Java Programming Lecture 9

Transcription

1 Java Programming Lecture 9 Alice E. Fischer February 16, 2012 Alice E. Fischer () Java Programming - L9... 1/14 February 16, / 14

2 Outline 1 Object Files Using an Object File Alice E. Fischer () Java Programming - L9... 2/14 February 16, / 14

3 Object Files What is an Object File? Using Object Files See demo programs: FileDemo.java Password.java Alice E. Fischer () Java Programming - L9... 3/14 February 16, / 14

4 What is a Text File? Text files contain a representation of data that is NOT the same as an in-memory representation of the same data. Numbers in the memory are represented in binary. When you write an number to a text file, the binary number is converted to ASCII, and the ASCII characters are written out. When you read in a number, the ASCII codes are converted to binary. When you write out a string to a text file, the null terminator is NOT written out. When you read in a string, a null terminator is put at its end. If you read a whole line from a text file, the newline at the end is changed into a null terminator. Alice E. Fischer () Java Programming - L9... 4/14 February 16, / 14

5 What is an Object File? In an object file, the data is represented the same way as it was in memory, byte by byte. In addition, type information about the object is written in the file. Numbers in the memory are represented in binary. When you write an object to a file, no conversion takes place. The file contains the same information as the memory, in the same form. Numbers are NOT converted to ASCII when they are written out. Strings are NOT changed. Newline and null characters are NOT removed or introduced. If you try to read an object file in a text editor, you see garbage because the information is stored in binary. Alice E. Fischer () Java Programming - L9... 5/14 February 16, / 14

6 A Binary File is an Image of the Memory A text file may contain the same information, but in a different format. In the memory (14 bytes): age weight name A n g i e \0 Written to an object file (14 bytes): Written to a text file (13 ASCII chars): A n g i e \n Angie\n Alice E. Fischer () Java Programming - L9... 6/14 February 16, / 14

7 Why do we need object files? When can we Serialize? Example: ObjectFileDemo Alice E. Fischer () Java Programming - L9... 7/14 February 16, / 14

8 Why have Object Files? To save a great lot of programmer effort when it is necessary to write out an application s data and read it back in the next day. Because sometimes we need to save binary data (photographs, compressed files, encrypted files). To enable moving data from one JVM to another. The same reason that XML files are used. Alice E. Fischer () Java Programming - L9... 8/14 February 16, / 14

9 Java provides a mechanism, called object serialization through which an object can be represented as a sequence of bytes that includes the object s data as well as information about the object s type and the types of data stored in the object. After a serialized object has been written into a file, it can be read from the file and realized (deserialized). The type information and bytes that represent the object and its data can be used to recreate the object in memory. The entire process is JVM independent, meaning an object can be serialized on one platform and deserialized on an entirely different platform. Alice E. Fischer () Java Programming - L9... 9/14 February 16, / 14

10 Serialization is JVM independent. Classes ObjectInputStream and ObjectOutputStream are high-level streams that contain the methods for serializing and deserializing an object. The ObjectOutputStream class contains many write methods for writing primitive data types, and one method for objects: void writeobject(object x) throws IOException The ObjectInputStream class contains this method for realizing an object: Object readobject() throws IOException, ClassNotFoundException You must cast the resulting Object to its proper type. Alice E. Fischer () Java Programming - L /14 February 16, / 14

11 When can we Serialize? The class must implement the java.io.serializable interface. Each standard Java class documents whether or not it is serializable. Primitive types, Strings, arrays, and ArrayLists are. All of the fields in the class must be serializable. If a field is not serializable, it must be marked transient. For a JVM to be able to deserialize an object, it must be able to find the byte code for the class. Otherwise, it throws a ClassNotFoundException. Alice E. Fischer () Java Programming - L /14 February 16, / 14

12 Note these facts: The standard encoding method is a simple translation of the data fields of the object into a byte stream. Data and metadata are transmitted, along with versioning information to make the object portable. The transient property is used for data members that would be meaningless when serialized (a Scanner). and also for information that should be kept secret, such as social security numbers and encryption keys. Alice E. Fischer () Java Programming - L /14 February 16, / 14

13 Opening an Object Output File Using an Object File Create an output stream to write to a disk file. Discards the current contents of the file if it already exists. Throws FileNotFoundException if a writeable file of this name does not exist and cannot be created. import java.io.* PrintWriter pw = new PrintWriter ( new File("results.txt") ); Use the stream: pw.print("use this like System.out." ); pw.println("to output strings and numbers "+ 17); Close the file. This is not optional. pw.close(); Alice E. Fischer () Java Programming - L /14 February 16, / 14

14 Using an Object File Opening an Object Input File Create an input stream to read a disk file. Throws FileNotFoundException if source file is not found. import java.io.* Scanner sc = new Scanner( new File("data.txt") ); Use the scanner normally: String s1 = sc.nextline(); Close the file to release the resources used by the stream: sc.close(); Alice E. Fischer () Java Programming - L /14 February 16, / 14