程序一直运行，直到它尝试访问updateUserSettings（）方法中的SharedPreferences。我花了最近2个小时搜索互联网，没有任何解释。请帮忙！
我有一条混乱的线路在我的onCreate（）的底部评论;
这是 FuelEconomyCalculatorActivity.java
package com.example.fuelcalculator;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.support.v7.app.ActionBarActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.TextView;
import android.widget.Toast;
public class FuelEconomyCalculatorActivity extends ActionBarActivity implements
OnClickListener {
private EditText fuelEditText;
private EditText distanceEditText;
private TextView mpgTextView;
private TextView litresTextView;
private Button calculateButton;
private Button clearButton;
private RadioButton gallonsRadio;
private RadioButton litresRadio;
private RadioButton milesRadio;
private RadioButton kmRadio;
private String mpg = " ";
private String kp100 = " ";
private boolean metricChecked;
private boolean imperialChecked;
private boolean usGallonChecked;
private boolean ukGallonChecked;
private static final int RESULT_SETTINGS = 1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_fuel_economy_calculator);
fuelEditText = (EditText) findViewById(R.id.fuelEditText);
distanceEditText = (EditText) findViewById(R.id.distanceEditText);
mpgTextView = (TextView) findViewById(R.id.mpgTextView);
litresTextView = (TextView) findViewById(R.id.litresTextView);
calculateButton = (Button) findViewById(R.id.calculateButton);
clearButton = (Button) findViewById(R.id.clearButton);
gallonsRadio = (RadioButton) findViewById(R.id.gallonsRadio);
litresRadio = (RadioButton) findViewById(R.id.litresRadio);
milesRadio = (RadioButton) findViewById(R.id.milesRadio);
kmRadio = (RadioButton) findViewById(R.id.kmRadio);
calculateButton.setOnClickListener(this);
clearButton.setOnClickListener(this);
if (savedInstanceState != null) {
mpg = savedInstanceState.getString("mpg");
kp100 = savedInstanceState.getString("kp100");
initializeViews();
}
// updateUserSettings();
}
protected void onSaveInstanceState(Bundle savedInstanceState) {
super.onSaveInstanceState(savedInstanceState);
savedInstanceState.putString("mpg", (String) mpgTextView.getText());
savedInstanceState.putString("kp100", (String) litresTextView.getText());
litresRadio.setChecked(true);
kmRadio.setChecked(true);
}
private void initializeViews() {
mpgTextView.setText(mpg);
litresTextView.setText(kp100);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.fuel_economy_calculator, 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.
switch (item.getItemId()) {
case R.id.action_settings: {
Intent intent = new Intent(FuelEconomyCalculatorActivity.this,
FuelEconomySettingsActivity.class);
startActivityForResult(intent, RESULT_SETTINGS);
return true;
}
case R.id.action_about: {
Intent intent = new Intent(getApplicationContext(), FuelEconomyAboutPageActivity.class);
startActivity(intent);
return true;
}
default:
return super.onOptionsItemSelected(item);
}
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
switch (requestCode) {
case RESULT_SETTINGS:
updateUserSettings();
break;
}
}
private void updateUserSettings() {
SharedPreferences sharedPrefs = PreferenceManager
.getDefaultSharedPreferences(this);
if(sharedPrefs.getString("measuringUnits", null).equalsIgnoreCase("metric"))
{
metricChecked = true;
imperialChecked = false;
}
else if(sharedPrefs.getString("measuringUnits", null).equalsIgnoreCase("imperial"))
{
imperialChecked = true;
metricChecked = false;
}
if(sharedPrefs.getString("gallons", null).equalsIgnoreCase("uk"))
{
ukGallonChecked = true;
usGallonChecked = false;
}
else if(sharedPrefs.getString("gallons", null).equalsIgnoreCase("us"))
{
usGallonChecked = true;
ukGallonChecked = false;
}
if(metricChecked)
{
litresRadio.setChecked(true);
kmRadio.setChecked(true);
}
else if(imperialChecked)
{
milesRadio.setChecked(true);
gallonsRadio.setChecked(true);
}
}
@Override
public void onClick(View v) {
if (v.getId() == R.id.calculateButton) {
float mpg = 0;
float kmLitres = 0;
// it doesn't matter which you use here, as long
// as you use a fuel and a distance method
if (ukGallonChecked && !usGallonChecked) {
if (getKM() > 0 && getLitres() > 0) {
mpg = getMiles() / getGallons();
kmLitres = getLitres() / (getKM() / 100);
mpgTextView.setText(String.format("%.2f", mpg));
litresTextView.setText(String.format("%.2f", kmLitres));
}
} else if (usGallonChecked && !ukGallonChecked) {
if (getKM() > 0 && getLitres() > 0) {
