Providing Your Own Security Manager

Implementing equals

Object class have methods like equals() and hashcode(). These methods have their own contracts which have to be fulfilled to override them.

 

Default implementation of equals() in object class

The default implementation of equals() in Object class is based on the == operator: Two objects are equal if and only if they are the same object. Naturally, most classes should define their own alternative implementation of this important method.

 

equals() vs == operator (click here)

 

Contract of equals method:

Implementing equals() correctly is not straightforward. The equals() method has a contract that says the equality relation must meet these demands:

• It must be reflexive. For any reference x, x.equals(x) must return True.
• It must be symmetric. For any two nonnull references x and y, x.equals(y) should return the exact same value as y.equals(x).
• It must be transitive. For any three references x, y, and z, if x.equals(y) and y.equals(z) are True, then x.equals(z) must also return True.
• It should be consistent. For any two references x and y, x.equals(y) should return the same value if called repeatedly (unless, of course, either x or y were changed between the repeated invocations of equals()).
• For any nonnull reference x, x.equals(null) should return False.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any reference values x and y, this method returns true if and only if x and y refer to the same object (x==y has the value true).
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

Steps to be taken when implementing equals()

Working with Directory tutorial

Following topics can be covered in this tutorial

• Creating a Directory recursively
• Copying a Directory or file
• Traversing files and directories
• Printing the directory hierarchy
• Display all the files in the directory
• Display all the directories in the directory
• Getting the Current Working Directory
• Getting the last modified time of a File or Directory
• Listing Files or Subdirectories 
• Deleting a File
• Deleting a directory in java
• Checking whether directory is empty or not
• Getting the parent directory of a file in java
• Searching all files inside directory

Serialization tutorial index

Internalization tutorial index

Java IO tutorial

Java IO : Introduction

What is Stream in java IO?

• Type of Steams in java
• The Stream Classes (Covers various classes present in java IO)
• Where do streams come from?

Reading from Standard input in java

Control flow in java

Conditional statement in java - if else statement and ? operator

Switches in java
Loops in java
Continue, break and return

Regex tutorial in java

• Regular expression introduction in java
• Common symbols to represent regex pattern in java
• Using Regular expressions with String in java
  Using regular expressions with String.matches()
  
  
• Regular expressions using the Matcher Class
• Regular expressions using the Pattern class
• Application of regex or regular expression

OOP in java

Encapsulation
Class in java

• Field Values or Properties
• Methods
• Types of methods
• Pass by value or reference

Object creation and destruction
Access Control in java
Nested classes
Inheritance in java

• Inheritance : cpp vs java
• Using super in java
• Multilevel inheritance : Calling order of Constructors
• Abstract classes
• Interfaces in java
• Multiple inheritance in java
• Inheritance among interfaces in java
• Multiple inheritance in java
• Abstract classes : cpp vs java
• Abstract classes vs Interfaces
• Abstract-Interface or skeletal implementations
• Various interfaces
• Abstract-Interface or skeletal implementations 
• Constant Interfaces 
• Marker Interfaces
• Final in java: Preventing inheritance

Polymorphism in java

• Array of base class objects
• Casting and instanceof

Java Tutorial - File IO

 

Nested Class : Index

Nesting of classes : Introduction

Types of nested classes

1. Static member classes
   
2. Member classes
   
3. Local classes
   
4. Anonymous classes

Scope and inner ness of nested classes

How inner classes work in java?

Generics : Index or TOC

Generics in java 

Motivation behind Generics-Dealing with casting of objects
The Generics Facility  
Creating a Generic class
Writing a Generic Method

Naming convention in generics
Generics are syntactic sugar - Type Erasure with Generics
Advantage of Generics



Subtyping a generic type

Wildcards in Generics
Generics classes in java vs templates in c++


Some mistakes to be avoided with generics:
Beginner's mistake of using Object as type parameter to make method generic
Generic methods: How generic are generic method?
Wildcards in Generics 
Bounded parametric types 
Covariance, contravariance, invariance
Upper bound boundedness in generics
Lower bound boundedness in generics
Multiple bounds in generics
Get and Set principle in generics
Restrictions on wildcards in generics
Expressing dependencies in type parameters : Wildcards vs type parameters

Class objects as type literals
Generic types are not covariant
Type erasure in generics
Making return type of method as generic
Covariant parameter types

Set tutorial in java

A Set is a Collection that cannot contain duplicate elements. Set models the mathematical set abstraction. The Set interface contains no methods other than those inherited from Collection. It adds the restriction that duplicate elements are prohibited. Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set objects with different implementation types to be compared meaningfully. Two Set objects are equal if they contain the same elements.

 

Set Interfaces

• Set Interface in java
• SortedSet Interface

 

Set Implementations

• LinkedHashSet
  Creating a set that retains the order of insertion
• HashSet
HashSet class in java
Converting ArrayList to HashSet
• TreeSet
  TreeSet class in java – constructors

 

Operations on Sets

• Operating on a set
• Bulk Operations : Operations between the 2 sets in java

 

SortedSet

• SortedSet Interface
• SortedSet Operations
• Standard constructors for SortedSet interface
• Range-view operations on SortedSet interface
• Creating a sorted set

 

How is set implemented internally?

Set and serializations

Sets and synchronization

Iterators returned by set implementation and concurrent modification

When to use which set implementation?

Performance of set interface implementations

 

Enum : Toc

Pre-java5 enums in java

Java5 - arrival of enum 

Enum with additional fields and custom constructor

Enum with additional fields and custom custructor

Enum that implements interface

Enum and their super class

Constant specific methods for enums

Switch statements on enums

toString() for enums

Convert string to enums

Datastructures related to enums

Restriction on enum types

Serializations of enums

Some points to note about enums

Servlets (index)

JSP (index)

Reflections (toc or index)

Before we being reflections we should note the following:

Note of caution for reflection

Now we can proceed further.

Examining Classes

Following is required to examine a class:

Retrieving Class Objects
First things first. Before you can find out anything about a class, you must first retrieve its corresponding Class object.

Getting the Class Name
It's easy to find out the name of a Class object. All you have to do is invoke the getName method.

Discovering Class Modifiers
This section shows you the methods you need to call to find out what modifiers a particular class has.

Finding Superclasses
In this section you'll learn how to retrieve all of the Class objects for the ancestors of a given class.

Identifying the Interfaces Implemented by a Class
If you want to find out what interfaces a class implements, then check out this section.

Examining Interfaces
In this section you'll learn how to tell if a Class object represents an interface or a class. You'll also get some tips on how to get more information about an interface.

Identifying Class Fields
This section shows you how to discover what fields belong to a class, and how to find out more about these fields by accessing Field objects.

Discovering Class Constructors
This section, which introduces the Constructor class, explains how to get information about a class's contructors.

Obtaining Method Information
To find out about a class's methods, you need to retrieve the correspondingMethod objects. This section shows you how to do this.

Manipulating Objects

Software development tools, such as GUI builders and debuggers, need to manipulate objects at run time. For example, a GUI builder may allow the end-user to select a Button from a menu of components, create the Buttonobject, and then click the Button while running the application within the GUI builder. If you're in the business of creating software development tools, you'll want to take advantage of the reflection API features described in this lesson.

• Creating Objects
  How can you create an object if you don't know its class name until run time? You'll find the answer in this section.
• Getting Field Values
  In this section you'll learn how to get the values of an object's fields, even if you don't know the name of the object, or even its class, until run time.
• Setting Field Values
  Not only can you get field values at run time, you can also set them. This section shows you how.
• Invoking Methods
  This section shows you how to dynamically invoke methods. Given an object, you can find out what methods its class defines, and then invoke the methods.

Creating Objects

• Using No-Argument Constructors
• Using Constructors that Have Arguments
• Getting Field Values
• Setting Field Values
• Invoking Methods

Working with Arrays

• Identifying Arrays
• Retrieving Component Types
• Creating Arrays
• Getting and Setting Element Values

Summary of Classes

The SortedSet interface

A SortedSetis a Setthat maintains its elements in ascending order, sorted according to the elements' natural order, or according to a Comparator provided at SortedSet creation time. (Natural order and Comparators are discussed in the previous section, on Object Ordering.) In addition to the normal Set operations, the Set interface provides operations for:

• Range-view: Performs arbitrary range operations on the sorted set.
• Endpoints: Returns the first or last element in the sorted set.
• Comparator access: Returns the Comparator used to sort the set (if any).

The SortedSet interface is shown below:

public interface SortedSet extends Set {
    // Range-view
    SortedSet subSet(Object fromElement, Object toElement);
    SortedSet headSet(Object toElement);
    SortedSet tailSet(Object fromElement);
    // Endpoints
    Object first();
    Object last();
    // Comparator access
    Comparator comparator();
}

SortedSet Operations 

Standard Constructors 

Range-view Operations on SortedSet

Example – Creating a SortedSet

Exceptions (TOC)

Introduction to exception
How java implements Exception handling?
Types of Exceptions in java
            Checked vs unchecked vs Error type exceptions

Java's catching or throwing exception

This section discusses the reasoning behind this requirement and what it means to you and your Java programs.

Dealing with Exceptions

• Example of class which has to deal with example
• Catching and handling exception with try-catch-finally
• Specifying the exceptions thrown by method using throws clause

How to throw Exceptions

• The throw statement
• Creating custom exception

Custom Exceptions

• Adding custom exceptions to the project design
• Creating the custom exception example
• Naming convention for exceptions in java

Nested Exceptions

Runtime Exceptions--The Controversy

Exceptions and control flow

Finally and catch

Some practices related to exception:

• Avoid @throws in javadoc

• Javadoc all exceptions

• Be specific in throws clause

• Stack trace as String

• Use template for repeated try-catch

• Pass all pertinent data to exceptions

Classloaders in java

What is class loading?
What is class loader?
Getting the classloader of a class
Classloader types
Process of class loading
Writing a custom class loader
Namespaces of classloading
Usage of class loaders