OCP - 3
Concurrency & IO
Concurrency & IO
Fichier Détails
| Cartes-fiches | 110 |
|---|---|
| Langue | English |
| Catégorie | Informatique |
| Niveau | Autres |
| Crée / Actualisé | 08.03.2019 / 08.01.2020 |
| Lien de web |
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WHat is the skip method for on an INputstream()
Skip n bytes of data
How can you determine EOF when using a FileInputStream?
When the read() method returns -1
How can you determine EOF when using a BufferedInputStream?
When the return value of read is 0 since it returns the number of bytes read
How can you determine EOF when using a FileReader?
When the readLine() method returns null or or read() returns -1
What variants of read() and write() do FileInputStream and FileOutputStream privde?
An override to pass a byte array. This is used by the BufferedIn/OuputStream classes to improve performance
What convenience method does BufferedWriter provde?
the newLine method
Converting an in-memory object to a stored data format is refered to as ____
Converting an in-memory object to a stored data format is refered to as serialization
Converting stored data into an object is known as ____.
Converting stored data into an object is known as deserialization
What methods does the Serializable interface declare?
None, it's a marker interface.
What happens whe serializing a class that does not implement Serializable
A checked NotSerializableException (subclass of IOException) is thrown
How can you prevent fields from being serialized?
Using the transient keyword on the fields. The values are then lost during the serialization process
What happens with static members during serialization?
They are ignored as they do not belong to the particular instance
What is the serialVersionUID for? Do you have to declare it?
It is good practice to identify a unique version of the class. If it changes, an exception can be throw or it can be migrated.
If not declared, the compiler does it for you. Because different compilers have different strategies, you should manage it yourself
How can you Serialize and Deserialize objects?
Using the ObjectInputStream and ObjectOutputStream classes and the coresponding methods readObject() and writeObject
What has to be minded when using readObject()?
It throws additionally to the checked IOExcption a checked ClassNotFoundException
Do you have to manually cast the results of the readObject() method?
Yes, the return type is of type Object
How can you determine EOF when using a ObjectInputStream?
Catch the EOFException!
is it safe to use inputsream.available() to check if a stream is EOF?
NO! Use the specific strategies for each kind of stream
How is a serialized object created?
- Java calls the first non-arg constructor for the first nonserializable parent class, skipping the constructors of all serialized classes in between!
- Static variables are ignored
- default intializers (eg private String myField = "123" and {}) are ignored
What are the PrintStream and PrintWriter classes for?
They offer a high-level interface to write formatted representation of Java objects to a text-based outputs stream
What is a common use of PrintStream?
The System.err and Sytstem.out fields
What is special about the print, println, printf and format methods found on the PrintStream/PrintWriter classes?
They do not throw an exception! The checkError() method can be used for that instead
What line separator does the println method od PrintWriter/PrintStream use?
It depends on the os. You can access the value using System.getProperty("line.separator")
What is the difference between format and printf on PrintWriter/PrintStream
They are identical, printf calls format behind the scenes
What is the Console class for?
It is convenience for the Readers/Writers System.in, System.out and System.err
What is special about the Console class?
It is a Singleton that is accessed using the System.console() method
What common methods does the Console class provide?
- reader()
- writer()
- format() / printf()
- Locale override is not supported! Use writer.format instead!
- flush()
- readLine()
- readPassword()
When should you use the Console.flush() method?
Before using readLine and readPassword to ensure only user input is read
Why does Console.readPassword return a byte array?
If a string were returned, the passwourd might be available in the String pool long after the user entered it. By using a byte array, the password can be wiped as soon as it is not required anymore using Arrays.fill(passwordArray, 'x')
Does the method Writer.append exist
Yes
