Download Chapter 6 Java Generic Type

Chapter 6 Java Generic Type
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[email protected]
Xiang Zhang
Content
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 Significance of Generic Type
 Definition of Generic Type
 Usage of Generic Type
Java CoSE
Significance of Generic Type
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 What is Generic Type
 Why Generic Type?
 How about the inconvenience without GT?
Java CoSE
Significance of Generic Type
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 Example

Write a MyArrayList

Without GT

The element is Object

Think: Is there any
problem? How to solve?

If we add a Person to
MyArrayList, what can
we get() later?

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Type loss
Significance of Generic Type
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 Example

Write
MyArrayList
using GT

GT improves
the simplicity
and
correctness
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Significance of Generic Type
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 Merit of GT

Type-safe

Eliminate Type Casting

Usually used in defining type of elements in Collections
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Definition of Generic Type
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 Generic Class
 Generic Interface
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Definition of Generic Type
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 Bounded Type Parameter (受限类型参数)

Sometimes, parameter type in GT should be limited

Can Person be the parameter type in SortedCollection?
Java CoSE
Definition of Generic Type
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 Bounded Type Parameter

The meaning Keyword extends is generic

Comparable interface is called upper bound

E is called a Bounded Type Parameter
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泛型的定义
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 Bounded Type Parameter

Multiple-bounded
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Usage of Generic Type
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 Example: Write a Static Method to Sum a List

1. without GT

Short: We do not know the element type, and cannot
guarantee each element in List L is able to sum up.
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Usage of Generic Type
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
2. With GT
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What We Got
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 The runtime type of an object is determined by:

Its class

Its class parameter
 So

Passing ArrayList<Number> to List<Number>, OK!

Passing List<Integer> to List<Number>, NO!!
 List<Integer> is not a subclass of List<Number>
 What should we do to maintain versatality?
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Usage of GT
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
3. With GT and Wildcard
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Some Tips
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 ArrayList<Number> a = new ArrayList<Integer>();
// right or not?
 ArrayList<? extends Number> a = new
ArrayList<Integer>(); // right or not?
 In invocation of method public void add(Number a),
what type of objects can we transfer into a method as
parameter

Just the type Number // right or not?

Number and all its subtypes // right or not?
Java CoSE
Usage of Generic Type
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 Usage of Wildcard

Define Upper bound


Define Lower bound


List<? super Integer> // Pls give 5 matching types
Multiple-bounded is not allowed


List<? extends Number>
List<? extends Number & Serializable> // Compiling error
Guess: is List<?> different with List<Object>?
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Example
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Example
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Example
Self-study
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 Text Processing

Basic elements in text processing (chapter 7)

String / StringBuffer / StringBuilder // try to benchmark

Scanner

StringTokenizer

Java and IR (Information Retrieval)

Tokenizing (分词)

Stopwords Removal (去除停用词)

Rooting (取词根)

Indexing (索引)
Java CoSE
Forecast
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 AWT and Swing Introduction
 Swing Container（JFrame, JPanel）
 Swing Components
 Layout Manager
 Event and Event-based Programming
 Menu
Java CoSE