Difference hashcode and equal, JAVA Programming

 

The hashCode() and equals() methods have to be very important, when objects implementing these two functions are added to collections. If not implemented or implemented incorrectly at all then your objects added in a collection like a Set, Map or List can behave strangely and also is hard to debug.

 

Equals ():

This method performs if some other object passed to it as an argument is same the object in which this method is called. It is easy to implement the equals() method incorrectly, if you do not understand the design. The contract can be stated in terms of 6 easy principles as given:

 

1. l1.equals(l1)  which defines an Object could be equal to itself.

 

2. l1.equals(l2) if and only if l2.equals(l1) So it may be incorrect to have your own class define "MySet" to have a equals() function that has a comparison with an Object of class "java.lang.String" class or with any other in built Java class.

 

3. l1.equals(l2) && l2.equals(l3) implies that l1.equals(o3) as well It defines that if the first object l1 same to the second object l2 and the second object l2 is similar to the third object l3 then the first object l1 is similar to the third object l3.  

 

4. l1.equals(l2) gives the same as long as l1 and l2 are unmodified if two objects are same, they must remain equal as long as they are not changed. Similarly, if they are not same, they must remain non-equal as long as they are not changed.

 

5. l1.equals(null)  which defines that any instantiable object is not same to null. So if you pass a null as an argument to your object l1, then it could give false.

 

6. l1.equals(l2) implies l1.hashCode() == o2.hashCode() ?? That is very important. If you define a equals() function then you must describe a hashCode() method as well. Also it gives that if you have two objects that are same then they must have the same hashCode, however the opposite is not true. So, If a field is not used in equals(), then it cannot be used in hashCode() function.

 

hashCode():

 

This method gives a hashCode() value as an Integer and is supported for the benefit of hashing based java.util.Collection classes like HashMap, Hashtable, HashSet etc. If a class overrides the equals() method, it must define the hashCode() method as well. The general design of the hashCode() method is that:

 

1. Whenever hashCode() method is called on the similar types object more than once during an execution of a Java program, this method must consistently give the same integer result. The integer result has not remained consistent from one execution of the program to the next execution of the same program.

 

2. If two objects are same as per the equals() method, then using the hashCode() method in each of the two objects must give the similar integer result. So, If a field is not used in equals(), then it cannot be used in hashCode() method.

3. If two objects are non equal as per the equals() method, each of the two objects can give either two same integer results or different integer results.

 

Posted Date: 7/25/2012 1:56:55 AM | Location : United States







Related Discussions:- Difference hashcode and equal, Assignment Help, Ask Question on Difference hashcode and equal, Get Answer, Expert's Help, Difference hashcode and equal Discussions

Write discussion on Difference hashcode and equal
Your posts are moderated
Related Questions
I want ProSport Savings Plans Project Description: To prepare an application on iPhone, Mac, iPad and Android, which does the subsequent:- Provides a calculation of the am

The File class encapsulates the files and directories of the local file system. The RandomAccessFile class gives the methods required to directly access data contained in any part

I can''t figure out how to do this question: "Given two linear equations: ax + by + c = 0 and dx + ey + f = 0" solve for x and y. A, b, c, d, e, and f being inputted from the user.

Ask question how to implement a decision tree in java #Minimum 100 words accepted#

Implement a program to query and update the text database for a library using a client/server model allowing multiple clients safe access to the database, and using a producer-cons

Component Developer to code in Java: Worked for Sansri's Gateway as Software Engineer . Here I Worked as a Component Developer to code in Java and J2EE technologies Here Ensu

What are the Sub categories of Artificial Intelligence? Briefly explain any two. 1. Robotics :  These are the machines that are computer programmed and perform work that was pr

what is join in java Thread

Socket and ServerSocket. ServerSocket is used for normal two-way socket communication. Socket class permits us to read and write by the sockets. getInputStream() and getOutputStrea

Preventing Applet Based Social Engineering Attacks To help avoid this, Java applet windows are specifically labeled as such along with an ugly bar that says: "Warning: Applet W