android - Android如何通过意图将变量从选项卡传递给活动

Question

抱歉，我是 android 新手并为此自学.. 我有一个标签列表，用于循环输出.. 所有标签都链接到同一个活动，但只需要为数据传递一些不同的 url。 . 我尝试了很多方法

TabSpec page = tabHost.newTabSpec(title);
                Intent pageIntent = new Intent(this, mainpage.class);
                pageIntent.putExtra("url", url);
                page.setIndicator(title);
                page.setContent(pageIntent);

并通过

String text = getIntent().getStringExtra("url");

但它不能很好地工作，并且 logcat 显示了很多错误..

这是我的标签代码..

for(int i = 0; i < category.length(); i++){
                JSONObject c = category.getJSONObject(i);   

                String title = c.getString(TAG_TITLE);
                String url = "http://www.abcde.com.my/list3.php?json&"+c.getString(TAG_URL);

                TabSpec page = tabHost.newTabSpec(title);
                Intent pageIntent = new Intent(this, mainpage.class);
                pageIntent.putExtra("url", url);
                page.setIndicator(title);
                page.setContent(pageIntent);


                tabHost.addTab(page); 
            }

这里是活动的代码......

String text = getIntent().getStringExtra("url");
    private String url = text;

来自 logcat 的错误代码

10-22 02:14:24.313: E/AndroidRuntime(1660): FATAL EXCEPTION: main
10-22 02:14:24.313: E/AndroidRuntime(1660): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.androidhive.jsonparsing/com.androidhive.jsonparsing.kwongwahtab}: java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.androidhive.jsonparsing/com.androidhive.jsonparsing.mainpage}: java.lang.NullPointerException
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.os.Looper.loop(Looper.java:123)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.main(ActivityThread.java:3683)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at java.lang.reflect.Method.invokeNative(Native Method)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at java.lang.reflect.Method.invoke(Method.java:507)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at dalvik.system.NativeStart.main(Native Method)
10-22 02:14:24.313: E/AndroidRuntime(1660): Caused by: java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.androidhive.jsonparsing/com.androidhive.jsonparsing.mainpage}: java.lang.NullPointerException
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1569)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.startActivityNow(ActivityThread.java:1487)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:654)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.widget.TabHost.setCurrentTab(TabHost.java:326)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.widget.TabHost.addTab(TabHost.java:216)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at com.androidhive.jsonparsing.kwongwahtab.onCreate(kwongwahtab.java:55)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
10-22 02:14:24.313: E/AndroidRuntime(1660):     ... 11 more
10-22 02:14:24.313: E/AndroidRuntime(1660): Caused by: java.lang.NullPointerException
10-22 02:14:24.313: E/AndroidRuntime(1660):     at com.androidhive.jsonparsing.mainpage.<init>(mainpage.java:25)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at java.lang.Class.newInstanceImpl(Native Method)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at java.lang.Class.newInstance(Class.java:1409)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
10-22 02:14:24.313: E/AndroidRuntime(1660):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1561)
10-22 02:14:24.313: E/AndroidRuntime(1660):     ... 20 more

任何人都可以帮助我吗？这是我的标签活动

package com.androidhive.jsonparsing;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.TabActivity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TabHost;
import android.widget.TabHost.TabSpec;

@SuppressWarnings("deprecation")
public class kwongwahtab extends TabActivity {
    /** Called when the activity is first created. */
    private static String url = "http://www.kwongwah.com.my/channels2.plist";
    private static final String TAG_CATEGORY = "categorylist";
    private static final String TAG_TITLE = "title";
    private static final String TAG_URL = "url";
    public static String url2;
    JSONArray category = null;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        TabHost tabHost = getTabHost();

        // Creating JSON Parser instance
        JSONParser jParser = new JSONParser();

        // getting JSON string from URL
        JSONObject json = jParser.getJSONFromUrl(url);

        try {
            // Getting Array of Contacts
            category = json.getJSONArray(TAG_CATEGORY);

            // looping through All Contacts
            for(int i = 0; i < category.length(); i++){
                JSONObject c = category.getJSONObject(i);

                // Storing each json item in variable

                String title = c.getString(TAG_TITLE);
                String url = "http://www.kwongwah.com.my/plist3.php?json&"+c.getString(TAG_URL);

