Review Generics and the ArrayList Class

Generics Added to Java v5.0 Generics = class and method

definitions may include parameters for types

Generic program – type parameter allows one to write code that applies to any class

Ex. list of items of type T When T=Double, it’s a list of Doubles, etc.

ArrayList Advantage:

Can grow or shrink (arrays can’t) import java.util.ArrayList; See

http://java.sun.com/j2se/1.5.0/docs/api/java/util/ArrayList.html

ArrayList Disadvantage:

1. Less efficient than arrays2. Lacks familiar [] syntax (Java

limitation, supported by C++)3. Base type must be an object (not

primitive) type Can have ArrayList of Integer but can’t

have an ArrayList of int. Less of a problem w/ automatic boxing and

unboxing Java limitation, supported by C++

ArrayList class Define an ArrayList of Strings:

ArrayList<String> list = new ArrayList<String>();

Define an ArrayList of Employees:

ArrayList<Employee> empList= new

ArrayList<Employee>( 20 ); Initial capacity is specified as 20.

ArrayList class Set an element:

list.set( 12, “Hi, Mom.” );

Get an element:

System.out.println( list.get( 12 ) ); String s = list.get( 5 );

ArrayList class

Add an element: add( value ) form

list.add( “One” ); list.add( “Two” ); list.add( “Three” );

“One” is at position 0, “Two” is at position 1, and “Three” is at position 2.

ArrayList class Add an element: add( i, value ) form

i must be a used position or the first unused position

list.add( “One” ); list.add( “Two” ); list.add( “Three” ); list.add( 1, “Fred” ); “One” is at position 0, “Fred is at position 1,

“Two” is at position 2, and “Three” is at position 3.

ArrayList class

size() method

for (int i=0; i<list.size(); i++) {System.out.println( list.get(i) );

}

ArrayList class methods

ArrayList class methods public ArrayList<Base_Type>( int

initialCapacity )

public ArrayList<Base_Type>() initial capacity is 10

ArrayList class methods

Array-like methods

public Base_Type set ( int index, Base_Type newElement )

where 0<=index<size() (or exception)

public Base_Type get ( int index ) where 0<=index<size() (or exception)

ArrayList class methods

Methods to add elements

public boolean add ( Base_Type newElement )

adds the new element to the end size() increases by 1 capacity increases if necessary

ArrayList class methods Methods to add elements

public void add ( int index, Base_Type newElement ) 0<=index<=size() when index==size(), inserts at end size() increases by 1 capacity increases if necessary when 0<=index<size(), current elements at

index move to index+1, at index+1 move to index+2, …, at size()-1 move to size()

ArrayList class methods

Methods to remove elements public Base_Type remove ( int index )

0<=index<size() (or exception) Removes element at index; shifts to the left

remaining elements at index+1 … size()-1. protected void removeRange ( int fromIndex, int

toIndex ) Removes elements with index i s.t.

fromIndex<=i<toIndex; shifts to the left remaining elements at toIndex … size()-1

ArrayList class methods

Methods to remove elements public boolean remove( Object

theElement ) if found then removes the first occurrence of

theElement; shifts the remaining elements to the left; size() becomes size()-1; returns true.

if not found then returns false. public void clear ( )

Removes all elements; size() becomes 0

ArrayList class methods Search methods

public boolean contains ( Object target ) Uses equals() method of target True if ArrayList contains target; false otherwise.

public int indexOf ( Object target ) Uses equals() method of target Returns index of first occurrence of target in

ArrayList; -1 otherwise. public int lastIndexOf ( Object target )

Same as above except index of last occurrence is returned.

ArrayList class methods Memory management (size & capacity)

public boolean isEmpty ( ) True if empty; false otherwise.

public int size ( ) Returns number of elements in ArrayList

public void ensureCapacity ( int newCapacity ) Increases the capacity (if necessary)

public void trimToSize ( ) Reduces capacity to current size

ArrayList class methods Make a copy

public Object[] toArray ( ) Returns an array of list elements (in order).

Public<Type> Type[] toArray ( Type[] a ) Returns an array of list elements (in order). If list will fit in array a, it will be used; remaining

elements of a will be null. If list won’t fit in array a, a new array will be created

and used. public Object clone ( )

Returns a shallow copy of the ArrayList.

ArrayList class methods

Equality public boolean equals ( Object other )

True only when both are of the same size, and both have the same elements (in the same order).

for-each loop

for (Array_Base_Type var : Collection_Object)

Statement;

Let myList be an ArrayList of Strings.for (String element : myList)

System.out.println( element );

for-each loop

The for-each loop also works with arrays:// Returns the sum of the elements of aint sum ( int[] a ) {

int result = 0;for (int item : a)

result += item;return result;

}

Generics

Generics

Classes and methods can have a type parameter (actually, more than one).

The type parameter can be any reference (class) type.

(started with Java version 5.0; in C++ as well but much restricted in Java)

Generic class = parameterized class = class with a parameter for a type

Generic methods

public class Utility {…public static<T> T getMidpoint ( T[] a ) {

return a[ a.length/2 ];}public static<T> T getFirst ( T[] a ) {

return a[0];}…

}

Generic methodspublic class Utility {

public static<T> T getMidpoint ( T[] a ) {return a[ a.length/2 ];

}public static<T> T getFirst ( T[] a ) {

return a[0];}

}//usage:String midString = Utility.<String>getMidPoint( b );double firstNumber = Utility.<Double>getFirst( c );

Generic (parameterized) classes

public class Sample<T> {private T data;public void setData ( T newData ) {

data = newData;}public T getData ( ) {

return data;}

}

Generic (parameterized) classespublic class Sample<T> {

private T data;public void setData ( T newData ) {

data = newData;}public T getData ( ) {

return data;}

}

//usage:Sample<String> object = new Sample<String>();object.setData( “Hello” );

