java - 致命异常 : main (School Project)

Question

这个问题在这里已经有了答案:





What is a NullPointerException, and how do I fix it?

(12 个回答)


6年前关闭。




我正在为我的类(class)做一个期末项目，一切进展顺利，然后我尝试运行该应用程序，在我的手机上崩溃了:

04-29 13:36:24.185 3507-3507/com.example.jessewmoore.database2 E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.jessewmoore.database2, PID: 3507 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.jessewmoore.database2/com.example.jessewmoore.database2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3184) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3294) at android.app.ActivityThread.access$1000(ActivityThread.java:210) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1704) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:145) at android.app.ActivityThread.main(ActivityThread.java:6938) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116) at org.json.JSONTokener.nextValue(JSONTokener.java:94) at org.json.JSONArray.(JSONArray.java:92) at org.json.JSONArray.(JSONArray.java:108) at com.example.jessewmoore.database2.MainActivity.onCreate(MainActivity.java:72) at android.app.Activity.performCreate(Activity.java:6575) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1134) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3137) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3294)  at android.app.ActivityThread.access$1000(ActivityThread.java:210)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1704)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:145)  at android.app.ActivityThread.main(ActivityThread.java:6938)  at java.lang.reflect.Method.invoke(Native Method)  at java.lang.reflect.Method.invoke(Method.java:372)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) 

每件事都应该是好的，没有错误。

Activity 主

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
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=".MainActivity">

<HorizontalScrollView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:focusableInTouchMode="true"
    android:focusable="true">

    <TableLayout android:id="@+id/table"
        android:layout_width="wrap_content"
        android:layout_height="fill_parent"
        android:focusableInTouchMode="true"
        android:focusable="true"></TableLayout>

 </HorizontalScrollView>
 </ScrollView>

AndroidManifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.jessewmoore.database2">
<uses-permission android:name="android.permission.INTERNET"/>
<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:exported="true">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

</manifest>

主要 Activity

package com.example.jessewmoore.database2;

 import java.io.BufferedReader;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import org.apache.http.HttpEntity;
 import org.apache.http.HttpResponse;
 import org.apache.http.client.HttpClient;
 import org.apache.http.client.methods.HttpPost;
 import org.apache.http.impl.client.DefaultHttpClient;
 import org.json.JSONArray;
 import org.json.JSONException;
 import org.json.JSONObject;
 import android.app.Activity;
 import android.graphics.Color;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
 import android.widget.Button;
 import android.widget.TableLayout;
 import android.widget.TableRow;
 import android.widget.TextView;
 import android.widget.Toast;
 import android.widget.TableRow.LayoutParams;

 public class MainActivity extends Activity {
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    String result = null;
    InputStream is = null;
    try {
        HttpClient httpclient = new DefaultHttpClient();
        HttpPost httppost = new HttpPost("http: busdata.pe.hu/myfile.php");
        HttpResponse response = httpclient.execute(httppost);
        HttpEntity entity = response.getEntity();
        is = entity.getContent();

        Log.e("log_tag", "connection_success");
        //   Toast.makeText(getApplicationContext(), “pass”,                           Toast.LENGTH_SHORT).show();
    } catch (Exception e) {
        Log.e("log_tag", "Error in http connection" + e.toString());
        Toast.makeText(getApplicationContext(), "Connection fail", Toast.LENGTH_SHORT).show();

    }
    //convert response to string
    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(is, "ISO-8859-1"), 8);
        StringBuilder sb = new StringBuilder();
        String line = null;
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
            //  Toast.makeText(getApplicationContext(), “Input Reading pass”, Toast.LENGTH_SHORT).show();
        }
        is.close();

        result = sb.toString();
    } catch (Exception e) {
        Log.e("log_tag", "Error converting results" + e.toString());
        Toast.makeText(getApplicationContext(), "Input reading fail", Toast.LENGTH_SHORT).show();

    }

    //parse json data
    try {
        JSONArray jArray = new JSONArray(result);
        TableLayout tv = (TableLayout) findViewById(R.id.table);
        tv.removeAllViewsInLayout();
        int flag = 1;
        for (int i = -1; i < jArray.length() - 1; i++) {
            TableRow tr = new TableRow(MainActivity.this);
            tr.setLayoutParams(new LayoutParams(
                    LayoutParams.FILL_PARENT,
                    LayoutParams.WRAP_CONTENT));
            if (flag == 1) {
                TextView b6 = new TextView(MainActivity.this);
                b6.setText("Id");
                b6.setTextColor(Color.BLUE);
                b6.setTextSize(15);
                tr.addView(b6);
                TextView b19 = new TextView(MainActivity.this);
                b19.setPadding(10, 0, 0, 0);
                b19.setTextSize(15);
                b19.setText("Name");
                b19.setTextColor(Color.BLUE);
                tr.addView(b19);
                TextView b29 = new TextView(MainActivity.this);
                b29.setPadding(10, 0, 0, 0);
                b29.setText("Status");
                b29.setTextColor(Color.BLUE);
                b29.setTextSize(15);
                tr.addView(b29);
                tv.addView(tr);
                final View vline = new View(MainActivity.this);
                vline.setLayoutParams(new TableRow.LayoutParams(TableRow.LayoutParams.FILL_PARENT, 2));
                vline.setBackgroundColor(Color.BLUE);
                tv.addView(vline);
                flag = 0;
            } else {
                JSONObject json_data = jArray.getJSONObject(i);
                Log.i("log_tag", "id: " + json_data.getInt("Id") + ", Username: " + json_data.getString("username") + ", No:" + json_data.getString("comment"));
                TextView b = new TextView(MainActivity.this);
                String stime = String.valueOf(json_data.getInt("Id"));
                b.setText(stime);
                b.setTextColor(Color.RED);
                b.setTextSize(15);
                tr.addView(b);
                TextView b1 = new TextView(MainActivity.this);
                b1.setPadding(10, 0, 0, 0);
                b1.setTextSize(15);
                String stime1 = json_data.getString("username");
                b1.setText(stime1);
                b1.setTextColor(Color.BLACK);
                tr.addView(b1);
                TextView b2 = new TextView(MainActivity.this);
                b2.setPadding(10, 0, 0, 0);
                String stime2 = json_data.getString("comment");
                b2.setText(stime2);
                b2.setTextColor(Color.BLACK);
                b2.setTextSize(15);
                tr.addView(b2);
                tv.addView(tr);
                final View vline1 = new View(MainActivity.this);
                vline1.setLayoutParams(new TableRow.LayoutParams(TableRow.LayoutParams.FILL_PARENT, 1));
                vline1.setBackgroundColor(Color.WHITE);
                tv.addView(vline1);
            }
        }
    } catch (JSONException e) {
        Log.e("log_tag", "Error parsing data" + e.toString());
        Toast.makeText(getApplicationContext(), "JsonArray_fail", Toast.LENGTH_SHORT).show();
    }

    }

   }

Answer 1

您的错误是 NullPointerException，由 MainActivity.java 中的第 72 行引起，即:

JSONArray jArray = new JSONArray(result);

为了进一步了解您的错误，JSONArray 似乎试图在结果上调用 length()，可能是在它试图从字符串构造适当的 JSON 数组时。能够理解这些错误对于解决您的问题很重要，并且可以帮助您提出更好的问题!

您最初将结果设置为 null - 在调用新 JSONArray(result) 之前似乎没有正确设置。一个好的开始是检查直接在它之前初始化结果的 try block 是否正确完成并且没有失败。在“将响应转换为字符串”try block 上的 catch 语句出现错误之前，您的日志中可能有一些内容。