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the Android Studio OnClickListener doesn't work

Nothing happens when I click the "start" button, the app suddenly closes. I think the View.OnClickListener doesn't work, but I can't find how to fix it.
package com.example.myquiz;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.content.res.ResourcesCompat;
import android.content.Intent;
import android.graphics.Typeface;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private TextView title;
private Button start;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
title = findViewById(R.id.main_title);
start = findViewById(R.id.ma_startB);
Typeface typeface = ResourcesCompat.getFont(this,R.font.blacklist);
title.setTypeface(typeface);
start.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this,CategoryActivity.class);
startActivity(intent);
}
});
}
}
I got this error in Logcat:
error screenshot
This is the code for the CategoryActivity class, this is where I get the error from, but I really don't see where the problem is.
package com.example.myquiz;
import android.os.Bundle;
import android.view.MenuItem;
import android.widget.GridView;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import java.util.ArrayList;
import java.util.List;
public class CategoryActivity extends AppCompatActivity {
private GridView catGrid;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_category);
androidx.appcompat.widget.Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
getSupportActionBar().setTitle("Categories");
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
catGrid = findViewById(R.id.catGridView);
List<String> catList = new ArrayList<>();
catList.add("Cat 1");
catList.add("Cat 2");
catList.add("Cat 3");
catList.add("Cat 4");
catList.add("Cat 5");
catList.add("Cat 6");
CatGridAdapter adapter = new CatGridAdapter(catList);
catGrid.setAdapter(adapter);
}
#Override
public boolean onOptionsItemSelected(#NonNull MenuItem item) {
if(item.getItemId() == android.R.id.home)
{ CategoryActivity.this.finish();}
return super.onOptionsItemSelected(item);
}
}

The click listener is working correct, looking at the error log the launching of the CategoryActivity class is causing the app crash:
Caused by: java.lang.IllegalStateException: This Activity already has an action bar supplied by the window decor...
This tells the ActionBar / ToolBar inside the CategoryActivity is setup twice. Probably (if not, please share the class code) you can either remove setSupportActionBar() or change the theme of the CategoryActivity to something like: Theme.MaterialComponents.Light.NoActionBar to see if you get the Activity running after the button click.

How to fix "error: cannot find symbol method startActivity(Intent)" in a class of Listener?

I can make an Intent to open other Activity with writing the code in MainActivity.java.
Then I try to make an Intent using a class and called it in MainActivity.java. But it becomes error.
How to solve this problem?
When I write startActivity(numberIntent); in MainActivity.java there is no error but when I move this line of code to NumbersClickListener.java
Errors come:
error: cannot find symbol method startActivity(Intent)
error: not an enclosing class: MainActivity
This my code
In MainActivity.java
package com.example.android.*****;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.widget.TextViewCompat;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
NumbersClickListener clickListener = new NumbersClickListener();
TextView numbers = (TextView)findViewById(R.id.numbers);
numbers.setOnClickListener(clickListener);
}
in NumbersClickListener.java
package com.example.android.*****;
import android.content.Intent;
import android.view.View;
android.widget.Toast first
import android.widget.Toast;
OnClickListener should be written in capital letter
public class NumbersClickListener implements View.OnClickListener {
#Override
public void onClick(View view) {//.makeText(view.getContext(),
"open the list of numbers", Toast.LENGTH_SHORT).show();
Intent numberIntent = new Intent(MainActivity.this,
NumbersActivity.class);
startActivity(numberIntent);
}
}

error: cannot find symbol method startActivity(Intent)” in a class of Listener?
Because if startActivity(Intent) is a method of activity and its required call from context
If you want call startActivity(Intent) outside activity you need to use
Context.startActivity(numberIntent);
Use this
view.getContext().startActivity(numberIntent);
instead of this
startActivity(numberIntent);
SAMPLE CODE
public class NumbersClickListener implements View.OnClickListener {
#Override
public void onClick(View view) {
Intent numberIntent = new Intent(view.getContext(),
NumbersActivity.class);
view.getContext().startActivity(numberIntent);
}
}

