Question

我正在尝试将变量mCurrentIndex保存到savedInstanceState捆绑包中，以便在旋转屏幕时不会重启我的应用程序。我应该如何将这个变量放入包中？每次尝试时，我都会不断得到一个空对象引用。这是我正在使用的当前代码：

基本上，我尝试使用onSaveInstanceState方法存储mCurrentIndex的值，然后在onCreate方法中检索该值。如果将saveInstanceState.putInt（）放在onCreate方法中的任何位置，则会得到空对象引用。

package com.example.geoquiz;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

public class QuizActivity extends AppCompatActivity {
    private Button mTrueButton;
    private Button mFalseButton;
    private Button mNextButton;
    private TextView mQuestionTextView;
    private Question[] mQuestionBank = new Question[] {
            new Question(R.string.question_australia, true),
            new Question(R.string.question_oceans, true),
            new Question(R.string.question_mideast, false),
            new Question(R.string.question_africa, false),
            new Question(R.string.question_americas, true),
            new Question(R.string.question_asia, true),
    };
    public int mCurrentIndex = 0;
// ...
@Override
protected void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_quiz);
    mQuestionTextView = (TextView)    findViewById(R.id.question_text_view);
    mTrueButton = (Button) findViewById(R.id.true_button);
    mTrueButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) { checkAnswer(true); }
    });
    mFalseButton = (Button) findViewById(R.id.false_button);
    mFalseButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) { checkAnswer(false); }
    });
    mNextButton = (Button) findViewById(R.id.next_button);

    //savedInstanceState.putInt("index", mCurrentIndex);

    mNextButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.length;
            //mCurrentIndex = currentIndex;
            updateQuestion();
        }
    });
    //savedInstanceState.putInt("index",mCurrentIndex);
    updateQuestion();


}
    @Override
    public void onSaveInstanceState(Bundle outState){
        super.onSaveInstanceState(outState);
        outState.putInt("index",mCurrentIndex);
    }
    private void updateQuestion() {

        //mCurrentIndex = savedInstanceState.getInt("index");
        int question = mQuestionBank[mCurrentIndex].getTextResId();
        mQuestionTextView.setText(question);
    }
    private void checkAnswer(boolean userPressedTrue) {
        boolean answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue();
        int messageResId = 0;
        if (userPressedTrue == answerIsTrue) {
            messageResId = R.string.correct_toast;
        } else {
            messageResId = R.string.incorrect_toast;
        }
        Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show();
    }

} // to close the entire class

Answer 1

为了做到这一点而又没有空指针异常，您可以这样做：

if (savedInstanceState != null){
        mCurrentIndex = savedInstanceState.getInt("index");
}

此外，onSavedInstanceState方法具有以下格式：


 @Override
    public void onSaveInstanceState(Bundle outState){
        outState.putInt("index",mCurrentIndex);
        super.onSaveInstanceState(outState);
}

如何在onCreate方法中使用捆绑包saveInstanceState？

1 个答案: