Android和Java新手。以下是一个例子。使用以下日志时出错：
04-19 02:05:25.920: E/AndroidRuntime(791): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-19 02:05:25.920: E/AndroidRuntime(791): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-19 02:05:25.920: E/AndroidRuntime(791): at dalvik.system.NativeStart.main(Native Method)
04-19 02:05:25.920: E/AndroidRuntime(791): Caused by: java.lang.NullPointerException
04-19 02:05:25.920: E/AndroidRuntime(791): at com.example.crazytipcalc.CrazyTipCalc.onCreate(CrazyTipCalc.java:57)
04-19 02:05:25.920: E/AndroidRuntime(791): at android.app.Activity.performCreate(Activity.java:5231)
04-19 02:05:25.920: E/AndroidRuntime(791): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-19 02:05:25.920: E/AndroidRuntime(791): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-19 02:05:25.920: E/AndroidRuntime(791): ... 11 more
CrazyTipCalc.java
package com.example.crazytipcalc;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.text.Editable;
import android.text.TextWatcher;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.SeekBar;
import android.widget.SeekBar.OnSeekBarChangeListener;
import android.os.Build;
public class CrazyTipCalc extends ActionBarActivity {
private static final String TOTAL_BILL = "TOTAL_BILL";
private static final String CURRENT_TIP = "CURRENT_TIP";
private static final String BILL_WITHOUT_TIP = "BILL_WITHOUT_TIP";
private double billBeforeTip;
private double tipAmount;
private double finalBill;
EditText billBeforeTipET;
EditText tipAmountET;
EditText finalBillET;
SeekBar tipSeekBar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_crazy_tip_calc);
if (savedInstanceState == null) {
// getSupportFragmentManager().beginTransaction()
// .add(R.id.container, new PlaceholderFragment()).commit();
billBeforeTip = 0.00;
tipAmount = 0.00;
finalBill = 0.00;
} else {
billBeforeTip = savedInstanceState.getDouble(BILL_WITHOUT_TIP);
tipAmount = savedInstanceState.getDouble(CURRENT_TIP);
finalBill = savedInstanceState.getDouble(TOTAL_BILL);
}
billBeforeTipET = (EditText) findViewById(R.id.billEditText);
tipAmountET = (EditText) findViewById(R.id.tipEditText);
finalBillET = (EditText) findViewById(R.id.finalBillEditText);
tipSeekBar = (SeekBar) findViewById(R.id.changeTipSeekBar);
tipSeekBar.setOnSeekBarChangeListener(tipSeekBarListener);
billBeforeTipET.addTextChangedListener(billBeforeTipListner);
}
private TextWatcher billBeforeTipListner = new TextWatcher() {
@Override
public void afterTextChanged(Editable arg0) {
// TODO Auto-generated method stub
}
@Override
public void beforeTextChanged(CharSequence arg0, int arg1, int arg2,
int arg3) {
// TODO Auto-generated method stub
}
@Override
public void onTextChanged(CharSequence arg0, int arg1, int arg2,
int arg3) {
try {
billBeforeTip = Double.parseDouble(arg0.toString());
} catch (NumberFormatException e) {
billBeforeTip = 0.00;
}
updateTipAndFinalBill();
}
};
private void updateTipAndFinalBill(){
double tipAmount = Double.parseDouble(tipAmountET.getText().toString());
double finalBill = billBeforeTip + (billBeforeTip * tipAmount);
finalBillET.setText(String.format("%.02f", finalBill));
}
protected void onSavedInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putDouble(TOTAL_BILL, finalBill);
outState.putDouble(CURRENT_TIP, tipAmount);
outState.putDouble(BILL_WITHOUT_TIP, billBeforeTip);
}
private OnSeekBarChangeListener tipSeekBarListener = new OnSeekBarChangeListener(){
@Override
public void onProgressChanged(SeekBar arg0, int arg1, boolean arg2) {
// TODO Auto-generated method stub
tipAmount = (tipSeekBar.getProgress()) * .01;
tipAmountET.setText(String.format("%.02f", tipAmount));
updateTipAndFinalBill();
}
@Override
public void onStartTrackingTouch(SeekBar arg0) {
