prof. dr. max mühlhäuser dr. guido rößling
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Telecooperation/RBG
Technische Universität Darmstadt
Copyrighted material; for TUD student use only
Introduction to Computer Science ITopic 15: Class properties, access rights
and CollectionsProf. Dr. Max MühlhäuserDr. Guido Rößling
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Structure of this lecture
• Class properties
• Access privileges
• Collections (Arrays and collection types)
2
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
• Some properties should be shared by all instances of a class:– A counter stores how many instances of a
class were created– The next unique identification for an instance
(e.g., customer number)– Constants which may be useful to all objects
• Class attributes and class methods are features associated with the class itself (not its objects)
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
• Class features are declared with the keyword static
• Access with <Class>.<Identifier>– Within the class, one may omit the class name
• They may be accessed from anywhere the class is visible
• Known examples– Class attribute: System.out– Class method: public static void main(...)
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
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class Person {
static int personCount = 0;
Person( .. ) {
personCount++;
// ...
}
//...
initialization occurs before any method is executedinitialization occurs before any method is executed
static variable only exists once for all objects
static variable only exists once for all objects
Upon creation of a new person, the counter is updatedUpon creation of a new person, the counter is updated
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
// class Person (continued)
// ...
static int personCount() {
return personCount;
}
//...
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Retrieve the number of existing person objects
Retrieve the number of existing person objects
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
• Access via class name int count = Person.personCount(); System.out.println(count +" objects");
• Another typical applicationboolean isLargerThan(Circle b) { .. }static boolean isLargerThan(Circle a, Circle b) {..}
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compares two circlescompares two circles
receiver compared withanother circle objectreceiver compared withanother circle object
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class PropertiesClass methods may not
access instance features
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class Mistake {
int i;
static void main() {
something(i);
// …
}
}
Not possible unless an objectis created or "i" is aclass variable
Not possible unless an objectis created or "i" is aclass variable
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
Static Initialization<Static-Initializer> ::= static { <Statements> }
• Purpose: initialization of class attributes• Similar to constructors for objects• Useful if simple initialization is insufficient
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class Properties
execution upon loading the class definition
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public class InitializationExample { private static double[] vector
= new double[6]; // static initialization block static { for (int i = 0; i < vector.length; i++) { vector[i] = 1.0 / (i + 1); } } // ...}
public class InitializationExample { private static double[] vector
= new double[6]; // static initialization block static { for (int i = 0; i < vector.length; i++) { vector[i] = 1.0 / (i + 1); } } // ...}
Static Initialization
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Constants
• May only receive a value once – after that, read access only => immutable
• Constants are declared like variables but using the keyword final– private final int MAXIMUM_NUMBER_CARDS = 5;– A constant declaration must always have an
initialization part– Convention: capital letters using underscores to
separate words
11
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Structure of this lecture
• Class properties
• Access privileges
• Collections (Arrays and collection types)
12
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
• Features are declared with their intended visibility– <Modifier> ::= private | ε | protected | public– with the help of visibility modifiers one can determine which
clients may access an attribute, method or class
• Four levels of visibility– hidden elements (private)– package elements (no modifier)– internal elements (protected)– freely accessible elements (public)
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
private• Access possible only from within the class itself
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package (implicit; no modifier given)
• The class element is visible for all classes in the same package
• No access from outside the package is allowed
• Not even from heirs in another package!