                TabSpec page = tabHost.newTabSpec(title);
                Intent pageIntent = new Intent(this, mainpage.class);
                pageIntent.putExtra("url", url);
                page.setIndicator(title);
                page.setContent(pageIntent);


                tabHost.addTab(page); 
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }

    }
}

mainpage.java 包 com.androidhive.jsonparsing；

import java.util.ArrayList;
import java.util.HashMap;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;

public class mainpage extends ListActivity {

    // url to make request
    String text = getIntent().getStringExtra("url");
    private String url = text;


    // JSON Node names
    private static final String TAG_CONTACTS = "new_list";

    private static final String TAG_SUBJECT = "subject";
    private static final String TAG_HTML = "html";
    private static final String TAG_ADDED = "added";

    // contacts JSONArray
    JSONArray contacts = null;

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

        // Hashmap for ListView
        ArrayList<HashMap<String, String>> contactList = new ArrayList<HashMap<String, String>>();

        // Creating JSON Parser instance
        JSONParser jParser = new JSONParser();

        // getting JSON string from URL
        JSONObject json = jParser.getJSONFromUrl(url);

        try {
            // Getting Array of Contacts
            contacts = json.getJSONArray(TAG_CONTACTS);

            // looping through All Contacts
            for(int i = 0; i < contacts.length(); i++){
                JSONObject c = contacts.getJSONObject(i);

                // Storing each json item in variable

                String subject = c.getString(TAG_SUBJECT);
                //String html = "<img src='http://www.kwongwah.com.my/news/2012/10/18/32_1.jpg'>";
                String html = c.getString(TAG_HTML);
                String added = c.getString(TAG_ADDED);

                // Phone number is agin JSON Object


                // creating new HashMap
                HashMap<String, String> map = new HashMap<String, String>();

                // adding each child node to HashMap key => value

                map.put(TAG_SUBJECT, subject);
                map.put(TAG_HTML, html);
                map.put(TAG_ADDED, added);

                // adding HashList to ArrayList
                contactList.add(map);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }


        /**
         * Updating parsed JSON data into ListView
         * */
        ListAdapter adapter = new SimpleAdapter(this, contactList,
                R.layout.list_item,
                new String[] { TAG_SUBJECT, TAG_HTML, TAG_ADDED }, new int[] {
                        R.id.subject, R.id.html, R.id.added});

        setListAdapter(adapter);

        // selecting single ListView item
        ListView lv = getListView();

        // Launching new screen on Selecting Single ListItem
        lv.setOnItemClickListener(new OnItemClickListener() {

            public void onItemClick(AdapterView<?> parent, View view,
                    int position, long id) {
                // getting values from selected ListItem
                String subject = ((TextView) view.findViewById(R.id.subject)).getText().toString();
                String html = ((TextView) view.findViewById(R.id.html)).getText().toString();
                String added = ((TextView) view.findViewById(R.id.added)).getText().toString();

                // Starting new intent
                Intent in = new Intent(getApplicationContext(), SingleMenuItemActivity.class);
                in.putExtra(TAG_SUBJECT, subject);
                in.putExtra(TAG_HTML, html);
                in.putExtra(TAG_ADDED, added);

                startActivity(in);

            }
        });



    }

}

Answer 1

这就是问题。您不能getIntent()在声明成员变量时引用该方法。它将在类的构造函数中被隐式调用。构造函数将在活动每次绑定到上下文或给定意图之前发生。

public class mainpage extends ListActivity {

    // url to make request ... -- This will only be valid from onCreate() to onDestroy()
    String text = getIntent().getStringExtra("url");

将这种类型的代码移动到生命周期方法中，例如onCreate()：

public class mainpage extends ListActivity {

    // url to make request
    String text;

    @Override
    public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      // At this point the Context is bound and the Intent is set on it.
      text = getIntent().getStringExtra("url");
      setContentView(R.layout.layout);

      // Hashmap for ListView
      ArrayList<HashMap<String, String>> contactList = new ArrayList<HashMap<String, String>>();

      // Creating JSON Parser instance
      JSONParser jParser = new JSONParser();