Generic class for ordered pairs

Pair<String> secretPair= new Pair<String>( “Happy”, “Day” );

Pair<Integer> rollOfDice= new Pair<Integer>( new Integer(2),

new Integer(3) );Pet male = new Pet();Pet female = new Pet();Pair<Pet> breedingPair

= new Pair<Pet>( male, female );

Defining the ordered pair classpublic class Pair<T> {

private T first;private T second;public Pair ( ) {

first = null;second = null;

}public Pair ( T f, T s ) {

first = f;second = s;

}…

}

Defining the ordered pair class

public class Pair<T> {private T first;private T second;…//These are suggested by our author.//Why are they bad?public T getFirst ( ) { return first; }public T getSecond ( ) { return second; }…

}

Defining the ordered pair classpublic class Pair<T> {

private T first;private T second;…//These are suggested by our author.//Why are they bad?//potential privacy leaks!public T getFirst ( ) { return first; }public T getSecond ( ) { return second; }…

}

Defining the ordered pair classpublic class Pair<T> {

private T first;private T second;…public boolean equals ( Object other ) {if (other==null) return false;if (getClass() != other.getClass()) return false;Pair<T> o = (Pair<T>)other;return first.equals(o.first) && second.equals(o.second);}…

}

More then one type parameter can be specified

Pair<String,Integer>p = new Pair<String,Integer>( “Kyle Jones”,

new Integer(123456789) );

Defining the ordered pair classpublic class Pair<T1,T2> {

private T1 first;private T2 second;public Pair ( ) {

first = null;second = null;

}public Pair ( T1 f, T2 s ) {

first = f;second = s;

}…

}

Defining the ordered pair class

public class Pair<T1,T2> {private T1 first;private T2 second;…public T1 getFirst ( ) { return first; }public T2 getSecond ( ) { return second; }…

}

Defining the ordered pair classpublic class Pair<T1,T2> {

private T1 first;private T2 second;…public boolean equals ( Object other ) {if (other==null) return false;if (getClass() != other.getClass()) return false;Pair<T1,T2> o = (Pair<T1,T2>)other;return first.equals(o.first) && second.equals(o.second);}…

}

Bounds for type parameters

Say I wish to restrict my type parameter T to only those things that implement the Comparable interface. Can I do this?

What happens if I use the compareTo() method without stating this restriction?

Bounds for type parameters

Say I wish to restrict my type parameter T to only those things that implement the Comparable interface. Can I do this?

Yes. What happens if I use the compareTo()

method without stating this restriction? A compiler error occurs.

Bounds for type parameters

Example of stating this restriction:public class Pair<T extends

Comparable> {…

}The keyword extends is used rather than implements because T must be a class (not an interface).

Multiple bounds Multiple bounds may also be specified. For example,

public class AnotherGeneric<T1 extends Employee & Comparable,T2 extends Comparable> {

…}

But only 1 may be a class. The rest must be interfaces. Why?

Problem 1 In the sport of diving, seven judges award a

score between [0.0,10.0]. The highest and lowest scores are discarded. The remaining scores are added together. The sum is then multiplied by the degree of difficulty [1.2,3.8] for that dive. The result is then multiplied by 0.6 to determine the diver’s score.

Write a program that inputs the degree of difficulty and the 7 judges’ score, and outputs the diver’s score.

You are required to use an ArrayList of Double for the judges’ scores.

Problem 2 Write a program that uses an ArrayList of parameter

type Contact to store a database of contracts. The Contract class should store the contact’s first and

last name, phone number, and email address. Add appropriate accessor and mutator methods.

Your program should present a textual menu that allows the user to add a contact, display all contacts, search for a specific contact and displays it, or search for a specific contact and give the user the option to delete it. The searches should find any contact where any member variable contains a target search string. For example, if “elmore” is the search target then any contact where the first name, last name, phone number, or email address contains “elmore” should be returned for display or deletion.

Use the “for-each” loop to iterate through the ArrayList.

Problem 3 Many GPS’ can record waypoints. The waypoint

marks the coordinates of a location on a map along with a timestamp. Our GPS stores waypoints in terms of an (X,Y) coordinate on a map together with a timestamp t that records the number of seconds that have elapsed since the unit was turned on.

Write a program that allows the user to enter as many waypoints as desired, storing each in an ArrayList of a class that you design.

As waypoints are entered, calculate the average speed of the trip so far. (The distance between (0,0) to (1,0) is 0.1 miles.)

Problem 4 Write a generic class, MyMathClass, with a type

parameter T where T is a numeric object type (e.g., Integer, Double, or any class that extends java.lang.Number).

Add a method named standardDeviation that takes an ArrayList of type T and returns as a double the standard deviation of the values in the ArrayList.

Use the doubleValue() method in the Number class to retrieve the value of each number as a double.

Your program should generate a compile-time error if your standard deviation method is invoked on an ArrayList that is defined for non-numeric elements (e.g., String).

Additionally, use the for-each in the standardDeviation method.

Problem 5 Create a generic class with a type parameter that

simulates drawing an item at random out of a box. This class could be used for simulating a random drawing.

For example the box might contain Strings representing names written on a slip of paper, or the box might contain Integers representing a random drawing for a lottery based on numeric lottery picks.

Create an add method that allows the user of the class to add an object of the specified type along with an isEmpty method that determines whether or not the box is empty.

Finally, your class should have a drawItem method that randomly selects an object from the box and returns it. If the user attempts to draw an item out of an empty box, return null.

Write a main method that tests your class. To generate a random number x, where 0<=x<=1, use x = Math.random();.