public class Activity implements OnCheckBoxSelectedListener
{
public void SetCheck(bool checked)
{
check_locateCar = (CheckBox)findViewById(R.id.check_locateCar);
check_locateCar.setChecked(true);
}
}
3）在片段中声明OnCheckBoxSelectedListener的私有成员
4）在Fragment的onAttach覆盖中，将OnCheckBoxSelectedListener成员设置为Activity参数。
public static class FragmentA extends ListFragment {
OnCheckBoxSelectedListener mListener;
...
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
mListener = (OnArticleSelectedListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
}
}
...
}
然后从onResponse调用监听器
@Override
public void onResponse(String response) {
final Fragment nextFrag;
final CheckBox check_locateCar, check_carOff,check_getImg, check_tase, check_carOn;
try {
check_locateCar = (CheckBox)findViewById(R.id.check_locateCar);
check_carOff = (CheckBox)findViewById(R.id.check_carOff);
check_getImg = (CheckBox)findViewById(R.id.check_getImg);
check_tase = (CheckBox)findViewById(R.id.check_tase);
check_carOn = (CheckBox)findViewById(R.id.check_carOn);
JSONObject jsonObject = new JSONObject(response);
if(jsonObject.getString("user").equals("logined")){
Toast.makeText(getApplicationContext(),"SUCCESS "+jsonObject.getString("user"),Toast.LENGTH_SHORT).show();
//startActivity(new Intent(getApplicationContext(),Welcome.class));
if(!jsonObject.getString("car_plate").equals("0")){
loading.hide();
if(jsonObject.getString("car_stat").equalsIgnoreCase("true")){
//////////////HERE//////////////////////////////////
if(jsonObject.getString("loc_stat").equalsIgnoreCase("true")){
/*THE ERROR IS AROUND THIS LINE*/
//check_locateCar.setChecked(true);
mListener.SetChecked(true)
}else{
check_locateCar.setChecked(false);
}
// ...
}
}
}
}
}
}
注意：以上建立了Fragment to Activity方向的沟通。换句话说（来自监听器的“回调”），我定义了一个回调接口，在Fragment中实现它，并使用对作为参数传入的Fragment的引用从监听器中调用它（通过{{1指针）。例如
this然后从onRequest调用它如下（在这种情况下你可能不需要回调，但在某些时候你可能会，例如从主活动调用startActivityForRequest / onActivityRequest时，因为它们在不同的线程上运行）< / p>
interface OnCheckBoxSelectedListener
{
public void SetCheck(OnCheckBoxSelectedCallback callingFragment, attachmentDialog2EventListener, bool checked);
}
interface OnCheckBoxSelectedCallback
{
SetCheckedCallback();
}
public class Activity implements OnCheckBoxSelectedListener
{
public void SetCheck(OnCheckBoxSelectedCallback callingFragment, bool checked)
{
check_locateCar = (CheckBox)findViewById(R.id.check_locateCar);
check_locateCar.setChecked(true);
callingFragment.SetCheckedCallback();
}
}
public static class FragmentA extends ListFragment implements OnCheckBoxSelectedCallback {
OnCheckBoxSelectedListener mListener;
...
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
mListener = (OnCheckBoxSelectedListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement OnCheckBoxSelectedListener");
}
}
...
public void SetCheckedCallback()
{
// ...
}
}