Java Android-无法启动activity ComponentInfo、Java.lang.NullPointerException

我完全是Android和Java的初学者。我创建了一个货币转换应用程序，当设备旋转时会崩溃

原代码：

package typeone.jiahuei.currencyconvert;

import android.content.res.Resources;
import android.os.Bundle;
import android.os.PersistableBundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

private Spinner unitSpinner;
private EditText amountEditText;
private String itemSelected;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    amountEditText = (EditText) findViewById(R.id.MainAct_InputEditText);
    unitSpinner = (Spinner) findViewById(R.id.MainAct_unit_spinner);
    addItemsToUnitSpinner();
    addListenerToUnitSpinner();
    addListenerToAmountEditText();
}

private void addListenerToAmountEditText() {

    amountEditText.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            addListenerToUnitSpinner();
            Quantity inputQuantity = new Quantity(getInputAmount(), Quantity.Unit.valueOf(itemSelected));
            updateTextViews(inputQuantity);
        }

        @Override
        public void afterTextChanged(Editable s) {
        }
    });
}


private void addItemsToUnitSpinner() {

    // Create an ArrayAdapter using the string array and a default spinner layout
    ArrayAdapter<CharSequence> unitSpinnerAdapter = ArrayAdapter.createFromResource(this,
            R.array.currency_type, android.R.layout.simple_spinner_item);

    // Specify the layout to use when the list of choices appears
    unitSpinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

    // Apply the adapter to the spinner
    unitSpinner.setAdapter(unitSpinnerAdapter);
}

private void addListenerToUnitSpinner() {

    unitSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

            itemSelected = parent.getItemAtPosition(position).toString();
            Quantity inputQuantity = new Quantity(getInputAmount(), Quantity.Unit.valueOf(itemSelected));
            updateTextViews(inputQuantity);

            Resources res = getResources();
            String[] currencyNameArray = res.getStringArray(R.array.currency_name);
            String toastString = "Converting " + currencyNameArray[position];
            Toast.makeText(MainActivity.this, toastString, Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onNothingSelected(AdapterView<?> parent) {
            // TODO lalala
        }
    });
}

private double getInputAmount() {
    String inputString = amountEditText.getText().toString();
    double inputAmount;

    if ( inputString.isEmpty() || inputString.equals(".") ) {
        inputAmount = 0;
    } else {
        inputAmount = Double.parseDouble(inputString);
    }
    return inputAmount;
}

private void updateTextViews(Quantity inputQuantity) {

    String currentViewName;
    for (Quantity.Unit unit : Quantity.Unit.values()) {
        currentViewName = unit.name() + "_TextView";
        int currentId = getResources().getIdentifier(currentViewName, "id",
                MainActivity.this.getPackageName());
        TextView currentTV = (TextView) findViewById(currentId);
        currentTV.setText(inputQuantity.toQuantity(unit).toString());
    }

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

在修改代码几个小时后，我发现通过更改下面的代码，错误得到了解决

public class MainActivity extends AppCompatActivity {

private Spinner unitSpinner;
private EditText amountEditText;
private static String itemSelected;


private void addListenerToAmountEditText() {

    amountEditText.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            Quantity inputQuantity = new Quantity(getInputAmount(), Quantity.Unit.valueOf(MainActivity.itemSelected));
            updateTextViews(inputQuantity);
        }

        @Override
        public void afterTextChanged(Editable s) {
        }
    });
}

我只在字符串变量的开头添加了“static”，并将itemSelected变量更改为MainActivity.itemSelected

我只是不明白为什么一开始就出现了错误，为什么做出这些更改可以解决这个问题？ 顺便说一下，该单位是Quantity类中的枚举

---

非常感谢您的回答，谢谢

itemSelected在调用onTextChanged时仍然为空

我知道这一点，但是为什么字符串在第一次启动时不为空？在调用onTextChanged之前，我尝试调用addListener方法，但它仍然返回null（当我旋转设备时）。只有在我将变量转换为静态变量后，错误才得以解决。谢谢，但您在onTextChangedBTW之前没有触发onItemSelected，只需调试它：添加日志或断点，然后查看它们的调用顺序以及在该点的不同值。我在开始时向字符串变量添加了“static”，并将onTextChanged中的itemSelected变量更改为MainActivity.itemSelected，错误消失了。你能告诉我它是如何解决这个问题的吗？我基本上是基于一种预感做出了这样的改变。我认为在旋转时会调用onCreate，但是出于我告诉过你的原因，它不会设置字符串。但是，在方向更改之前，您可能会执行一些其他操作。将其设置为“静态”时，可能会看到在方向更改之前设置的值。虽然它可能会在某些情况下修复崩溃，但也可能会破坏一些东西（您可能会看到错误的字符串），最重要的是，它可能无法修复问题。我知道当你第一次打开活动时，它仍然会发生。这就是为什么我建议您调试它

public class MainActivity extends AppCompatActivity {

private Spinner unitSpinner;
private EditText amountEditText;
private static String itemSelected;


private void addListenerToAmountEditText() {

    amountEditText.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            Quantity inputQuantity = new Quantity(getInputAmount(), Quantity.Unit.valueOf(MainActivity.itemSelected));
            updateTextViews(inputQuantity);
        }

        @Override
        public void afterTextChanged(Editable s) {
        }
    });
}