试图在当前位置上设置标记会给我空指针异常

Question

我在程序中做了以下事情:

1. 我在MenuItemsActivity类中以编程方式生成一些Buttons。我在MenuItemsActivity班的xml班有一个Listview。

2. 当我点击按钮时，适当的内容在Listview中加载。我只是刷新活动，即我使用相同的Listview来加载不同的内容基于被点击的按钮。

我想做以下事情:

1. 当按钮被点击时，我想将按钮的背景更改为'blue_tab '，并在相同的活动重新加载时保持相同的颜色。谁能指导我一步一步做什么，因为我是一个新手Android。

     i = getIntent();
   String Salad=i.getStringExtra("Salad");
   String cat_name_from_fragment=i.getStringExtra("category name");
   final ListView salad_list = (ListView) findViewById(R.id.salads);
   category = new ArrayList<HashMap<String, String>>();
   items = new ArrayList<HashMap<String, String>>();
   db = new DbHelper(MenuItemsActivity.this);
   category.clear();
   if (i.getStringExtra("category name") != null) {
       String getcategory = i.getStringExtra("category name").toString();
       items = db.retrieve_item_details(getcategory);
       Log.i("sub items", "" + items);
   }
   else if(cat_name_from_fragment!=null)
   {
       items = db.retrieve_item_details(cat_name_from_fragment);   
   }
   else
   {
       items = db.retrieve_item_details("Salads");
   }
   category = db.retrieve_category_name();
   
   count = category.size();
   Log.i("Sqlite database values", "" + count + " " + category);
   if (android.os.Build.VERSION.SDK_INT > 9) {
       StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder()
               .permitAll().build();
       StrictMode.setThreadPolicy(policy);
   }
   LinearLayout l1 = (LinearLayout) findViewById(R.id.tableRow1);
   int i = 0;
   for (HashMap<String, String> map : category)
       for (Entry<String, String> mapEntry : map.entrySet()) {
           String key = mapEntry.getKey();
           String value = mapEntry.getValue();
           TextView tv2 = new TextView(this);
           tv2.setLayoutParams(new LinearLayout.LayoutParams(40, 90));
           Log.i("map", "" + value);
           final Button tv1 = new Button(this);
           tv1.setId(i);
           tv1.setText(value);
           tv1.setTextSize(35);
           tv1.setTextColor(Color.parseColor("#1569C7"));
           tv1.setGravity(Gravity.CENTER);
           tv1.setBackgroundDrawable(getResources().getDrawable(R.drawable.popup));
           tv1.setLayoutParams(new LinearLayout.LayoutParams(300,90));
           tv1.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {
                   // TODO Auto-generated method stub
                   String text = tv1.getText().toString();
                   Log.e("text message", "" + text);
                   tv1.setBackgroundDrawable(getResources().getDrawable(R.drawable.blue_tab));
                   Toast.makeText(MenuItemsActivity.this, "clicked", 1000)
                           .show();
                   Intent i = new Intent(MenuItemsActivity.this,
                           MenuItemsActivity.class);
                   i.putExtra("category name", "" + text);
                   finish();
                   startActivity(i);
               }
           });
           /*TextView tv2 = new TextView(this);
           tv2.setText("      ");
           tv2.setTextSize(10);
           tv2.setGravity(Gravity.CENTER);*/
           l1.addView(tv1);
           l1.addView(tv2);
           i++;
           Log.e("i count ", "" + i);
       }
   final int imageArra[] = { R.drawable.leftbar_logo ,R.drawable.leftbar_logo};
   
   ListAdapter k = new SimpleAdapter(MenuItemsActivity.this, items,
           R.layout.menulist, new String[] { "Item_Name", "Desc",
                   "Currency", "Price","url","veggie","cat" }, new int[] { R.id.cat_name,
                   R.id.textView1, R.id.textView2, R.id.textView3,R.id.url,R.id.veggie,R.id.Category}) {
       @Override
       public View getView(int position, View convertView, ViewGroup parent) {
           // TODO Auto-generated method stub
           final View v = super.getView(position, convertView, parent);
           final ImageView im=(ImageView)v.findViewById(R.id.imageView1);
           TextView url=(TextView)v.findViewById(R.id.url);
           TextView veg=(TextView)v.findViewById(R.id.veggie);
           String vegg=veg.getText().toString();
           ImageView imagevegs=(ImageView)v.findViewById(R.id.veggies);
           Log.i("veggie",""+vegg);
           if(vegg.compareToIgnoreCase("Veg")==0)
           {
   
               imagevegs.setImageResource(R.drawable.veg);
               imagevegs.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
           }
           else
           {imagevegs.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
               imagevegs.setImageResource(R.drawable.non);
           }
           final String urls="http://166.62.17.208/"+url.getText().toString();
           Log.i("urls",""+urls);
           imageLoader.DisplayImage(urls,im);
           //return super.getView(position, convertView, parent);
           return v;
       }
   };
   salad_list.setAdapter(k);
   

Answer 1

您可以使用PreferenceManager保存数据，并在应用程序重新加载/重新启动时使用

示例:

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            String text = tv1.getText().toString();
            Log.e("text message", "" + text);
            if(PreferenceManager.getDefaultSharedPreferences(MenuItemsActivity.this).getString("button", "").length != 0)
                tv1.setBackgroundDrawable(getResources().getDrawable(R.drawable.blue_tab));
            else
            {
                Editor editor = PreferenceManager.getDefaultSharedPreferences(MenuItemsActivity.this).edit();
                editor.putString("button", "1");
                editor.commit(); 
                tv1.setBackgroundDrawable(getResources().getDrawable(R.drawable.blue_tab)); 
            }
            Toast.makeText(MenuItemsActivity.this, "clicked", 1000)
                    .show();
            Intent i = new Intent(MenuItemsActivity.this,
                    MenuItemsActivity.class);
            i.putExtra("category name", "" + text);
            finish();
            startActivity(i);
        }