java - android bootcamp 医疗计算器无法打开

Question

每当我尝试在模拟器上运行以下计算器应用程序时，它都会立即崩溃。不懂Java，看logcat有点吃力。

该程序的目的是创建一个重量转换应用程序，该应用程序使用单选组并将千克转换为磅，反之亦然。

这是我的 list :

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="net.androidbootcamp.medicalcalculator"

 <application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:theme="@style/AppTheme.NoActionBar">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

这是我的日志:

>02-26 11:14:42.147 1535-1535/? E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{net.androidbootcamp.medicalcalculator/net.androidbootcamp.medicalcalculator.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
    at android.app.ActivityThread.access$600(ActivityThread.java:130)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4745)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at net.androidbootcamp.medicalcalculator.MainActivity.onCreate(MainActivity.java:24)
    at android.app.Activity.performCreate(Activity.java:5008)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
    at android.app.ActivityThread.access$600(ActivityThread.java:130) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
    at android.os.Handler.dispatchMessage(Handler.java:99) 
    at android.os.Looper.loop(Looper.java:137) 
    at android.app.ActivityThread.main(ActivityThread.java:4745) 
    at java.lang.reflect.Method.invokeNative(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:511) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
    at dalvik.system.NativeStart.main(Native Method)

这是我的主要 Activity :

package net.androidbootcamp.medicalcalculator;

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

import java.text.DecimalFormat;

public class MainActivity extends AppCompatActivity {
    double conversionRate = 2.2;
    double weightEntered;
    double convertedWeight;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        assert getSupportActionBar() != null;
        getSupportActionBar().setDisplayShowHomeEnabled(true);
        getSupportActionBar().setLogo(R.mipmap.ic_launcher);
        getSupportActionBar().setDisplayUseLogoEnabled(true);
        final EditText weight = (EditText)findViewById(R.id.txtWeight);
        final RadioButton lbToKilo = (RadioButton)findViewById(R.id.radLbToKilo);
        final RadioButton kiloToLb = (RadioButton)findViewById(R.id.radKiloToLb);
        final TextView result = (TextView)findViewById(R.id.txtResult);
        Button convert = (Button)findViewById(R.id.btnConvert);

        convert.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                weightEntered=Double.parseDouble(weight.getText().toString());
                DecimalFormat tenth = new DecimalFormat("#.#");
                if(lbToKilo.isChecked()){
                    if (weightEntered <= 500){
                        convertedWeight = weightEntered / conversionRate;
                        result.setText(tenth.format(convertedWeight) + "kilograms");
                    } else {
                        Toast.makeText(MainActivity.this,"Pounds must be less than 500",Toast.LENGTH_LONG).show();
                    }
                }
                if(kiloToLb.isChecked()){
                    if(weightEntered <= 225) {
                        convertedWeight = weightEntered * conversionRate;
                        result.setText(tenth.format(convertedWeight) + "pounds");
                    } else {
                        Toast.makeText(MainActivity.this,"Kilos must be less than 225",Toast.LENGTH_LONG).show();
                    }
                }
            }
        });
    }
}

Answer 1

在您的 list 中，您使用android:theme="@style/AppTheme.NoActionBar"，因此主 Activity 将不包含操作栏，您将收到 NullPointerException。

assert getSupportActionBar() != null 会抛出异常，因为在这种情况下它始终为 null。在这种情况下，断言的工作方式如下:

if(!getSupportActionBar() != null) {
    throw new Exception(); //NullPointerException in this case
}