问题

以下是翻译好的内容：

第一个活动的Java代码：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button btn = (Button) findViewById(R.id.button_start_quiz);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openQuizActivity();
            }
        });
    }

    public void openQuizActivity() {
        Intent intent = new Intent(MainActivity.this, QuizActivity.class);
        startActivity(intent);
    }
}

第二个活动的Java代码：

public class QuizActivity extends AppCompatActivity {

    Button answer1, answer2, answer3, answer4;
    ImageView flag;
    List<CountryItem> list;
    Random r;
    int turn = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        answer1 = (Button) findViewById(R.id.answer_1);
        answer2 = (Button) findViewById(R.id.answer_2);
        answer3 = (Button) findViewById(R.id.answer_3);
        answer4 = (Button) findViewById(R.id.answer_4);

        flag = (ImageView) findViewById(R.id.flag);

        list = new ArrayList<>();

        r = new Random();

        for (int i = 0; i < new Database().answers.length; i++) {
            list.add(new CountryItem(new Database().answers[i], new Database().flags[i]));
        }

        Collections.shuffle(list);

        newQuestion(turn);

        answer1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // ...
            }
        });

        answer2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // ...
            }
        });

        answer3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // ...
            }
        });

        answer4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // ...
            }
        });
    }

    private void newQuestion(int number) {
        // ...
    }
}

英文:

I have created a project in Android Studio with two activities. In the first activity I have a button that opens the second activity, but when I press it the second activity crashes. However, if I write the java of the second activity alone into a new project it runs perfectly with no errors. Could someone please tell me what the problem is?

Java for the first activity:

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button btn = (Button) findViewById(R.id.button_start_quiz);
    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            openQuizActivity();
        }
    });
}

public void openQuizActivity() {
    Intent intent = new Intent(MainActivity.this, QuizActivity.class);
    startActivity(intent);
}
}

Java for the second activity:

public class QuizActivity extends AppCompatActivity {

Button answer1, answer2, answer3, answer4;

ImageView flag;

List&lt;CountryItem&gt; list;

Random r;

int turn = 1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    answer1 = (Button) findViewById(R.id.answer_1);
    answer2 = (Button) findViewById(R.id.answer_2);
    answer3 = (Button) findViewById(R.id.answer_3);
    answer4 = (Button) findViewById(R.id.answer_4);

    flag = (ImageView) findViewById(R.id.flag);

    list = new ArrayList&lt;&gt;();

    r = new Random();

    for (int i = 0; i &lt; new Database().answers.length; i++) {
        list.add(new CountryItem(new Database().answers[i], new Database().flags[i]));
    }

    Collections.shuffle(list);

    newQuestion(turn);

    answer1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (answer1.getText().toString().equalsIgnoreCase(list.get(turn - 1).getName())) {
                Toast.makeText(QuizActivity.this, &quot;Corect !&quot;, Toast.LENGTH_SHORT).show();
                if(turn &lt; list.size()) {
                    turn++;
                    newQuestion(turn);
                } else {
                    Toast.makeText(QuizActivity.this, &quot;you finished the game&quot;, Toast.LENGTH_SHORT).show();
                }
            } else {
                Toast.makeText(QuizActivity.this, &quot;wrong !&quot;, Toast.LENGTH_SHORT).show();
                Toast.makeText(QuizActivity.this, &quot;you lost the game !&quot;, Toast.LENGTH_SHORT).show();
            }
        }
    });

    answer2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (answer2.getText().toString().equalsIgnoreCase(list.get(turn - 1).getName())) {
                Toast.makeText(QuizActivity.this, &quot;Corect !&quot;, Toast.LENGTH_SHORT).show();
                if(turn &lt; list.size()) {
                    turn++;
                    newQuestion(turn);
                } else {
                    Toast.makeText(QuizActivity.this, &quot;you finished the game&quot;, Toast.LENGTH_SHORT).show();
                }
            } else {
                Toast.makeText(QuizActivity.this, &quot;wrong !&quot;, Toast.LENGTH_SHORT).show();
                Toast.makeText(QuizActivity.this, &quot;you lost the game !&quot;, Toast.LENGTH_SHORT).show();
            }
        }
    });

