关于android的坑

　　

坑1：

使用SQLiteOpenHelper的时候如果建立的表中存在不为空的字段，但是用ContentValues()的方式来插入数据的话恰好没有往这个字段里插入数据，那么执行后市没法往数据库里插入数据的。单是写在这里可能觉得这个问题很平常，下面以代码的形式展示出来。

MySqliteDatabase.java

 package com.jikexueyuan.hellonotes;
 
 import android.content.Context;
 import android.database.sqlite.SQLiteDatabase;
 import android.database.sqlite.SQLiteDatabase.CursorFactory;
 import android.database.sqlite.SQLiteOpenHelper;
 
 public class MySqliteDatabase extends SQLiteOpenHelper {
     public static final String _ID = "_id";
     public static final String CONTENT = "content";
     public static final String TIME = "time";
     public static final String TABLE = "mydiary";
     public static final String TABLE_DRAFT = "draft";
     public static final String DATABASENAME = "wyldb.db";
 
     public MySqliteDatabase(Context context, String name,
             CursorFactory factory, int version) {
         super(context, name, factory, version);
         // TODO Auto-generated constructor stub
 
     }
 
     /**
      * 三个参数的构造器
      *
      * @param context
      * @param name
      * @param factory
      */
     public MySqliteDatabase(Context context) {
         this(context, DATABASENAME, null, 1);
         System.out.println("MySqliteDatabase.MySqliteDatabase()构造器，新建表："
                 + DATABASENAME + "...");
     }
 
     @Override
     public void onCreate(SQLiteDatabase db) {
         // TODO Auto-generated method stub
         System.out.println("MySqliteDatabase.onCreate(),创建表...");
         db.execSQL("create table mydiary(_id integer primary key autoincrement,content text not null,time text not null)");
         db.execSQL("create table draft(_id integer primary key autoincrement,content text not null,time text not null)");
     }
 
     @Override
     public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
         // TODO Auto-generated method stub
 
     }
 
 }

DetailActivity.java

 package com.jikexueyuan.hellonotes;
 
 import java.text.SimpleDateFormat;
 import java.util.Date;
 
 import android.app.Activity;
 import android.content.ContentValues;
 import android.content.Intent;
 import android.database.sqlite.SQLiteDatabase;
 import android.os.Bundle;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.widget.Button;
 import android.widget.EditText;
 import android.widget.Toast;
 
 public class DetailActivity extends Activity implements OnClickListener {
     EditText et_detail;
     Button btn_edit, btn_delete, btn_detail_save;
     String val;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         // TODO Auto-generated method stub
         super.onCreate(savedInstanceState);
         setContentView(R.layout.detail);
         et_detail = (EditText) findViewById(R.id.et_detail);
         btn_edit = (Button) findViewById(R.id.btn_edit);
         btn_delete = (Button) findViewById(R.id.btn_delete);
         btn_detail_save = (Button) findViewById(R.id.btn_detail_save);
         btn_edit.setOnClickListener(this);
         btn_delete.setOnClickListener(this);
         btn_detail_save.setOnClickListener(this);
         Intent intent = getIntent();
         val = intent.getExtras().getString(FirstActivity.KEY_CONTENT);
         if (!val.equals("") || val == null) {
             // Toast.makeText(this, "获取的值不为空，已经设置，，", 0).show();
             et_detail.setText(val);
             et_detail.setSelection(val.length());// 设置光标到最后
         }
 
     }
 
     @Override
     public void onClick(View v) {
         // TODO Auto-generated method stub
         MySqliteDatabase db = new MySqliteDatabase(DetailActivity.this);
         SQLiteDatabase sdb = db.getWritableDatabase();
         switch (v.getId()) {
         case R.id.btn_edit:
             et_detail.setEnabled(true);// 设置文本克可编辑
             // 可编辑的时候删除按钮隐藏，保存按钮可见
             btn_delete.setVisibility(View.GONE);
             btn_detail_save.setVisibility(View.VISIBLE);
             System.out.println("===val:" + val + ",====");
             // 同时还要删除
             sdb.delete(MySqliteDatabase.TABLE, MySqliteDatabase.CONTENT + "=?",
                     new String[] { val });// 有报错，暂时不处理
             break;
         case R.id.btn_detail_save:
             String content = et_detail.getText().toString();
             ContentValues cv = new ContentValues();
 
             SimpleDateFormat format = new SimpleDateFormat();
             Date date = new Date();
             String time = format.format(date);
             cv.put(MySqliteDatabase.CONTENT, content);
             /*
              * 之前没有插入time这个值，但是因为这个表里的time字段不能够为空，导致下main的代码都
              * 没正确执行，即不能够按照预期把文本内容插入到数据库。
              */
             cv.put(MySqliteDatabase.TIME, time);
             System.out.println("-----btn_detail_save：content=  ----");
             sdb.insert(MySqliteDatabase.TABLE, null, cv);
             Intent intent = new Intent(DetailActivity.this, FirstActivity.class);
             startActivity(intent);
             finish();
             break;
 
         default:
             break;
         }
     }
 }

具体代码：http://pan.baidu.com/s/1sjIJRit

坑2：viewpager与fragment一起用的时候，需要传入fragment作为参数，在自己的Fragment实现类中有一个必须复写的方法，

public View onCreateView(LayoutInflater inflater,
@Nullable ViewGroup container, @Nullable Bundle savedInstanceState){

}

，具体的坑如下：

 public class Fragment2 extends Fragment {
     @Override
     public View onCreateView(LayoutInflater inflater,
             @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
         // return inflater.inflate(R.layout.view2, container, true);
         // 这里的第二个参数只能填写null,不能像上面一行注释的那样填写container，
         // 否则会报错：The specified child already has a parent. You must call
         // removeView() on the child's parent first.
         return inflater.inflate(R.layout.view2, null, true);
     }
 }

同时也可以参见：http://blog.csdn.net/mcy478643968/article/details/26812225 ，具体源码可见百度网盘里的Wyl.rar。