Maven and Java Generics Jan Jusko jan.jusko@fel.cvut.cz 24.2.2014

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Goal

1.  Establish java project format 2.  Show use of Generics in Java

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Java Project Structure

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Use Maven

•  Automation tool for java projects •  Makes dependency management easier •  Common build workflow •  And many more! Makes our job easier!

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Dependency Management

•  Need to import IDE project manually •  Different IDEs •  Very labor intensive for 50+ projects

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Dependency Management with Maven

<dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.8.1</version>

</dependency>

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How to Work with Maven

•  Maven installed •  Win: download, unpack & add to path •  Lin: aptitude install maven3 •  OSX: same same but different

•  Predefined directory structure •  Configuration file (pom.xml)

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Maven Directory structure

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Using Maven

•  Compile $ mvn compile •  Run tests $ mvn test •  Create jar $ mvn package

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Generics in Java

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Motivation

•  Type checking @ compile-time •  Limit explicit casting •  Implementation of generic algorithms

•  You probably encountered them in java Collections

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Generics in Collections

Set varSet = new HashSet(); •  may contain any object type •  notice the interface!

Set<String> varSet = new HashSet<>(); •  may contain only instances of String

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Raw Collections

•  No compile-time check •  Responsibility of programmer •  Runtime Exceptions

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Generic Collections

•  Compiler checks for type safety

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Arrays Can Be Generic As Well

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Generic Arrays vs. Generic Collections

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Invariance of Generic Collections

•  Integer extends Number •  Collection<Integer> does not extend Collection<Number>

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Variance of Generic Arrays

•  Integer extends Number •  Integer[] extends Number[]

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Generic Arrays vs. Generic Collections

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You Cannot Create Generic Arrays

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If it was legal…

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Wildcards with Generics

•  Sometimes you don’t care what type is in Collection •  Then you can use a wildcard <?>

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Wildcard Types

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Pure Wildcard

•  Collections with pure wildcard are effectively “read only”

•  Only null can be added to the collection •  For methods that do not care about contents of

collection

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Extending Wildcard

•  Also effectively “read only”, with null only possible addition

•  For methods that can work with base class or any other extending class

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Extending Wildcard as Method Parameter

•  Since Generic Collections Are Invariant

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Extending Wildcard vs. Superclass

•  Collection of Superclass contains any instance of extending classes

•  Collection of Extending Wildcard contains elements of specific extending class

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Super Wildcard

•  Can add any instance of Number or instance of its predecessor

•  Can fetch only Object •  Not very common

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Creating Generic Classes

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Creating Restricted Generic Classes

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Creating Restricted Generic Classes

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Recursive Generic Parameter

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Generic Methods

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Generic Class & Methods