Question of the Day

Question of the Day

Move one matchstick to produce a square

LECTURE 19:GENERIC TYPES

CSC 212 – Data Structures

Generic Types

Starting with 1.5 release, Java uses generics

Include type parameters in class definition Like methods, parameters can change each

time Fields independent of types can now be

written

Generic Types

On allocating instance, actual type is specified Must be reference type or String as actual

type Code runs as if were written using that type Type used by instance cannot be changed Type parameter becomes part of variable’s

type

Generics Before & After

Before Generics After Genericspublic class ONode {private Object data;ONode(Object d) { data = d;}Object getData() { return data;}void setData(Object d){ data = d;}

}

public class Node<T> {private T data;public Node(T d) { data = d;}T getData() { return data;}void setData(T d){ data = d;}

}

public class Bag<T, String> {private T data;private String name;

public Bag(T d, String newName) { data = d; name = newName;}

public T getData() { return data; }

public void setData(T d){ data = d; }

public String toString() { return name + “: ” + data.toString();}

}

Writing Generics

public class Bag<T> {private T data;private String name;





}

Writing Generics

Bag<Integer> earth = new Bag<Integer>(4, “The Answer”);public class Bag<T> {

private T data;private String name;





}

See Generics Behave

Bag<Integer> earth = new Bag<Integer>(4, “The Answer”);public class Bag {

private Integer data;private String name;

public Bag(Integer d, String newName) { data = d; name = newName;}

public Integer getData() { return data; }

public void setData(Integer d){ data = d; }


}

See Generics Behave

For earth, class written as if T were replaced by Integer

Bag<Car> matchbox = new Bag<Car>(Z4, “Dream”);public class Bag<T> {

private T data;private String name;





}

See Generics Behave

Bag<Car> matchbox = new Bag<Car>(Z4, “Dream”);public class Bag {

private Car data;private String name;

public Bag(Car d, String newName) { data = d; name = newName;}

public Car getData() { return data; }

public void setData(Car d){ data = d; }


}

See Generics Behave

For matchbox, T is Car

This can be at same time T is Integer when for earth

Using Generics

Without Generics With GenericsInteger i;Car c;Bag n;

...

n = new Bag(5,“B”);i = ((Integer)n.getData());c = ((Car)n.getData());

n.setData(c);

i = ((Integer)n.getData()); c = ((Car)n.getData());

Integer i;Car c;Bag<Integer> n;Bag<Car> m;

...

n = new Bag<Integer>(5,“B”);i = n.getData();c = n.getData();

n.setData(c);m = new Bag<Car>(c, “B”);

i = m.getData();c = m.getData();

public class Entry<MONKEY, TYPE> { private MONKEY key; private TYPE value; // And more goes here...

}Entry<String, Integer> a;Entry<String> b;Entry<String, String> c;Entry<String, boolean> d;Entry<Car, Boolean> e;

Can Use Multiple Generic Types


}Entry<String, Integer> a;Entry<String> b; Did not specify for each typeEntry<String, String> c;Entry<String, boolean> d;Entry<Car, Boolean> e;



}Entry<String, Integer> a;Entry<String> b; Did not specify for each typeEntry<String, String> c;Entry<String, boolean> d; Not reference typeEntry<Car, Boolean> e;


When To Specify Type

Whenever class name used (except constructors) Variable declarations:Stack<Integer> hogCount;

Object instantiation:hogCount = new ArrayStack<Double>();

Return type for method :private Stack<Pig> transport()

Parameter listing:public void cook(Stack<Meat> fd)

Used as type parameter:Stack<Stack<Meat>> bacon;

Type cannot be specified instantiating array Compiler error if type specified during

instantiation Can provide type theory explaining this

problem

Generics Annoyance

Type cannot be specified instantiating array

Can use generics with arrays, but need typecast Only needed once, use generics after instantiation Still checks when compiling, so get most benefits

public class Farm<T> { private ArrayList<Feed>[] troughs; private T[] animals; public Farm() { troughs = (ArrayList<Feed>[])new ArrayList[10]; animals = (T[])new Object[1034821]; }}

Generics Annoyance

Single method or class may not care about type Different than using typecasts to get to

work Only will work if does not need to

instantiate object Do not skip specifying type -- that is BAD

IDEA™ Instead use the generic wildcard ?

In Case of Unknown Type

Wildcard in Generic

public class StackHolder {private Stack<?> myStack; public void setStack(Stack<?> lst){myStack = lst;}

public void printStackSize() {System.out.println(myStack.size());}

public Stack<?> getStack() {return myStack;}

}

Wildcard in Generic




}

? matches any reference type(and String)

Wildcard in Generic




}

Any Stack can be passed in for lst

Wildcard in Generic




}

Can call methods as long asmissing type not important

Wildcard in Generic




}

Legal, but yucky. All type information is lost!

Typecasting Explained

99% of typecasts are incorrect 90% fix missing generic type specification

on variable Eclipse “Quick-Fix” on illegal code is 9% 0.95% instantiate arrays of generic type When using interfaces in an ADT is 0.05%

Typecasting Explained

Your Turn

Get into your groups and complete activity

For Next Lecture

Week#7 assignment due Wednesday Extra day given to you, since no classes on

Tuesday Help still available Tuesday; I will be in my

office Project #1 available from Angel; due in

2.5 weeks Based on time give, assume it to takes

much longer Do not wait until last second; cannot be

done in 1 hour Bring calendar Friday; will sign up for

templates Worth 8% of final grade, so need to do good

job

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