    answer3.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (answer3.getText().toString().equalsIgnoreCase(list.get(turn - 1).getName())) {
                Toast.makeText(QuizActivity.this, &quot;Corect !&quot;, Toast.LENGTH_SHORT).show();
                if(turn &lt; list.size()) {
                    turn++;
                    newQuestion(turn);
                } else {
                    Toast.makeText(QuizActivity.this, &quot;you finished the game&quot;, Toast.LENGTH_SHORT).show();
                }
            } else {
                Toast.makeText(QuizActivity.this, &quot;wrong !&quot;, Toast.LENGTH_SHORT).show();
                Toast.makeText(QuizActivity.this, &quot;you lost the game !&quot;, Toast.LENGTH_SHORT).show();
            }
        }
    });

    answer4.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (answer4.getText().toString().equalsIgnoreCase(list.get(turn - 1).getName())) {
                Toast.makeText(QuizActivity.this, &quot;Corect !&quot;, Toast.LENGTH_SHORT).show();
                if(turn &lt; list.size()) {
                    turn++;
                    newQuestion(turn);
                } else {
                    Toast.makeText(QuizActivity.this, &quot;you finished the game&quot;, Toast.LENGTH_SHORT).show();
                }
            } else {
                Toast.makeText(QuizActivity.this, &quot;wrong !&quot;, Toast.LENGTH_SHORT).show();
                Toast.makeText(QuizActivity.this, &quot;you lost the game !&quot;, Toast.LENGTH_SHORT).show();
            }
        }
    });
}

private void newQuestion(int number) {
    flag.setImageResource(list.get(number - 1).getImage());

    int correct_answer = r.nextInt(4) + 1;

    int firstButton = number - 1;
    int secondButton;
    int thirdButton;
    int forthButton;

    switch (correct_answer) {
        case 1:
            answer1.setText(list.get(firstButton).getName());

            do {
                secondButton = r.nextInt(list.size());
            } while (secondButton == firstButton);
            do {
                thirdButton = r.nextInt(list.size());
            } while (thirdButton == firstButton || thirdButton == secondButton);
            do {
                forthButton = r.nextInt(list.size());
            } while (forthButton == firstButton || forthButton == secondButton || forthButton == thirdButton);

            answer2.setText(list.get(secondButton).getName());
            answer3.setText(list.get(thirdButton).getName());
            answer4.setText(list.get(forthButton).getName());

            break;
        case 2:
            answer2.setText(list.get(firstButton).getName());

            do {
                secondButton = r.nextInt(list.size());
            } while (secondButton == firstButton);
            do {
                thirdButton = r.nextInt(list.size());
            } while (thirdButton == firstButton || thirdButton == secondButton);
            do {
                forthButton = r.nextInt(list.size());
            } while (forthButton == firstButton || forthButton == secondButton || forthButton == thirdButton);

            answer1.setText(list.get(secondButton).getName());
            answer3.setText(list.get(thirdButton).getName());
            answer4.setText(list.get(forthButton).getName());

            break;
        case 3:
            answer3.setText(list.get(firstButton).getName());

            do {
                secondButton = r.nextInt(list.size());
            } while (secondButton == firstButton);
            do {
                thirdButton = r.nextInt(list.size());
            } while (thirdButton == firstButton || thirdButton == secondButton);
            do {
                forthButton = r.nextInt(list.size());
            } while (forthButton == firstButton || forthButton == secondButton || forthButton == thirdButton);

            answer2.setText(list.get(secondButton).getName());
            answer1.setText(list.get(thirdButton).getName());
            answer4.setText(list.get(forthButton).getName());

            break;
        case 4:
            answer4.setText(list.get(firstButton).getName());

            do {
                secondButton = r.nextInt(list.size());
            } while (secondButton == firstButton);
            do {
                thirdButton = r.nextInt(list.size());
            } while (thirdButton == firstButton || thirdButton == secondButton);
            do {
                forthButton = r.nextInt(list.size());
            } while (forthButton == firstButton || forthButton == secondButton || forthButton == thirdButton);

            answer2.setText(list.get(secondButton).getName());
            answer3.setText(list.get(thirdButton).getName());
            answer1.setText(list.get(forthButton).getName());

            break;
    }
}
}

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