Summary of hashCode and equals methos in Java

Summary & Miscellaneous Tips about hashCode and equals method

• Equal objects must produce the same hash code as long as they are equal, however unequal objects need not produce distinct hash codes.
• The equals method provides "deep comparison" by checking if two objects are logically equal as opposed to the "shallow comparison" provided by the equality operator ==.
• However, the equals method in java.lang.Object class only provides "shallow comparison", same as provided by the equality operator ==.
• The equals method only takes Java objects as an argument, and not primitives; passing primitives will result in a compile time error.
• Passing objects of different types to the equals method will never result in a compile time error or runtime error.
• For standard Java wrapper classes and for java.lang.String, if the equals argument type (class) is different from the type of the object on which the equals method is invoked, it will return false.
• The class java.lang.StringBuffer does not override the equals method, and hence it inherits the implementation from java.lang.Object class.
• The equals method must not provide equality comparison with any built in Java class, as it would result in the violation of the symmetry requirement stated in the general contract of the equals method.
• If null is passed as an argument to the equals method, it will return false.
• Equal hash codes do not imply that the objects are equal.
• return 1; is a legal implementation of the hashCode method, however it is a very bad implementation. It is legal because it ensures that equal objects will have equal hash codes, it also ensures that the hash code returned will be consistent for multiple invocations during the same execution. Thus, it does not violate the general contract of the hashCode method. It is a bad implementation because it returns same hash code for all the objects. This explanation applies to all implementations of the hashCode method which return same constant integer value for all the objects.
• In standard JDK 1.4, the wrapper classes java.lang.Short, java.lang.Byte, java.lang.Character and java.lang.Integer simply return the value they represent as the hash code by typecasting it to an int.
• Since JDK version 1.3, the class java.lang.String caches its hash code, i.e. it calculates the hash code only once and stores it in an instance variable and returns this value whenever the hashCode method is called. It is legal because java.lang.String represents an immutable string.
• It is incorrect to involve a random number directly while computing the hash code of the class object, as it would not consistently return the same hash code for multiple invocations during the same execution.

How to get certified with Oracle

After spending around  year as a Java Developer  I thought of having a certification in Java. I  have started google to get the information.  So I want to share the information with you all which might helpful for you all.

Sun offers six levels of Certification exams for Java technology developers. In increasing level of difficulty, these are - 

1. Sun's Java Associate (SCJA)
2. Sun's Java Programmer Certification (SCJP)
3. Sun's web component Developer Certification (SCWCD)
4. Sun's business component Developer Certification (SCBCD)
5. Sun's Developer for Java Web Services (SCDJWS)
6. Sun's mobile application Developer Certification (SCMAD)
7. Sun's Java Developer Certification (SCJD)
8. Sun's Java Architect Certification (SCEA)

I have started searching the kind of certification I can go through. I found the relevant one for me is following :

Exam Number:1Z0-851

Associated Certifications:Oracle Certified Professional, Java SE 6 Programmer (SCJP 6.0)

Exam Registration:Register

About the Contents :

The Sun's Java Certification Programmer 6 exam consists of 60 questions. These need to be completed in 210 minutes. To clear the certification exam you need to get 61% correct answers.

There are three type of questions in the exam - 

1. Multiple choice questions in which you have to select one or more correct answers from a set of options. A majority of the questions are of this type. In the exam it would be clear from the question whether it has one correct answer or multiple correct answers. For questions with multiple correct answers, you must get all the correct answers, to get credit for that question. Also these type of questions will mention the number of correct answers.
2. True or False type questions
3. Questions that require you to drag and drop items from one column to another.

The test paper covers contents (what I found is) as follows :

	Declarations, Initialization and Scoping
	Flow Control
	API Contents
	Concurrency
	OO Concepts
	Collections / Generics
	Fundamentals

* Will discuss above points in my coming blogs.

The details can be found on the oracle's websites  Click Here

Where To take Exam

Now we know what should be the done but where and  how to get the exam. Lets talk about the same.

First get your nearest vendor affiliated with Oracle. you can find the nearest vendor from the Pearson VUE.

For scheduling your exam you need to have a web account with the Pearson VUE. once registering with the site you can get your login credentials with 24 hours.

After getting the credentials you can select the type of the certification you are going to join. In case of SCJP 6.0 we have to select following :

• Once you logged in with the specific vendor you can see all the certification available with that Vendor. 
• Select your's and you will be prompted for the search and select of the nearest center where you can give your exam.
• After selecting the centre you will be get options to get the appointment for the exam. 
• The selection of dates lands you on the payment page. Once you done with the payment you will get the e-receipt. Or if you don't want to pay online you can contact to your Test center.

For more help or guidelines Click Here