Question

Log Cat：

02-23 18:47:36.399: E/AndroidRuntime(25571): FATAL EXCEPTION: main
02-23 18:47:36.399: E/AndroidRuntime(25571): Process: com.example.calculator, PID: 25571
02-23 18:47:36.399: E/AndroidRuntime(25571): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.calculator/com.example.calculator.MainActivity}:    java.lang.NullPointerException
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.os.Handler.dispatchMessage(Handler.java:102)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.os.Looper.loop(Looper.java:136)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.ActivityThread.main(ActivityThread.java:5017)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at java.lang.reflect.Method.invokeNative(Native Method)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at java.lang.reflect.Method.invoke(Method.java:515)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at dalvik.system.NativeStart.main(Native Method)
02-23 18:47:36.399: E/AndroidRuntime(25571): Caused by: java.lang.NullPointerException
02-23 18:47:36.399: E/AndroidRuntime(25571):    at com.example.calculator.MainActivity.onCreate(MainActivity.java:39)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.Activity.performCreate(Activity.java:5231)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
02-23 18:47:36.399: E/AndroidRuntime(25571):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
02-23 18:47:36.399: E/AndroidRuntime(25571):    ... 11 more
02-23 18:47:48.010: D/AndroidRuntime(25647): Shutting down VM

activity_main.xml：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">


<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/screen"
    android:textSize="60sp"
    android:id="@+id/screen" />

<Button
    android:id="@+id/num0"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:text="@string/num0" />

<Button
    android:id="@+id/num1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/screen"
    android:layout_marginTop="16dp"
    android:text="@string/num1" />

<Button
    android:id="@+id/num2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num1"
    android:layout_alignBottom="@+id/num1"
    android:layout_marginLeft="16dp"
    android:layout_toRightOf="@+id/num1"
    android:text="@string/num2" />

<Button
    android:id="@+id/num3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num2"
    android:layout_alignBottom="@+id/num2"
    android:layout_marginLeft="20dp"
    android:layout_toRightOf="@+id/num2"
    android:text="@string/num3" />

<Button
    android:id="@+id/num4"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerVertical="true"
    android:layout_toLeftOf="@+id/num2"
    android:text="@string/num4" />

<Button
    android:id="@+id/num5"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num4"
    android:layout_alignBottom="@+id/num4"
    android:layout_alignLeft="@+id/num2"
    android:text="@string/num5" />

<Button
    android:id="@+id/num6"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num5"
    android:layout_alignBottom="@+id/num5"
    android:layout_alignLeft="@+id/num3"
    android:text="@string/num6" />

<Button
    android:id="@+id/num7"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/num4"
    android:layout_marginTop="65dp"
    android:layout_toLeftOf="@+id/num2"
    android:text="@string/num7" />

<Button
    android:id="@+id/num8"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num7"
    android:layout_alignBottom="@+id/num7"
    android:layout_alignLeft="@+id/num5"
    android:text="@string/num8" />

<Button
    android:id="@+id/num9"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num8"
    android:layout_alignBottom="@+id/num8"
    android:layout_alignLeft="@+id/num6"
    android:text="@string/num9" />

<Button
    android:id="@+id/add"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/num3"
    android:layout_alignBottom="@+id/num3"
    android:layout_alignParentRight="true"
    android:text="@string/add" />

<Button
    android:id="@+id/subtract"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentRight="true"
    android:layout_alignTop="@+id/num9"
    android:text="@string/subtract"
    android:textSize="30sp" />

<Button
    android:id="@+id/multiply"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/num7"
    android:layout_alignParentRight="true"
    android:text="@string/multiply" />

<Button
    android:id="@+id/divide"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:text="@string/divide" />

<Button
    android:id="@+id/clear"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_toLeftOf="@+id/num3"
    android:text="@string/clear" />

<Button
    android:id="@+id/equal"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/num9"
    android:layout_alignParentBottom="true"
    android:text="@string/equal" />

</RelativeLayout>

MainActivity.java：

package com.example.calculator;

import android.os.Bundle;
import android.app.Activity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

Button zero, one, two, three, four, five, six, seven, eight, nine, clear, add, subtract, multiply, divide, equal;
double num1, num2, answer;
String operation;
int twoDigit;
TextView screen;
@Override
protected void onCreate(Bundle savedInstanceState) {
    zero = (Button) findViewById(R.id.num0);
    one = (Button) findViewById(R.id.num1);
    two = (Button) findViewById(R.id.num2);
    three = (Button) findViewById(R.id.num3);
    four = (Button) findViewById(R.id.num4);
    five = (Button) findViewById(R.id.num5);
    six = (Button) findViewById(R.id.num6);
    seven = (Button) findViewById(R.id.num7);
    eight = (Button) findViewById(R.id.num8);
    nine = (Button) findViewById(R.id.num9);
    clear = (Button) findViewById(R.id.clear);
    add = (Button) findViewById(R.id.add);
    subtract = (Button) findViewById(R.id.subtract);
    multiply = (Button) findViewById(R.id.multiply);
    divide = (Button) findViewById(R.id.divide);
    equal = (Button) findViewById(R.id.equal);

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    screen.setText("0");
    num1 = 0; num2 = 0;
    firstNumberEntry();
}

public void firstNumberEntry() {
    one.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            num1 = 1;
            screen.setText("" + num1);
            if(add.isPressed()){
                operation = "add";
                operations(num1, operation);
            } else if(subtract.isPressed()){
                operation = "subtract";
                operations(num1, operation);
            }else if(multiply.isPressed()){
                operation = "multiply";
                operations(num1, operation);
            }else if(divide.isPressed()){
                operation = "divide";
                operations(num1, operation);
            }
        }


    });


}



public void operations(double num1, String operation) {


}
}

清单：

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

<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=".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>

我是android开发的初学者，我正在尝试制作一个两位数的计算器。我还没有完成，但我想测试我已编码的内容。每当我在设备上打开应用程序时，我都会收到消息“不幸的是，计算器已停止”。我附上了我的所有代码，包括LogCat。既然我是这个东西的新手，我真的不明白什么是错的。有帮助吗？

Answer 1

您没有将screen变量设置为任何内容，因此它会初始化为null，当您尝试调用null的方法时会出现异常。

你应该添加

screen = (TextView) findViewById(R.id.textView);

在此声明之前。

我一直得到致命的异常：主要

1 个答案: