Question

我是Android的新手，今天正在创建我的第一个Android应用程序，但它无法启动。当我启动应用程序时它来了upp，但是一个弹出窗口说“不幸的是[appname]已经停止了”，它就会关闭。

我在youtube上关注了Derek Banas的教程，作为我的第一个应用程序，我创建了一个小费计算器。

当搞砸问题时，问题似乎每次都取决于不同的事情。

我不知道为您提供什么类型的代码，但我会发布我认为最重要的代码。

我有点困惑的一点是，我通过fragment_main创建了我的所有GUI，而不是我在每个指南中看到的activity_main！另外，我正在使用android studio！

清单：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.emilime93.tipcalc"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="7"
        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.emilime93.tipcalc.MainActivity"
            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>

</manifest>

这是代码：

package com.emilime93.tipcalc;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.widget.CheckBox;
import android.widget.CompoundButton;
import android.widget.EditText;

public class MainActivity extends ActionBarActivity {

    private static final String FINAL_BILL = "FINAL_BILL";
    private static final String CURRENT_TIP = "CURRENT_TIP";
    private static final String PRE_TIP_BILL = "PRE_TIP_BILL";
    private static final String NICE_WAITRESS = "NICE_WAITRESS";

    private double finalBill;
    private double preTipBill;
    private double currentTip;
    private Boolean niceWaitress;

    private EditText finalBillET;
    private EditText preTipBillET;
    private CheckBox niceWaitressCB;

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

        if (savedInstanceState == null) {
            preTipBill = 0.0;
            currentTip = 0.1;
            finalBill = 0.0;
            niceWaitress = false;
        } else {
            preTipBill = savedInstanceState.getDouble(PRE_TIP_BILL);
            currentTip = savedInstanceState.getDouble(CURRENT_TIP);
            finalBill = savedInstanceState.getDouble(FINAL_BILL);
            niceWaitress = savedInstanceState.getBoolean(NICE_WAITRESS);
        }

        //adding actionlistener for the checkbutton
        preTipBillET = (EditText) findViewById(R.id.billTextEdit);
        finalBillET = (EditText) findViewById(R.id.finalBillTextEdit);
        niceWaitressCB = (CheckBox) findViewById(R.id.niceWatiressCheckBox);

        niceWaitressCB.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton compoundButton, boolean b) {
                if(niceWaitress) {
                    currentTip -= 0.05;
                } else {
                    currentTip += 0.05;
                }
                niceWaitress = b;
                updateBillAndTip();
            }
        });

        //adding actionlistener to the input filed for the bill
        preTipBillET.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int i, int i2, int i3) {

            }

            @Override
            public void onTextChanged(CharSequence charSequence, int i, int i2, int i3) {
                try {
                    preTipBill = Double.parseDouble(charSequence.toString());
                    updateBillAndTip();
                } catch(NumberFormatException e){

                }
            }

            @Override
            public void afterTextChanged(Editable editable) {

            }
        });

    }

    /*private TextWatcher billPreTipListener = new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence charSequence, int i, int i2, int i3) {

        }

        @Override
        public void onTextChanged(CharSequence charSequence, int i, int i2, int i3) {
            try {
                preTipBill = Double.parseDouble(s.toString());
            } catch(NumberFormatException e){

            }
        }

        @Override
        public void afterTextChanged(Editable editable) {

        }
    };*/

    private void updateBillAndTip() {
        this.finalBill = (currentTip+1)*preTipBill;
        finalBillET.setText(String.format("%.02f", finalBill));
    }

    public void increaseTip(double precentageAmmount) {
        this.currentTip += precentageAmmount;
    }

    protected void onSavedInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        outState.putDouble(FINAL_BILL, finalBill);
        outState.putDouble(CURRENT_TIP, currentTip);
        outState.putDouble(PRE_TIP_BILL, preTipBill);
        outState.putBoolean(NICE_WAITRESS, niceWaitress);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.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();
        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_main, container, false);
            return rootView;
        }
    }

}

启动应用时出现错误消息：

12-05 14:36:51.582    1092-1092/com.emilime93.tipcalc E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.emilime93.tipcalc, PID: 1092
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.emilime93.tipcalc/com.emilime93.tipcalc.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
            at android.app.ActivityThread.access$700(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4998)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.emilime93.tipcalc.MainActivity.onCreate(MainActivity.java:57)
            at android.app.Activity.performCreate(Activity.java:5243)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
            at android.app.ActivityThread.access$700(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4998)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

布局activity_main没有ID为niceWatiressCheckBox的视图，findViewById()返回null。尝试在空引用上调用setOnCheckedChangeListener()会导致NPE。

Answer 2

好吧，问题是当我的布局位于“fragment_main”时我使用了setContentView(R.layout.activity_main);。所以我将其更改为setContentView(R.layout.fragment_main);。

我不知道所有这些事情做了什么，这就是为什么我无法追踪它。我希望至少能帮助别人解决这个问题！

Stil不知道片段和活动_main之间的区别是什么！

不幸的是[app]已经停止了

2 个答案: