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Null pointer on not null index? [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 5 years ago.
Like in subject. I got null pointer, how it is possible ?
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
public class MainActivity extends AppCompatActivity {
Button button, button2;
TextView textView, textView2;
String abc;
public List list = new ArrayList<>();
int size;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
list.add("abc");
abc = (String) list.get(0);
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
textView.setText(abc);
}
});
button2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
size = list.size();
textView2.setText(size);
}
});
}
}

You just forgot to do findViewById to your Controls in OnCreate()
button2 =(Button) findViewById(R.id.button2);
button = (Button) findViewById(R.id.btnID);
textView2 =(textView) findViewById(R.id.textView2);
Sample code
#Override
protected void onCreate( Bundle savedInstanceState ){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.mybtnID);
button2 = (Button) findViewById(R.id.button2);
textView2 = (textView) findViewById(R.id.textView2);
list.add("abc");
abc = (String) list.get(0);
button.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick( View view ){
textView.setText(abc);
}
});
button2.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick( View view ){
size = list.size();
textView2.setText(size);
}
});
}

You are missing
findViewById for all your widget
i.e.
Button button, button2;
TextView textView, textView2;
button = findViewById(R.id.id_of_button_1); // same for button2,textview,textview2

You can initialize with findById(R.id.myBtn) like everyone is saying. Or programatically like button = new Button(this);

Try this
You forgot to add findViewById
public class MainActivity extends AppCompatActivity {
Button button, button2;
TextView textView, textView2;
String abc;
public List list = new ArrayList<>();
int size;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button=(Button)findViewById(R.id.button);
button2=(Button)findViewById(R.id.button2);
list.add("abc");
abc = (String) list.get(0);
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
textView.setText(abc);
}
});
button2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
size = list.size();
textView2.setText(size);
}
});
}
}

Try this are missing findViewById for all your widget
button = (Button) findViewById(R.id.btnID);
button2 =(Button) findViewById(R.id.button2);
textView = =(textView) findViewById(R.id.textView);
textView2 =(textView) findViewById(R.id.textView2);
Sample Code
public class MainActivity extends AppCompatActivity {
Button button, button2;
TextView textView, textView2;
String abc;
public List list = new ArrayList<>();
int size;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.btnID);
button2 =(Button) findViewById(R.id.button2);
textView = =(textView) findViewById(R.id.textView);
textView2 =(textView) findViewById(R.id.textView2);
list.add("abc");
abc = (String) list.get(0);
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
textView.setText(abc);
}
});
button2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
size = list.size();
textView2.setText(size);
}
});
}
}

Passing EditText integer to TextView via Intent app crashes after clicking (button) three

I am trying just to pass the int user input to the next class and print it, see that it works before continuing on using it or something else.
Home.java
start and exit button
public class Home extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button one = (Button) findViewById(R.id.b1);
one.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
goToSecondActivity();
}
});
Button two = (Button) findViewById(R.id.b2);
two.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
finish();
}
});
}
private void goToSecondActivity() {
Intent i = new Intent(this, SelectNumberOfPlayers.class);
startActivity(i);
}
}
SelectNumberOfPlayers.java
Taking only the numbers from the input and passing it to StartGame.class
public class SelectNumberOfPlayers extends AppCompatActivity {
EditText numberOfPlayers;
Button three;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.enter_number_of_players);
three = (Button) findViewById(R.id.button3);
three.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
String txt = numberOfPlayers.getText().toString();
Intent i = new Intent(getApplicationContext(), StartGame.class);
i.putExtra("players", txt);
startActivity(i);
}
});
}
}
StartGame.java
Receiving Int and printing to TextView
public class StartGame extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.start_game);
Intent i = getIntent();
TextView numOfPlayersVal = (TextView) findViewById(R.id.txt2);
numOfPlayersVal.setText(i.getStringExtra("Player number"));
}
}
Where is the error happening? I've set the input keyboard to take only numbers

1. you forgot to initialize youe editext first initialize it in your SelectNumberOfPlayers Activity
numberOfPlayers = (EditText) findViewById(R.id.numberOfPlayers);
2.
the key must be same to pass and receive data with intent
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.start_game);
Intent i = getIntent();
TextView numOfPlayersVal = (TextView) findViewById(R.id.txt2);
numOfPlayersVal.setText(i.getStringExtra("players"));
}

Trouble with Intent in Android Studio

I have been trying to get a string to go from one java class to another for a project of mine. The code I have been experimenting with is not working. When I press the button, I know it opens the other Java class because it creates the other layout, but it doesn't show the string. Please help me.
First Java Class:
public class MainActivity extends AppCompatActivity {
private Button button;
Context context;
private EditText editText;
String number = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
context = this;
editText = (EditText) findViewById(R.id.editText);
button = (Button) findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
if (editText.getText().toString() != null) {
String value = "value";
Intent secondscreenIntent = new Intent(context, SecondScreenJ.class);
secondscreenIntent.putExtra("Number", editText.getText().toString());
startActivity(secondscreenIntent);
}
}
});
}
}
Second Java Class:
public class SecondScreenJ extends Activity {
String number = null;
private TextView textView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.secondscreen);
textView = (TextView) findViewById(R.id.textView);
Bundle extras = getIntent().getExtras();
if (extras != null){
number = extras.getString("number");
}
textView.setText(number);
}
}

you are putting "Number" as key but, in your second activity you are trying to retrieve "number"
so change number to Number and it will work.
Its case Sensitive.

