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Saturday, January 15, 2011

enum (in Java)::::Interview Questions

Define enum 
Answer: In prior releases, the standard way to represent an enumerated type was the int Enum pattern:
// int Enum Pattern - has severe problems!
public static final int SEASON_WINTER = 0;
public static final int SEASON_SPRING = 1;
public static final int SEASON_SUMMER = 2;
public static final int SEASON_FALL = 3;
This pattern has many problems, such as:
  • Not typesafe - Since a season is just an int you can pass in any other int value where a season is required, or add two seasons together (which makes no sense).
  • No namespace - You must prefix constants of an int enum with a string (in this case SEASON_) to avoid collisions with other int enum types.
  • Brittleness - Because int enums are compile-time constants, they are compiled into clients that use them. If a new constant is added between two existing constants or the order is changed, clients must be recompiled. If they are not, they will still run, but their behavior will be undefined.
  • Printed values are uninformative - Because they are just ints, if you print one out all you get is a number, which tells you nothing about what it represents, or even what type it is.
It is possible to get around these problems by using the Typesafe Enum pattern (see Effective Java Item 21), but this pattern has its own problems: It is quite verbose, hence error prone, and its enum constants cannot be used in switch statements. In 5.0, the Java™ programming language gets linguistic support for enumerated types. In their simplest form, these enums look just like their C, C++, and C# counterparts:
But appearances can be deceiving. Java programming language enums are far more powerful than their counterparts in other languages, which are little more than glorified integers. The new enum declaration defines a full-fledged class (dubbed an enum type). In addition to solving all the problems mentioned above, it allows you to add arbitrary methods and fields to an enum type, to implement arbitrary interfaces, and more. Enum types provide high-quality implementations of all the Object methods. They are Comparable and Serializable, and the serial form is designed to withstand arbitrary changes in the enum type