Question

实际上我正在开发应用程序运行时的一个应用程序第一次它会创建一个txt文件，如果文件已经存在，那么它只会在文件中写入但我不知道如何以编程方式检查文件是否存在。我试过了，

try  
    {  
        FPartyCreation = new File ("/data/data/com.MediExcel/files/","dbPartyCreation.dat");  
        if (FPartyCreation.exists())  
        {  
            writeOnFile();
        }  
        else  
        {  
            FPartyCreation.createNewFile();  
            writeOnFile();
        }  
    }  
    catch (IOException ioe)  
    {  
        ioe.printStackTrace();  
    }

但它会产生以下错误，并且文件也不会在同一路径上创建。

07-06 01:39:26.183: E/AndroidRuntime(23107): FATAL EXCEPTION: main
07-06 01:39:26.183: E/AndroidRuntime(23107): java.lang.NullPointerException
07-06 01:39:26.183: E/AndroidRuntime(23107):    at com.MediExcel.AddMethodPartyCreation.getMethod(AddMethodPartyCreation.java:28)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at com.MediExcel.PartyCreationActivity$1.onClick(PartyCreationActivity.java:64)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at android.view.View.performClick(View.java:3511)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at android.view.View$PerformClick.run(View.java:14105)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at android.os.Handler.handleCallback(Handler.java:605)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at android.os.Handler.dispatchMessage(Handler.java:92)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at android.os.Looper.loop(Looper.java:137)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at android.app.ActivityThread.main(ActivityThread.java:4424)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at java.lang.reflect.Method.invokeNative(Native Method)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at java.lang.reflect.Method.invoke(Method.java:511)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
07-06 01:39:26.183: E/AndroidRuntime(23107):    at dalvik.system.NativeStart.main(Native Method)

请帮帮我

谢谢

Answer 1

创建SQLite数据库 -

public class DatabaseHelper extends SQLiteOpenHelper {

static final String dbName="demoDB";
static final String employeeTable="Employees";
static final String colID="EmployeeID";
static final String colName="EmployeeName";
static final String colAge="Age";
static final String colDept="Dept";

static final String deptTable="Dept";
static final String colDeptID="DeptID";
static final String colDeptName="DeptName";

static final String viewEmps="ViewEmps";

创建数据库

public void onCreate(SQLiteDatabase db) 
{
    // TODO Auto-generated method stub

    db.execSQL("CREATE TABLE "+deptTable+" ("+colDeptID+ " INTEGER PRIMARY KEY , "+
    colDeptName+ " TEXT)");

  db.execSQL("CREATE TABLE "+employeeTable+" 
    ("+colID+" INTEGER PRIMARY KEY AUTOINCREMENT, "+
        colName+" TEXT, "+colAge+" Integer, "+colDept+" 
    INTEGER NOT NULL ,FOREIGN KEY ("+colDept+") REFERENCES 
    "+deptTable+" ("+colDeptID+"));");


  db.execSQL("CREATE TRIGGER fk_empdept_deptid " +
    " BEFORE INSERT "+
    " ON "+employeeTable+

    " FOR EACH ROW BEGIN"+
    " SELECT CASE WHEN ((SELECT "+colDeptID+" FROM "+deptTable+" 
    WHERE "+colDeptID+"=new."+colDept+" ) IS NULL)"+
    " THEN RAISE (ABORT,'Foreign Key Violation') END;"+
    "  END;");

  db.execSQL("CREATE VIEW "+viewEmps+
    " AS SELECT "+employeeTable+"."+colID+" AS _id,"+
    " "+employeeTable+"."+colName+","+
    " "+employeeTable+"."+colAge+","+
    " "+deptTable+"."+colDeptName+""+
    " FROM "+employeeTable+" JOIN "+deptTable+
    " ON "+employeeTable+"."+colDept+" ="+deptTable+"."+colDeptID
    );
  //Inserts pre-defined departments
  InsertDepts(db);  
 }

查看此Using-SQLite-Database-with-Android博客。希望这个博客足以满足您的疑问。

如何以编程方式找出该文件是否已存在？

1 个答案: