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Question of the Day • Move one matchstick to produce a square

CSC 212 – Data Structures Lecture 12:Generic Types

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 Stringas actual type • Code runs as if were written using that type • Type used by instancecannot be changed • Type parameter becomes part of variable’s type

Generics Before & After Before Generics After Generics public class ONode {private Objectdata;ONode(Object d) {data = d;}ObjectgetData() { return data;}void setData(Objectd){data = d;} } public class Node<T> {private Tdata;public Node(T d) {data = d;}TgetData() { return data;}void setData(Td){data = d;} }

Writing Generics public class Bag<T, String>{private Tdata;private String name;public Bag(T d, String newName) {data = d;name = newName;}public TgetData() { return data; }public void setData(Td){ data = d; }public String toString() { return name + “: ” + data.toString();} }

Writing Generics public class Bag<T,String>{private Tdata;private Stringname;public Bag(T d, StringnewName) {data = d;name = newName;}public TgetData() { return data; }public void setData(Td){ data = d; }public StringtoString() { return name + “: ” + data.toString();} }

Writing Generics public class Bag<T, String>{private Tdata;private String name;public Bag(T d, String newName) {data = d;name = newName;}public TgetData() { return data; }public void setData(Td){ data = d; }public String toString() { return name + “: ” + data.toString();} }

Writing Generics public class Bag<T> {private Tdata;private String name;public Bag(T d, String newName) {data = d;name = newName;}public TgetData() { return data; }public void setData(Td){ data = d; }public String toString() { return name + “: ” + data.toString();} }

See Generics Behave Bag<Integer> earth = new Bag<Integer>(4, “The Answer”); public class Bag<T> {private Tdata;private String name;public Bag(T d, String newName) {data = d;name = newName;}public TgetData() { return data; }public void setData(Td){ data = d; }public String toString() { return name + “: ” + data.toString();} }

See Generics Behave For earth, class written as if T were replaced by Integer Bag<Integer> earth = new Bag<Integer>(4, “The Answer”); public class Bag {private Integerdata;private String name;public Bag(Integer d, String newName) {data = d;name = newName;}public IntegergetData() { return data; }public void setData(Integerd){ data = d; }public String toString() { return name + “: ” + data.toString();} }

See Generics Behave Bag<Car> matchbox = new Bag<Car>(Z4, “Dream”); public class Bag<T> {private Tdata;private String name;public Bag(T d, String newName) {data = d;name = newName;}public TgetData() { return data; }public void setData(Td){ data = d; }public String toString() { return name + “: ” + data.toString();} }

See Generics Behave For matchbox, T is Car This can be at same time T is Integer when for earth Bag<Car> matchbox = new Bag<Car>(Z4, “Dream”); public class Bag {private Cardata;private String name;public Bag(Car d, String newName) {data = d;name = newName;}public CargetData() { return data; }public void setData(Card){ data = d; }public String toString() { return name + “: ” + data.toString();} }

Using Generics Without Generics With Generics Integer 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();

Using Generics Without Generics With Generics Integer 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();

Using Generics Without Generics With Generics Integer 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();

Can Use Multiple Generic Types public class Entry<MONKEY, TYPE> { private MONKEYkey; 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 public class Entry<MONKEY, TYPE> { private MONKEYkey; private TYPE value;// And more goes here... } Entry<String, Integer> a; Entry<String> b;Did not specify for each type Entry<String, String> c; Entry<String, boolean> d; Entry<Car, Boolean> e;

Can Use Multiple Generic Types public class Entry<MONKEY, TYPE> { private MONKEYkey; private TYPE value;// And more goes here... } Entry<String, Integer> a; Entry<String> b;Did not specify for each type Entry<String, String> c; Entry<String, boolean> d; Not reference type Entry<Car, Boolean> e;

When To Specify Type • Whenever class name used(except constructors) • Variable declarations:ArrayList<Integer> hogCount; • Object instantiation:hogCount = new ArrayList<Double>(); • Return type for method :private ArrayList<Pig> transport() • Parameter listing:public void cook(ArrayList<Meat> fd) • Used as type parameter:ArrayList<ArrayList<Meat>> bacon;

Generics Annoyance • Type cannot be specified instantiatingarray • Compiler error if type specified during instantiation • Can provide type theory explaining this problem

Generics Annoyance • Type cannot be specified instantiatingarray • Can use generics with arrays, but need typecast • Only needed once, use generics after instantiation • Still checks when compiling, so get most benefitspublic class Farm<T> { private ArrayList<Feed>[] troughs; private T[] animals; public Farm() {troughs= (ArrayList<Feed>[])new ArrayList[10];animals = (T[])new Object[1034821]; }}

In Case of Unknown Type • 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 ?

Wildcard in Generic public class ListHolder {private ArrayList<?> myList;public void setList(ArrayList<?> lst){myList= lst;}public void printListSize() {System.out.println(myList.size());}public ArrayList<?> getList() { return myList;} }

Wildcard in Generic public class ListHolder {private ArrayList<?> myList;public void setList(ArrayList<?> lst){myList= lst;}public void printListSize() {System.out.println(myList.size());}public ArrayList<?> getList() { return myList;} } ?matches any reference type(and String)

Wildcard in Generic public class ListHolder {private ArrayList<?> myList;public void setList(ArrayList<?> lst){myList= lst;}public void printListSize() {System.out.println(myList.size());}public ArrayList<?> getList() { return myList;} } Any ArrayList can be passed in for lst

Wildcard in Generic public class ListHolder {private ArrayList<?> myList;public void setList(ArrayList<?> lst){myList= lst;}public void printListSize() {System.out.println(myList.size());}public ArrayList<?> getList() { return myList;} } Can call methods as long as missing type not important

Wildcard in Generic public class ListHolder {private ArrayList<?> myList;public void setList(ArrayList<?> lst){myList= lst;}public void printListSize() {System.out.println(myList.size());}public ArrayList<?> getList() { return myList;} } Legal, but yucky. All type information is lost!

Typecasting Explained

Typecasting Explained • 99% of typecasts are incorrect • 90% fix missing generic typespecification 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%

Your Turn • Get into your groups and complete activity

For Next Lecture • Midterm #1 next Monday for duration of class • Will be doing review on Friday, so bring your questions • Object-orientation quiz in class Wednesday • Will be just like quiz in class last week • Inheritance, chains, generics & more covered on quiz • As usual, there is weekly assignment on Angel • Due by 5PM tomorrow via Assignment Submitter • Problem #2 graded using provided JUnit tests • No week #5 assignment due to midterm

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