Package as a collection of classes which “trust" each other, have high cohesion
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
protected• All classes in the same package may access
the element• Same for all heirs of the class (even if in a
different package)
Heirs as "trusted" clients are allowed to access implementation details of the ancestor
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public• Any class may access the element
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
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Sameclass
public
protected
package
private
Classes in other
packages
Heirs inother
packages
Samepackage
Heirs insame
package
Overview
Have the same access rights
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
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package A;public class AA {
public int one;protected int two;void three() { … }private void four() { … }
}
package A;public class AA {
public int one;protected int two;void three() { … }private void four() { … }
}
package B;class BA extends AA {
// one, two}
package B;class BA extends AA {
// one, two}
package A;class AB {
// one, two, three}
package A;class AB {
// one, two, three}
package B;class BB {
// one}
package B;class BB {
// one}
package A;class AC extends AA {
// one, two, three}
package A;class AC extends AA {
// one, two, three}
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
Class Modifiers
• Either public– Unique—worldwide– Only one class may be specified public per file– Filename must be ClassName + “.java”
• or implicit (no keyword given)– Only visible within the same package
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
Modifiers and Redefinition
• If a method is redefined, its visibility may only be extended
• Reason: substitutability– It must be possible to substitute an object of a more
special type for an object of a more general type
– The subclass must offer at least as many features as its ancestor
It must not hide redefined features from clients
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Access Privileges
Visibility using UML • UML offers "public", "protected" and "private"• public +• protected #• private -• no modifier visibility is undefined
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Class
-privateAttribute : AnotherClass
#protectedAttribute : int
+publicMethod(x: int) : boolean
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Structure of this lecture
• Class properties
• Access privileges
• Collections (Arrays and collection types)
21
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collection Types
• A frequently needed kind of data structure is a collection for storing multiple data items – either of exactly the same type– or of the same type (including subtypes)– or of various types (i.e., of type Object)
• Depending on intended use, the collections types…– support quick access to individual elements– provide support for keeping elements sorted– provide a discipline for accessing certain elements only– may grow on demand – etc.
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Wrapper Classes
• Primitive data types are not reference types Are not derived from Object, can not respond to
messages
Can not be assigned to variables of type Object
• Sometimes it is useful (e.g., for collections) to treat a primitive piece of data as if it were an object Wrapper Classes
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Values versus References
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Wrapper Classes
• Class name is derived from primitive data type, using a capital first letter– java.lang.Byte, java.lang.Short,
java.lang.Integer, java.lang.Long,
java.lang.Float, java.lang.Double,
java.lang.Character
• Wrapper objects are immutable– Value assignment happens via constructor
– Value inquiry uses a message <primitiveType>Value()
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Autoboxing - Autounboxing
• Since Java 1.5, an implicit conversion from int to Integer is possible (same for other types)
// AUTOBOXING
Integer a = 5;
// AUTO-UNBOXING
int i = a;
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Integer a = new Integer(5);
int i = a.intValue();
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections
Java offers a number of collection types, implementing the java.util.Collection interface, or one of the following sub-interfaces thereof (this list is incomplete!):
• Collection – A general interface for collections of objects – No restrictions/guarantees regarding duplicates/ordering/sorting,
etc.
• List (has implementations ArrayList, LinkedList, Vector)– Objects are ordered– May contain duplicates– Enables direct access to objects via an index
• Set (has implementation HashSet)– Objects may be contained only once
• SortedSet (has implementation TreeSet)– Same as Set but ordered, user may specify custom
comparison strategy for elements
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CollectionCollection
SortedSetSortedSet
SetSetListList
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Lists
• In the following, we will assume the use of a collection of type „E“ (for element); this type can be replaced by „appropriate“ types.– Details follow in the slide set about Generics
• The interface java.util.List contains these methods:– boolean add(E e)
• Appends element e to the list– void add(int pos, E e)
• Adds element e to the list at position pos; moves all succeeding elements back by one position
– boolean addAll(Collection c)• Appends all elements stored in collection c to the list
– boolean addAll(int pos, Collection c)• Adds all elements of collection c to list, starting at position
pos27
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Lists• void clear()
– Removes all elements from the list
• boolean contains(Object o)– Returns true if object o is contained in the list
• boolean containsAll(Collection c)– Returns true if all objects contained in c belong to the list
• E get(int pos)– Returns the element at position pos
• int indexOf(Object o)– Returns the first position at which o occurs in the list, else
-1. For the last position, use int lastIndexOf(Object o)
• boolean isEmpty()– Returns true if the list is empty
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Lists
• E remove(int pos)– Removes the element at position pos and returns it
• boolean remove(Object O)– Tries to remove object o from the list; returns true if
successful
• int size()– Returns the number of elements stored in the list
• Object[] toArray()– Returns an array containing all elements of the list
• Constructors in the subclasses:– Parameterless– With a Collections as a parameter: copies all values into the
list– Special cases (see the subtypes)
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Implementations of List
• java.util.LinkedList– Adds the following methods (this is only a subset!):
• void addFirst(E)• void addLast(E)• E getFirst()• E getLast()
• java.util.ArrayList– Elements are stored in an array– New methods (selection):
• void ensureCapacity(int minCapacity) – if the list can contain less than minCapacity elements, the array is adapted to grow
• void trimToSize() – shrinks the array to the list size
– New constructor: ArrayList(int initialCapacity) for the size
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Implementations of List
• java.util.Vector:– Essentially identical to java.util.ArrayList ( array-based)– Accesses are synchronized
• Avoids problems if two objects want to write or read/write at the same time
– „Standard class“ for many applications– Constructors:
• Vector() – creates a Vector of a default size• Vector(int c) – creates a Vector of size c• Vector(int c, int inc) – creates a Vector of size c; if this is not
enough, the Vector will grow by “inc” elements each time• The last constructor is to be preferred
– You should determine beforehand how many elements you expect!
