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What is a NullPointerException, and how do I fix it?
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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);
}
});
}
}
Related
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"));
}
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
EditText textmsg;
static final int READ_BLOCK_SIZE = 100;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textmsg = (EditText) findViewById(R.id.editText1);
}
#Override
public void onClick(View v) {
Button noteBtn = (Button) findViewById(R.id.noteBtn);
Button resuBtn = (Button) findViewById(R.id.resuBtn);
Button agenBtn = (Button) findViewById(R.id.agenBtn);
noteBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, Notes.class);
}
});
resuBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, Results.class);
}
});
agenBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, Agenda.class);
}
});
When I run the application the buttons don't work. If I set the buttons as so..
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button agenBtn = (Button) findViewById(R.id.agenBtn);
Button resuBtn= (Button) findViewById(R.id.resuBtn);
Button noteBtn = (Button) findViewById(R.id.noteBtn);
agenBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, Agenda.class);
startActivity(intent);
}
}); etc...
If I use this code above, the code works fine and the buttons work correctly. But other functionality with different classes/activities won't run. Could someone please show me a solution or explain how to solve this issue.
You should place:
this.noteBtn = (Button) findViewById(R.id.noteBtn);
this.notBtn.setOnClickListener(this);
this.resuBtn = (Button) findViewById(R.id.resuBtn);
this.resuBtn.setOnClickListener(this);
this.timeBtn = (Button) findViewById(R.id.timeBtn);
this.timeBtn.setOnClickListener(this);
in onCreate() instead of onClick(). Make the buttons belong to the class, so you can reference them in onClick(). You should not be setting new onClickListeners in onClick(), but should rather have a switch statement based on the clicked view's id to determine which button was pressed.
this code
#Override
public void onClick(View v) {
Button noteBtn = (Button) findViewById(R.id.noteBtn);
Button resuBtn = (Button) findViewById(R.id.resuBtn);
Button timeBtn = (Button) findViewById(R.id.timeBtn);
...
}
is not working because it's never going to get call, since none of your buttons is implementing it, worse yet they have not even been initialized because the onClick has not and will never be called.
the correct way to do it is like:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button but1 = (Button) findViewById(R.id.but1);
Button resBtn = (Button) findViewById(R.id.resBtn);
Button noteBtn = (Button) findViewById(R.id.noteBtn);
agendaBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View viewTimeTable) {
Intent intent = new Intent(MainActivity.this, Agenda.class);
startActivity(intent);
}
}); etc...
because you are first getting a reference to your buttons and assigning them the onClickListener to each one of it.
I will suggest reading more about the Android Activity life cycle you can find it
here
I am new to Java and android programming.Can someone explain to me what is wrong here?
When i run it it gives me the errors:
Error:illegal start of expression, Error: identifier expected, Error:
not a statement, Error: ';' expected, Error: ')' expected,
Error:illegal start of type, Error:reached end of file while parsing.
Here is the code:
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Button
Button btn = (Button) findViewById(R.id.button);
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
}
});
//EditText
EditText nop = (EditText) findViewById(R.id.editText);
EditText cob = (EditText) findViewById(R.id.editText2);
cob.getText().toString();
nop.getText().toString();
public void total = cob+ nop;
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
tv.setText(final.total)
})
//TextView
TextView tv = (TextView) findViewById(R.id.textView);
}
}}
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
tv.setText(final.total)
})
Should be immediately followed by an ';', same goes for the setText call.
An error like that always means you forgot one of those, or misplaced/forgotten some brackets, ...
Since you don't state the exact error message: I'm just guessing it's this error, but it's possible there are more in your code.
EDIT:
And, as others already mentioned: public void total = cob+ nop; makes no sense at all. void is not a valid datatype in Java, so you can't declare a 'void'.
The second button click event is not right.
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
tv.setText(final.total);
}
});
total is also wrong.
It should be
int total = Integer.parseInt(cob.getText().toString()) + Integer.parseInt(nob.getText().toString()) ;
Putting all these together
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//EditText
EditText nop = (EditText) findViewById(R.id.editText);
EditText cob = (EditText) findViewById(R.id.editText2);
TextView tv = (TextView) findViewById(R.id.textView);
int a = Integer.parseInt(cob.getText().toString());
int b = Integer.parseInt(nop.getText().toString());
public void total = a + b;
Button btn = (Button) findViewById(R.id.button);
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
tv.setText(total);
}
});
}
int total = Integer.parseInt(cob.getText().toString())+ Integer.parseInt( nop.getText().toString());
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
tv.setText(String.valueOf(total));
});
remove
cob.getText().toString();
nop.getText().toString();
public void total = cob+ nop;
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);
}
I want to disable button B when making a treatement in button A.
public class GoJump extends Activity{
Button answerA, answerB;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.jump);
answerA = (Button) findViewById(R.id.button_A);
answerB = (Button) findViewById(R.id.button_B);
answerA.setOnClickListener(new OnClickListener()
{
public void onClick(View v)
{
// Do something
// Disable button B
}
});
The problem is button B is not visible inside button A treatement. I have to declare it another time to disable it.
Is there any other method to do? Make a variable visible in all class.
Thank you.
Initialize your variable before onCreate().
// Before onCreate
private Button b, b2;
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
b = (Button) findViewById(R.id.button);
b2 = (Button) findViewById(R.id.button2);
b.setOnClickListener(new OnClickListener()
{
public void onClick(View v)
{
b2.setEnabled(false);
}
});
{
You just need to setEnabled to false. Read this javadoc.
public void onClick(View v)
{
answerB.setEnabled(false);
}