Не понимаю в чем ошибка, не запускается MainActivity и SecondActivity?

Question

[PeJlToFaN]

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 22180
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3308)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3457)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2044)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7562)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference
at com.example.myapplication.MainActivity.onCreate(MainActivity.java:18)
at android.app.Activity.performCreate(Activity.java:7893)
at android.app.Activity.performCreate(Activity.java:7880)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1307)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3283)

Comments

[PeJlToFaN]

package com.example.myapplication;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends Activity {
Button l1, l2, l3, yl;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_one_more);
l1.findViewById(R.id.level1);
l2.findViewById(R.id.level2);
l3.findViewById(R.id.level3);
yl.findViewById(R.id.yourquiz);
l1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, OMActivity.class);
startActivity(intent);
}
});
l2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, SLevelActivity.class);
startActivity(intent);
}
});
l3.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, ThLevelActivity.class);
startActivity(intent);
}
});
yl.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, yLevelActivity.class);
startActivity(intent);
}
});

}
}

[PeJlToFaN]

package com.example.myapplication;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;

import androidx.annotation.Nullable;

import java.util.Random;

public class SecondActivity extends Activity {
Button accept, nButton;
EditText question, answer;
ProgressBar progressBar;
private yQuiz Quiz = new yQuiz();
int x = 0;
Random r;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);

accept.findViewById(R.id.accept);
question.findViewById(R.id.question);
answer.findViewById(R.id.answer);
progressBar.findViewById(R.id.progressBar);
nButton.findViewById(R.id.nbutton);
nButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(SecondActivity.this, yLevelActivity.class);
startActivity(intent);
}
});
accept.setOnClickListener(new View.OnClickListener() {
public void onClick(View view){
if(question.getText().length() != 0 && answer.getText().length() !=0 ){
Quiz.nQuestions[x] = String.valueOf(question.getText());
Quiz.nCorrectAnswer[x] = String.valueOf(answer.getText());
if(x==7) {
nButton.setVisibility(View.VISIBLE);
nButton.setClickable(true);
}
if(Integer.parseInt(Quiz.nCorrectAnswer[x])>=10 && Integer.parseInt(Quiz.nCorrectAnswer[x])<100){
Quiz.nChoices[x][0] = String.valueOf(r.nextInt()%100);
Quiz.nChoices[x][1] = String.valueOf(r.nextInt()%100);
Quiz.nChoices[x][2] = String.valueOf(answer.getText());
Quiz.nChoices[x][3] = String.valueOf(r.nextInt()%100);
}
else if(Integer.parseInt(Quiz.nCorrectAnswer[x])>=100 && Integer.parseInt(Quiz.nCorrectAnswer[x])<1000){
Quiz.nChoices[x][0] = String.valueOf(r.nextInt()%1000);
Quiz.nChoices[x][1] = String.valueOf(answer.getText());
Quiz.nChoices[x][2] = String.valueOf(r.nextInt()%1000);
Quiz.nChoices[x][3] = String.valueOf(r.nextInt()%1000);
}
else if(Integer.parseInt(Quiz.nCorrectAnswer[x])>=1000 && Integer.parseInt(Quiz.nCorrectAnswer[x])<10000){
Quiz.nChoices[x][0] = String.valueOf(r.nextInt()%10000);
Quiz.nChoices[x][1] = String.valueOf(r.nextInt()%10000);
Quiz.nChoices[x][2] = String.valueOf(r.nextInt()%10000);
Quiz.nChoices[x][3] = String.valueOf(answer.getText());
}
else if(Integer.parseInt(Quiz.nCorrectAnswer[x])>=10000 && Integer.parseInt(Quiz.nCorrectAnswer[x])<100000){
Quiz.nChoices[x][0] = String.valueOf(answer.getText());
Quiz.nChoices[x][1] = String.valueOf(r.nextInt()%100000);
Quiz.nChoices[x][2] = String.valueOf(r.nextInt()%100000);
Quiz.nChoices[x][3] = String.valueOf(r.nextInt()%100000);
}
else if(Integer.parseInt(Quiz.nCorrectAnswer[x])>=1 && Integer.parseInt(Quiz.nCorrectAnswer[x])<10){
Quiz.nChoices[x][0] = String.valueOf(answer.getText());
Quiz.nChoices[x][1] = String.valueOf(r.nextInt()%10);
Quiz.nChoices[x][2] = String.valueOf(r.nextInt()%10);
Quiz.nChoices[x][3] = String.valueOf(r.nextInt()%10);
}
x++;
progressBar.setProgress(x);
}
}

});
}
}

Answer 1

[Денис Загаевский]

Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference

потому что

setContentView(R.layout.activity_one_more);
l1.findViewById(R.id.level1);

l1 никто не заполняет, но ты на нем вызываешь метод.
нужно так l1 = findViewById(R.id.level1)