Don't make your keys hard-coded in that way.
Just declare public static variable in MainActivity and use it from SecondScreenJ
public class MainActivity extends AppCompatActivity {
private Button button;
Context context;
private EditText editText;
public static String NUMBER_KEY = "Number";
String number = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
context = this;
editText = (EditText) findViewById(R.id.editText);
button = (Button) findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
if (editText.getText().toString() != null) {
String value = "value";
Intent secondscreenIntent = new Intent(context, SecondScreenJ.class);
secondscreenIntent.putExtra(NUMBER_KEY , editText.getText().toString());
startActivity(secondscreenIntent);
}
}
});
}
}
Second Java Class:
public class SecondScreenJ extends Activity {
String number = null;
private TextView textView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.secondscreen);
textView = (TextView) findViewById(R.id.textView);
Bundle extras = getIntent().getExtras();
if (extras != null){
number = extras.getString(MainActivity.NUMBER_KEY);
}
textView.setText(number);
}
}

Beware when using the key for putting and getting the extras. They're case sensitive. Replace "number" with "Number" in your second activity.

Disabling edittext by using a checkbox error

I have a problem here. I want to disable a edittext by using a checkbox. If checkbox is checked, edittext isa available, if not, it is disabled. But I'm having problems. Here is my code: Can anyone check on this. Accdng to eclipse, no error. But when I run it on my phone, the enabling/disabling is not functioning.
public class Order extends Activity {
private Button button1;
private EditText txtbox1,txtbox2;
private TextView tv;
CheckBox check1;
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.order);
txtbox1= (EditText) findViewById(R.id.editText1);
button1 = (Button) findViewById(R.id.button1);
tv = (TextView) findViewById(R.id.editText5);
txtbox2= (EditText) findViewById(R.id.editText2);
check1 = (CheckBox)findViewById(R.id.checkcheck);
button1.setOnClickListener(new clicker());
}
public void addListenerOncheck1() {
check1.setOnCheckedChangeListener(new OnCheckedChangeListener() {
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
// TODO Auto-generated method stub
if(isChecked){
txtbox1.setFocusable(true);
txtbox1.setFocusableInTouchMode(true);
} else {
txtbox1.setFocusable(false);
txtbox1.setFocusableInTouchMode(false);
}
}
});
}
class clicker implements Button.OnClickListener
{
public void onClick(View v)
{
String a,b;
Integer vis;
a = txtbox1.getText().toString();
b = txtbox2.getText().toString();
vis = Integer.parseInt(a)*2+Integer.parseInt(b)*3;
tv.setText(vis.toString());
}
}
}

you are nowhere adding your Listener by calling ur method addListenerOncheck1();
so put addListenerOncheck1(); add the end of you OnCreate method.
But I highly recommend you not to use all these additional self-made Classes for just adding listeners. Use the following code instead and add it into your OnCreate Method:
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.order);
txtbox1= (EditText) findViewById(R.id.editText1);
button1 = (Button) findViewById(R.id.button1);
tv = (TextView) findViewById(R.id.editText5);
txtbox2= (EditText) findViewById(R.id.editText2);
check1 = (CheckBox)findViewById(R.id.checkcheck);
button1.setOnClickListener(new clicker());
check1.setOnCheckedChangeListener(new OnCheckedChangeListener() {
#Override
public void onCheckedChanged(CompoundButton buttonView,
boolean isChecked) {
if (isChecked) {
txtbox1.setEnabled(false);
} else {
txtbox1.setEnabled(true);
}
}
});
button1.setOnClickListener(new OnClickListener(){
#Override
public void onClick(View v) {
String a,b;
Integer vis;
a = txtbox1.getText().toString();
b = txtbox2.getText().toString();
vis = Integer.parseInt(a)*2+Integer.parseInt(b)*3;
tv.setText(vis.toString());
}
});
// This will make sure the user can only type numbers into the EditTexts:
txtbox1.setInputType(InputType.TYPE_CLASS_NUMBER);
txtbox2.setInputType(InputType.TYPE_CLASS_NUMBER);
// Code to disable editTexts at start once:
txtbox1.setEnabled(false);
txtbox2.setEnabled(false);
}

How to show user entered EditText value in a android dialogue box

my modified sample of my work basically below i have 2 editboxes i want the values of the both to be shown in an alertbox, rest of the code works fine, the alertbox does show up but without the entered values.
public class Main extends Activity {
EditText username = null;
EditText password = null;
Button login;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
EditText usern = (EditText) findViewById(R.id.username);
EditText pass = (EditText) findViewById(R.id.password);
Button login = (Button) findViewById(R.id.login);
String usernm = usern.getText().toString();
String passnm = pass.getText().toString();
final Builder alert = new AlertDialog.Builder(this)
.setTitle("SHOW FIELDS")
.setMessage("USERNAME:" + usernm + " PASSWORD:" + passnm)
.setNegativeButton("CLOSE", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
// do nothing
closeContextMenu();
}
});
login.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
alert.show();
}
});

you will need to get EditText's value on button so show in AlertDialog as :
login.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
usernm = usern.getText().toString();
passnm = pass.getText().toString();
alert.show();
}
and declare both usernm and passnm at class level
EDIT :
or best way create a method to show alert and call it on button click as:
public void showalert(){
usernm = usern.getText().toString();
passnm = pass.getText().toString();
alert = new AlertDialog.Builder(Main.this)
.setTitle("SHOW FIELDS")
.setMessage("USERNAME:" + usernm + " PASSWORD:" + passnm)
.setNegativeButton("CLOSE", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
// do nothing
closeContextMenu();
}
});
alert.show();
}
and call this method on button click as:
login.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
showalert();
}:
});
and declare all variables at class level by which you can access it in whole class as :
public class Main extends Activity {
EditText usern ;
EditText pass ;
Button login;
String usernm ,passnm; //<<< Declare here
Builder alert;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
usern = (EditText) findViewById(R.id.username);
pass = (EditText) findViewById(R.id.password);
login = (Button) findViewById(R.id.login);
login.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
showalert();
}
});
}