1. Packages, Interfaces & Exception Handling
Unit II-Chapter No. : 4-Interfaces
Packages, Interfaces & Exception Handling

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Content
Explain packages, Interfaces and Exception handling mechanism of java
Write java program to create code library using packages
Write a java program to manage run time errors using java exception handling mechanism.
Write a java program to implement interfaces for a given problem specification.
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Content
Packages: Define, create, access protection and importing packages
Interfaces: Define, create interface and implement Method overriding
Exceptions: Define, Exception handling fundamentals, classification: Exception Types.
Using throw, throws, finally keywords to manage exceptions.
User Defined Exceptions
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Interfaces: Basics
Background: abstract classes
The “interface” in java supports to fully abstract the class from its implementation (where as abstract class consists of concrete methods)
Using interface we can specify what a class must do , but not how it does it.
Example: Suppose : BankInterface { void deposit(double amt); // what, not how}
class BankAccount implements BankInterface{ double balance;
void deposit(double amt){balance+=amt;} // how to deposit
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5. Interfaces: Definition
Define: Interfaces are syntactically similar to classes but they lack instance variables and their methods are declared without any body.
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Interfaces: Basics
One interface can be implemented by any number of classes.
One class can implement any number of interfaces.
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Interfaces: Basics
Few conditions on classes which implements interface
To implement an interface , a class must create the complete set of methods defined by the interface
Each class is free to determine the details of its own implementation.
Interfaces are designed to support dynamic method resolution at run time.
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8. Interfaces: Define/ General syntax
access interface name{ return-type method-name1(parameter-list); return-type method-name2(parameter-list); type final-varname1 = value; type final-varname2 = value; //…. return-type method-nameN(parameter-list); type final-varnameN = value; } Note: all methods are abstract in interface All interface variables are final and static
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Interface: Example
Example
create interface
interface CallBack{
void callBack(int param); }
Implementing interface
General syntax
class ClassName [extends superclass][implements interface [,interface]] { // class-body }
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10. Interface: create and implement
Example
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11. Interface: create and implement
It is both permissible
and common for classes
that implement interface
to define additional
members of their own
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12. Interfaces: Polymorphic Power
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13. Interfaces: partial implementation
If a class includes an interface but does not fully implement the methods defined by that interface, that class must be declared as abstract.
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Interfaces: Nested
An interface can be declared a member of a class or another interface, such interface is called a member interface or a nested interface.
A nested interface can be declared as public, private, or protected.
When a nested interface is used outside of its enclosing , it must be qualified by the class or interface of which it is a member.
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15. Nested Interfaces: Example
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Interface: Variables
Apart from having methods declared in interfaces, it can also have “variables”.
The variables declared in interface are by default “final and static” and they can be shared by multiple classes
Example
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Interface:Variables
Example :
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Extending Interface
Example :
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19. Interface: Multiple Inheritance
An interface can extend more than interface
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Interfaces
Caution: Because dynamic lookup of a method at run time incurs a significant overhead when compared with normal method invocation in java, we should be careful not to use interfaces casually in performance-critical code.
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Exercises
Consider customer can have multiple accounts like SavingsAccount and ProvidentFundAccount, the customer can withdraw and deposit amount to savings account but not with provident fund account (PF account), however he can transfer amount from saving account to PF account. Write a test program to simulate the above scenario.
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