参考：https://www.xuebuyuan.com/3244495.html

原来错误的程序，app会崩溃（请大神指点错误的原因是啥？_(:з)∠)_）

 1 btn_search.setOnClickListener(new View.OnClickListener() {
 2             @Override
 3             public void onClick(View v) {
 4                 String key = et_username.getText().toString();
 5                 Cursor cursor = dbOpenHelper.getReadableDatabase().query("tb_user",null,"username=?",new String[]{key}, null,null,null);
 6 
 7                 if (cursor.getCount() == 0) {
 8                     Toast.makeText(MainActivity.this,"很遗憾，没有相关记录",Toast.LENGTH_SHORT).show();
 9                 } else {
10 //                    while (cursor.moveToNext()) {
11 //                        Log.i("cccccccccc", cursor.getString(1)   " "   cursor.getString(2));
12 //                    }
13                     Log.i("dddddddd",cursor.getString(1));
14                 }
15             }
16         });

改正后的程序，用循环来输出

 1 btn_search.setOnClickListener(new View.OnClickListener() {
 2             @Override
 3             public void onClick(View v) {
 4                 String key = et_username.getText().toString();
 5                 Cursor cursor = dbOpenHelper.getReadableDatabase().query("tb_user",null,"username=?",new String[]{key}, null,null,null);
 6 
 7                 if (cursor.getCount() == 0) {
 8                     Toast.makeText(MainActivity.this,"很遗憾，没有相关记录",Toast.LENGTH_SHORT).show();
 9                 } else {
10                     while (cursor.moveToNext()) {
11                         Log.i("cccccccccc", cursor.getString(1)   " "   cursor.getString(2));
12                     }
13 //                    Log.i("dddddddd",cursor.getString(1));
14                 }
15             }
16         });