You are defining NumbersClickListener in a separate java file. It has no way compiler will know that when u call startActivity you are referring to the Activity.startActivity
Unless you have a deeper purpose for NumbersClickListener.java, simply do inline declaration of View.Listener will do
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
NumbersClickListener clickListener = new NumbersClickListener();
TextView numbers = (TextView)findViewById(R.id.numbers);
numbers.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View view) {
Intent numberIntent = new Intent(MainActivity.this,NumbersActivity.class);
startActivity(numberIntent);
}
});
}

In place MainActivity.this, use its context.
Intent numberIntent = new Intent(context, NumbersActivity.class);
startActivity(numberIntent);
Notice the changes I have made
MainActivity.java
package com.example.android.*****;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.widget.TextViewCompat;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
NumbersClickListener clickListener = new NumbersClickListener(MainActivity.this); // Context while creating ClickListener Object
TextView numbers = (TextView)findViewById(R.id.numbers);
numbers.setOnClickListener(clickListener);
}
NumbersClickListener.java
package com.example.android.*****;
import android.content.Intent;
import android.view.View;
import android.widget.Toast;
public class NumbersClickListener implements View.OnClickListener {
Context context;
NumbersClickListener(Context c){
this.context = c;
}
#Override
public void onClick(View view) {
Intent numberIntent = new Intent(context, NumbersActivity.class);
startActivity(numberIntent);
}
}

To startActivity you need Context.
It will be like this context.startActivity()
In MainActivity it is not giving error because Activity internally extends Context.
NumbersClickListener is not extended Context.
So, you can start activity using View context
Replace startActivity(numberIntent) with
view.getContext().startActivity(numberIntent);

inside your NumberClickListener class you can do the following
Context context = view.getContext();
Intent numberIntent = new Intent (context, NumberActivity.class);
context.startActivity(numberIntent);
By using this code you can use your NumberClickListener with any other activity.
happy codding :)

How to use new class wiith media player in main activity?

I'm trying to create new class, becouse I will have there many of sound file, but I don't know how to use it in main ativity. Here is my example code of sound.java:
package app.damian.komunikat_v1;
import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
public class sound extends MainActivity{
final MediaPlayer sound1 = MediaPlayer.create(this,R.raw.miau);
final MediaPlayer sound2 = MediaPlayer.create(this,R.raw.lol);
}
And now i'm trying to use sound1.start(); in main activity, but i don't know how. Any suggestions?
EDIT:
This is my MainActivity.java:
this is my MainActivity.java:
package app.damian.komunikat_v1;
import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button guzik = findViewById(R.id.button);
final MediaPlayer sound1 = MediaPlayer.create(this,R.raw.miau);
guzik.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
sound1.start();
};
});
}
}

I found a different solution. Instead of creating new class with list of sound file, I had created an array of sounds in main activity class.

IN my case Button1.setEnable(True); is not working

in my case when i try to set the button enable using BUTTON1.setEnable(false);
so the emulator does not start the apk...
i want to create a game level menu with some buttons Enable and rest of the buttons Disable .as each button is a game level .. so if my integer flag has value 5 so first five levels buttons should be active and rest of the buttons be disable.... but i could not even set a single button disable ... please help am i missing some concept with buttons ... where to change their state in OnResume() or in OnCreate();
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
Button b1=(Button)findViewById(R.id.button);
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1.setEnabled(false);
}
}

You should add b1 = (Button) findViewById(R.id.button) into your onCreate() method.
Try this:
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
Button b1;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1 = (Button) findViewById(R.id.button);
b1.setEnabled(false);
}
}

I don't have an android development environment at hand right now, but I could imagine, that it has somethin to do, with the view (and therefore the button) not beining available at the time of the initiation of the attribute b1. Could you try the following:
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getButton1().setEnabled(false);
}
private Button getButton1() {
return (Button)findViewById(R.id.button);
}
}

so in order to set buttons Enable/Disable one have to declare the buttons into OnCreate ...otherwise it wont find the view of the button and show errors ...
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
Button b1;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1 = (Button) findViewById(R.id.button);
b1.setEnabled(false);
}
}
is the right way .............thanks everone

main cannot be resolved or is not a field # helloworld

So I just started programming in eclipse, and I got the error ``main cannot be resolved or is not a field" with this code:
package com.example.wouter1;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
import com.example.wouter1.R;
public class HelloWorldActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main); // PROBLEM HERE <<<< # MAIN
TextView myText = new TextView(this);
myText.setText("hello world");
setContentView(myText);
}
}