// TODO Auto-generated method stub
}
@Override
public void onStopTrackingTouch(SeekBar arg0) {
// TODO Auto-generated method stub
}
};
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.crazy_tip_calc, 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();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_crazy_tip_calc,
container, false);
return rootView;
}
}
}
fragment_crazy_tip_calc.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.crazytipcalc.CrazyTipCalc$PlaceholderFragment" >
<TextView
android:id="@+id/billTextView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="18dp"
android:layout_marginTop="23dp"
android:text="@string/bill_text_view" />
<EditText
android:id="@+id/billEditText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/billTextView"
android:layout_alignBottom="@+id/billTextView"
android:layout_marginLeft="23dp"
android:layout_toRightOf="@+id/billTextView"
android:ems="5"
android:inputType="numberDecimal"
android:text="@string/bill_edit_text" />
<TextView
android:id="@+id/tipTextView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/billEditText"
android:layout_alignBottom="@+id/billEditText"
android:layout_marginLeft="15dp"
android:layout_toRightOf="@+id/billEditText"
android:text="@string/tip_text_view" />
<EditText
android:id="@+id/tipEditText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/tipTextView"
android:layout_alignBottom="@+id/tipTextView"
android:layout_toRightOf="@+id/tipTextView"
android:ems="4"
android:inputType="numberDecimal" >
<requestFocus />
</EditText>
<TextView
android:id="@+id/finalBillTextView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/tipTextView"
android:layout_below="@+id/tipEditText"
android:layout_marginTop="26dp"
android:text="@string/final_text_view" />
<EditText
android:id="@+id/finalBillEditText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/finalBillTextView"
android:layout_below="@+id/finalBillTextView"
android:layout_marginTop="16dp"
android:ems="5"
android:inputType="numberDecimal" />
<TextView
android:id="@+id/changeTipTextView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/finalBillEditText"
android:layout_alignLeft="@+id/billTextView"
android:text="@string/change_tip_text_view" />
<SeekBar
android:id="@+id/changeTipSeekBar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/finalBillEditText"
android:layout_alignParentLeft="true"
android:layout_alignRight="@+id/billEditText"
android:progress="15" />
</RelativeLayout>
的AndroidManifest.xml
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="19" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.crazytipcalc.CrazyTipCalc"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
答案 0 :(得分：0)
你有
setContentView(R.layout.activity_crazy_tip_calc);
但您的观点位于fragment_crazy_tip_calc.xml。因此初始化失败导致NullPointerException。
您需要更改为
setContentView(R.layout.fragment_crazy_tip_calc)
// you also have the fragment code. so instead of this i recommend the below
或
将视图初始化移动到Fragment的onCreateView并移动相关代码
EditText billBeforeTipET;
EditText tipAmountET;
EditText finalBillET;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_crazy_tip_calc,
container, false);
billBeforeTipET = (EditText) rootView.findViewById(R.id.billEditText);
tipAmountET = (EditText) rootView.findViewById(R.id.tipEditText);
finalBillET = (EditText) rootView.findViewById(R.id.finalBillEditText);
tipSeekBar = (SeekBar) rootView.findViewById(R.id.changeTipSeekBar);
return rootView;
}
您还需要取消注释以下内容
// getSupportFragmentManager().beginTransaction()
// .add(R.id.container, new PlaceholderFragment()).commit();
findViewById查找当前视图层次结构中提到的视图。视图属于片段布局而不属于活动。因此，您需要使用infalted视图对象来初始化视图。
答案 1 :(得分：0)
你正在初始化错误的xml文件：改变它如下：
setContentView(R.layout.fragment_crazy_tip_calc);