Input and Output Streams

Some Important points on Input and OutPut Streams:

• InputStream and OutputStream are abstract classes.
• FileInputStream and FilterInputStream both inherit directly from InputStream.
• FileOutputStream and FilterOutputStream both inherit directly from OutputStream.
• DataInputStream inherits directly from FilterInputStream.
• DataOutputStream inherits directly from FilterOutputStream.
• When you create a Filter stream, you must specify the stream to which it will attach.
• A Filter stream processes a stream of bytes in some way. By “chaining” any number of Filter streams, you can add any amount of processing to a stream of bytes.
• DataInputStream, DataOutputStream and RandomAccessFile know how to work with Java data types because they implement the
• DataInput and DataOutput interfaces, whereas FileInputStream and FileOutputStream know only how to work with individual bytes.
• RandomAccessFile implements both DataInput and DataOutput methods - RandomAccessFile objects can read from and write to files.
• The File class is not used to create files. Can create a file using an instance of class RandomAccessFile and FileOutputStream.
• To test if a File object refers to an existing file, you can invoke exists() which returns true or false. The File methods canRead() and canWrite() return boolean values that indicate whether the application can read from or write to the file. (Note: applets can’t write to a file.)
• Can use File methods to make a permanent change to the file system. For example, you can call delete() or rename(). For rename(), need to supply a File object which embodies the new name.
• You can create a directory using the File class. The method mkdir() does this.
• It is possible to navigate the filing system using the File class. Methods getParent(), getPath() and getName() are provided, and also getAbsolutePath().
• The method getAbsolutePath() returns the name of the current user directory (the full pathname included) with the file name concatenated. See Practice Exam 1 Q.45
• Creating a FileOutputStream object creates the appropriate file. If the file already exists, FileOutputStream replaces it (unless the FileOutputStream object is created using the constructor which takes a String and a boolean - see later)
• Upon creation of a RandomAccessFile object, need to supply a file object plus a mode. Alternatively, can supply a filename plus a mode.
• Valid modes for RandomAccessFile are “r” and “rw”.
• Constructors for FileInputStream: one takes a String, one takes a File, one takes a FileDescriptor
• Constructors for FileOutputStream: one takes a String, one takes a File, one takes a FileDescriptor, one takes a String and a boolean (which indicates whether or not to append).
• Constructors for FilterInputStream: one only, which takes an InputStream
• Constructors for FilterOutputStream: one only, which takes an OutputStream

Toast message in Android

Toast is a transient message that appears and disappears without any interaction from the user and with no notification to the program that it disappeared. The Toast can display a simple text message or a complex view.

Displaying simple text: To display a simple toast that displays a text message we use the following code:

1	Toast toast=Toast.makeText(this, "This is toast message", 2000);

2	toast.setGravity(Gravity.TOP, -30, 50);

3	toast.show();

we create the toast using the static Toast.makeText(Context con,String message, int duration) method to create a toast in the current context to display a text message for a duration specified in milli seconds or we can use the constant values Toast.LENGTH_SHORT to display for a short duration or Toast.LENGTH_LONG for longer duration. The toast by default appears in the center of the screen, you can change the default position by specifying the Gravity and the X and Y offsets. finally we call the Show() method to display the toast.

Displaying complex views: Toasts can also display complex views. this is done like this:

First: create a layout xml file in res/layout directory. the file must be named toast_layout.xml.

01	<?xml version="1.0" encoding="utf-8"?>

02	<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

03	android:orientation="vertical"

04	android:layout_width="fill_parent"

05	android:layout_height="fill_parent"

06	android:id="@+id/toastView"

07

>

08

<TextView

09	android:layout_width="fill_parent"

10	android:layout_height="wrap_content"

11	android:text="Hello toast"

12	android:textColor="#000"

13

/>

14

<TextView

15	android:layout_width="fill_parent"

16	android:layout_height="wrap_content"

17	android:id="@+id/txtDate"

18	android:textColor="#000"

19

/>

20

21

22	</LinearLayout>

then from the code

1	Toast toast=new Toast(this);

2	LayoutInflater inflater=this.getLayoutInflater();

3	View toastView=inflater.inflate(R.layout.toast_layout, (ViewGroup)findViewById(R.id.toastView));

4	TextView txtDate=(TextView)toastView.findViewById(R.id.txtDate);

5	txtDate.setText("toast appeared at "+Calendar.getInstance().getTime().toLocaleString());

6	toast.setGravity(Gravity.CENTER, 0, 0);

7	toast.setView(toastView);

8	toast.show();

the toast will be like this:

Notes:

• In the toast_layout.xml width, if you put any buttons or any control that has a callback, it would appear disabled and the user cannot interact with it.
• The toast can be created in two ways: by calling Toast.makeText method or by specifying a view via setView method. when you want to display a simple text use the first one otherwise use the second. if you try to interchange or combie between the two methods an exception will be thrown.