common mistakes best practices coding standards

8/14/2019 Common Mistakes Best Practices Coding Standards

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Common mistakesBest Practices

Coding standards

Govind Krishna Mekala


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What we wont cover

J2ee/EJB antipatternsDesign Patterns


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What we will cover

Interactive session with lots of exampleson:

Core Java mistakes and solutions.

Best Practices for Java and JDBC.

Why coding standards are so

important?


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Object Creation

Static factory methods are moreappealing than constructors.Public static Boolean valueOf(boolean b) {

return (b?Boolean.TRUE:Bolean.FALSE);}

Use private constructor when defining a

class for grouping static fields andmethods.


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Avoid creating duplicate objects:String s = new String(No);

Eliminate obsolete reference by nullingout to avoid memory leaks due tounitentional object reference. (Specially incollections and Maps).

Nothing time critical should ever be doneby a finalizer. Never depend on Finalizerand we cant time GC. Use finalizer as

safe net to terminate noncritical nativeresources without forgettingsuper.finalize.


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Providing a good toStringimplementation makes your class muchmore pleasant to use.

System.out.println(Failed to connect: +phoneNumber);

Always override hashCode method whenyou override equals. Failure to do so will

result in violation of the general contractfor Object.hashCode which will preventyour class from functioning properly withconjunction with all hash-basedcollections, including HashMap, HashSetand HashTable.


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Composition vs Inheritance

Lets discuss this firstNever extend a class unless documented todo so. Always document if you write a classfor extending. Reason: Subclasses will breakwhen super class changes. Unintentional trapfor super class just because some ignorantprogrammers written several subclasses.

Must read link:http://www.artima.com/designtechniques/compoinh.html


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Prefer Interface over Abstract

Multiple inheritance problem.Interface enables safe, power funtionalityenhancement without using hierarchical typeframework.

You can combine the virtue of interface andabstract classes by proving as abstractskeleton implementation class to go with

each nontrivial interface you export. E.gAbstractCollection, AbstractSet,AbstractList..

Comparative analysis (Not official):

http://mindprod.com/jgloss/interfacevsabstract.html


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Use overloading carefully.Return zero-length arrays instead of nulls.

Minimize the Use of Finalizers

Avoid float and double if exact answersare required. Use BigDecimal, int or longfor monetary calculations.

System.out.println(1.03 0.42);

System.out.println(1.00 - .10);

Money matters in financial calculations.htm

//0.6100000000000001

//0.09999999999999995


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Avoid string concatenation for morethan few strings. Use stringBufferinstead as string is immutable.

Refer to objects by their interfaces ifappropriate interface exists.

(flexibility).List myList = new vector();


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Exception Handling common

mistakes

Empty Catch BlocksLoss of Stack Information. Use exceptionchaining (or nesting) supported in JDK 1.4

Incomprehensible and IncompleteException Logs.

Because, a catch statement that uses asuperclass will catch exceptions of that type

plus any of its subclasses. So, the subclasswould never be reached if it come after itssuperclass. For example,

ArithmeticException is a subclass ofException


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Enhanced Exception Handling

Support in Java 1.4

Exception Chaining: In Java version 1.4support for it has therefore been built intothe uppermost exception base class:

Throwable( String message, Throwable cause )

Throwable( Throwable cause )

Assertions: They make it possible toimprove software quality by abundant use ofruntime checks for internal preconditions,postconditions and invariants similar to Cassertion macros.


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Thread Safety

Synchronize access to shared mutable data.In a multi-threaded environment, anymutable data visible to more than one threadmust be referenced within a synchronized

block. This includes all primitive data. All getand set methods for shared data which canchange must be synchronized.

It is a misconception that all primitives exceptlong and double do not need synchronizedaccess.


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Thread Safety cont..

Avoid excessive synchronization. Excessivesynchronization might causes deadlock. Do as little as possible inside synchronized region.

Obtain the lock, examine the shared data,

transform the data as necessary, and drop thelock.

If a class could be used both in circumstancesrequiring synchronization and circumstances

where synchronization not required, provide boththe variants through wrapper class or subclasswith synchronization.

To avoid deadlock and data corruption, never call

an alien method from within synchronized region.


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Never invoke wait outside a loop and prefernotifyAll in preference to notify.

Dont depend on thread scheduler and threadpriorities. They are not portable across VM

and OS.Avoid thread groups. Thread groups wereintroduced for isolating applets for security

which didnt fulfill the promise. The APIs toget list of active threads and subgroups arebuggy. Thread groups are obsolete.


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Singleton pitfalls. To decide whether to createa singleton class, first ask these questions: 1. Does there need to one global entry point to

this class?

2. Should there be fixed number of instance inVM?

Never use singleton class as global variables asthey might become non singleton.

Read link:http://www.artima.com/designtechniques/threadsafety.html


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JDBC

Use connection pooling.

Choose appropriate isolation levels.


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JDBC Cont..

Use Statement pooling.


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JDBC Cont..

Statements versus Prepared Statements:When to Use What?

Tune the SQL to minimize the data returned

(e.g. not 'SELECT *').Avoid the Usage of Metadata Methods (e.gDatabaseMetaData.getColumns() whenever

Possible. They are expensive.Try to combine queries and batch updates.


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JDBC Cont..

Use stored procedures.Cache data to avoid repeated queries.

Close resources (Connections, Statements,ResultSets) when finished with.

Select the fastest JDBC driver.

http://www.oreil ly.com/catalog/jorajdbc/chapter/ch19.html#74469

Link to read:

http://www.precisejava.com/javaperf/j2ee/JDBC.htm


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Why coding conventions are

important?

We all have our own coding style but ourcode is not a piece of art for others.

Use most widely used conventions formethod, fields naming. Refer Sun site.

Make set of coding standards mandatoryrelevant to your product and projectmaintenance.

Document inheritance, overload, staticchecked exceptions without fail.

common mistakes best practices coding standards

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