mpg = getMiles() / getGallonsUS();
kmLitres = getLitresUS() / (getKM() / 100);
mpgTextView.setText(String.format("%.2f", mpg));
litresTextView.setText(String.format("%.2f", kmLitres));
}
}
} else if (v.getId() == R.id.clearButton) {
resetValues();
}
}
public float CheckValues(EditText input) {
float value = 0;
try {
value = Float.parseFloat(input.getText().toString());
if (value < 1) {
Toast toast = Toast.makeText(this,
"Please enter a number that is larger than 0",
Toast.LENGTH_SHORT);
toast.show();
}
} catch (Exception ex) {
Toast toast = Toast.makeText(this, "Please enter a number",
Toast.LENGTH_SHORT);
toast.show();
}
return value;
}
public void resetValues() {
mpg = " ";
kp100 = " ";
fuelEditText.setText("");
distanceEditText.setText("");
mpgTextView.setText("");
litresTextView.setText("");
}
public float getKM() {
float distance = CheckValues(distanceEditText);
if (milesRadio.isChecked()) {
distance = (float) (distance * 1.60934);
}
return distance;
}
public float getMiles() {
float distance = CheckValues(distanceEditText);
if (kmRadio.isChecked()) {
distance = (float) (distance * 0.62137);
}
return distance;
}
public float getLitres() {
float fuel = CheckValues(fuelEditText);
if (gallonsRadio.isChecked()) {
fuel = (float) (fuel * 4.54609);
}
return fuel;
}
public float getLitresUS() {
float fuel = CheckValues(fuelEditText);
if (gallonsRadio.isChecked()) {
fuel = (float) (fuel * 3.785411784);
}
return fuel;
}
public float getGallons() {
float fuel = CheckValues(fuelEditText);
if (litresRadio.isChecked()) {
fuel = (float) (fuel * 0.219969);
}
return fuel;
}
public float getGallonsUS() {
float fuel = CheckValues(fuelEditText);
if (litresRadio.isChecked()) {
fuel = (float) (fuel * 0.264172);
}
return fuel;
}
}
这是我的 FuelEconomySettingsActivity.java
package com.example.fuelcalculator;
import android.app.Activity;
import android.os.Bundle;
public class FuelEconomySettingsActivity extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getFragmentManager().beginTransaction().replace(android.R.id.content,
new FuelEconomySettingsFragment()).commit();
}
}
这是我的 LogCat
07-18 14:35:37.014: E/AndroidRuntime(3084): FATAL EXCEPTION: main
07-18 14:35:37.014: E/AndroidRuntime(3084): Process: com.example.fuelcalculator, PID: 3084
07-18 14:35:37.014: E/AndroidRuntime(3084): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fuelcalculator/com.example.fuelcalculator.FuelEconomyCalculatorActivity}: java.lang.NullPointerException
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.ActivityThread.access$900(ActivityThread.java:161)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.os.Handler.dispatchMessage(Handler.java:102)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.os.Looper.loop(Looper.java:157)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.ActivityThread.main(ActivityThread.java:5356)
07-18 14:35:37.014: E/AndroidRuntime(3084): at java.lang.reflect.Method.invokeNative(Native Method)
07-18 14:35:37.014: E/AndroidRuntime(3084): at java.lang.reflect.Method.invoke(Method.java:515)
07-18 14:35:37.014: E/AndroidRuntime(3084): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
07-18 14:35:37.014: E/AndroidRuntime(3084): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
07-18 14:35:37.014: E/AndroidRuntime(3084): at dalvik.system.NativeStart.main(Native Method)
07-18 14:35:37.014: E/AndroidRuntime(3084): Caused by: java.lang.NullPointerException
07-18 14:35:37.014: E/AndroidRuntime(3084): at com.example.fuelcalculator.FuelEconomyCalculatorActivity.updateUserSettings(FuelEconomyCalculatorActivity.java:128)
07-18 14:35:37.014: E/AndroidRuntime(3084): at com.example.fuelcalculator.FuelEconomyCalculatorActivity.onCreate(FuelEconomyCalculatorActivity.java:63)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.Activity.performCreate(Activity.java:5426)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
07-18 14:35:37.014: E/AndroidRuntime(3084): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
07-18 14:35:37.014: E/AndroidRuntime(3084): ... 11 more
我是Android的新手，所以我不擅长调试，所以有人可以帮助我解决困境吗？
活动已包含在我的清单中，所以我知道这不是问题。
任何帮助都很有帮助。
答案 0 :(得分：0)
您收到NullPointerException，因为null
SharedPreference.getString()
因为getString()获取一个空字符串作为默认值...
所以试试这个......
if(sharedPrefs.getString("measuringUnits", "").equalsIgnoreCase("metric"))
而不是
if(sharedPrefs.getString("measuringUnits", null).equalsIgnoreCase("metric"))