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Set
• A Set represents a mathematical set– Thus, a given object can be contained at most once
• Also supports most of the methods we already know– boolean add(E e)– boolean addAll(Collection c)– void clear()– boolean contains(Object O)– boolean containsAll(Collection c)– boolean isEmpty()– boolean remove(Object O)– boolean removeAll(Collection c)– int size()– Object[] toArray()
• Insertion fails if the set already contains the object32
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collections: Set
• Concrete instances of Set:– java.util.HashSet: stores the data in a hash table (very
efficient access)– java.util.TreeSet: stores the data in a tree with an access
time of O(log n).
• There are additional specialized implementations– Please check the JDK API Documentation!
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Interface Map
Sometimes objects should not be accessible via a numeric index, but via a key (some unique, but otherwise arbitrary value) of some sort,e.g., access telephone number via "surname + name"
• This is supported by the interface java.util.Map• … and its sub-interface SortedMap
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MapMap
SortedMapSortedMap
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class: java.util.Hashtable• implements java.util.Map• Enables access to elements via a computed
key, e.g., "surname + name“• Key is first converted into a numeric index and
then used for efficient access– Very efficient access– Theory of hashing in ICS II
• Useful constructors– HashMap()
default constructor– HashMap(int size)
creates a HashMap of size size– HashMap(Map t)
creates a HashMap, containing all elements contained in the map referenced by "t"
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Class java.util.Hashtable
• Useful methods include
– Object put(Object key, Object value)stores "value" for retrieval with "key"
– Object get(Object key)retrieves object stored with "key"
– boolean containsKey(Object key)answers whether an object is stored with
"key"– boolean containsValue(Object value) answers whether "value" is stored in the table
– Object remove(Object key)removes "key" and the object associated
with it
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Iterators
• Java uses an Iterator to step through the elements in a collection
• Usually, you get the iterator by calling iterator() on the collection– This is true for all subclasses of the Collection interface– For a HashMap, retrieve the keys() first and then use
iterator()
• iterator() returns an instance of java.util.Iterator
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Iterators
• The Iterator type offers only three operations:– boolean hasNext() – are there more elements?
– Object next() – returns the next element, if there is one• Otherwise, a NoSuchElementException is thrown (see T18)• You should check first by using hasNext()
– void remove() – removes the last retrieved element• May not be supported by all implementing types• In this case, you get a UnsupportedOperationException
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Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Iterators
• Here is a practical example for using Iterators:
• There is also a variant of the for loop for this purpose:
• The loop uses an iterator to retrieve next()– This value is then assigned to o in each iteration
• Usable for iterable elements – most collections, arrays
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List intList = Arrays.asList(1, 2, 3); // create listint s = 0; for (Iterator it = intList.iterator(); // get Iterator it.hasNext(); ) // continue while elements exist s += (Integer)it.next(); // add next element to sum
for (Object o : intList) // get Iterator // continue while elements exist s += (Integer)o; // add next element to sum
Dr. G. RößlingProf. Dr. M. Mühlhäuser
RBG / Telekooperation©
Introduction to Computer Science I: T15
Collection Framework
• Due to the extent of this subject, and our limited time, we could only partially cover it
• Find out more on collections athttp://java.sun.com/docs/books/tutorial/collections/index.html
• Several specialized classes are available
• Before implementing a type yourself, check http://java.sun.com/javase/6/docs/technotes/guides/collections/index.html
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