Java Language Features

Transcription

1 Java Language Features References: Object-Oriented Development Using Java, Xiaoping Jia Internet Course notes by E.Burris Computing Fundamentals with Java, by Rick Mercer Beginning Java Objects - From Concepts to Code by Jacquie Barker Object-Oriented Design & Patterns, Cay Horstmann Sun white papers on Java 9/7/ Basic elements Character set Identifiers Primitive types and literals Floating point types Character type Character literals 9/7/ Data Types Two categories of data types in Java: 1. primitive : hold exact value 2. reference : hold pointer to object A reference data type represents a reference to an instance of a class. primitive types are mainly used to manipulate data but they can't be stored in a collection or passed as a reference 9/7/

2 Primitive Types Char 2 bytes Unicode no unsigned int type boolean and int are not interchangeable default type for floating point literals is double 9/7/ Unicode universal character encoding standard used for the representing and processing text. provides a unique number for every character, for any platform, for any program, for any language 16-bit encoding standard Can represent the characters and symbols included in most of the languages of the world. For more information about Unicode check out the Unicode web site at 9/7/ Variables A variable is an instance of a primitive or reference type. In the diagram, i is primitive. References act like pointers but don't support all of the features associated with pointers as they are used in other languages such as C++ (e.g. pointer arithmetic). 9/7/

3 Variables Variable names are represented in Unicode. default values for built-in types: boolean answer = false; int i=0; double rate = 0.0; char ch = ; //blank space String name = null; 9/7/ Naming conventions item productnumber nameofthiscustomer A long name is preferable to a confusing short one! 9/7/ Variable, Object, Class, Type Variables have types, objects have classes. A variable is a storage location and has an associated type. An object is an instance of a class or an array. The type of a variable is determined at compilation time. The class of an object is determined at run time. null reference: an invalid object object reference: an object whose class is assignment compatible with the type of the variable. 9/7/

4 Constants final int MAX_VALUE = 3; You can assign a value to a final variable only once. You don't have to assign a value in the definition (as shown), but it's good programming style. Final variables are ALL CAPS. 9/7/ Other Operators Array declaration/access ([ ]) new (create new object or array) String [] names = new String[3]; 9/7/ Other Operators instanceof Tests whether a reference is of a particular type or is to a class that implements a particular interface Object obj = e.nextelement(); if (obj instanceof Shape) { Shape s = (Shape) obj; 9/7/

5 Other Operators cast operator (type). Converts from one type to another. This example casts a reference to an Object to a reference to a Shape. Shape s = (Shape) e.nextelement(); Precedence chart - use of parentheses overrides 9/7/ Blocks Statements within {'s form a block. A block can be used anywhere a statement can be used. Blocks define the scope of variables. variables defined within a block are not known outside of the block they are defined in 9/7/ Blocks public class test { public static void main(string[] args) { if (args.length > 1) { int x = 0; // x is only known within this block else {... 9/7/

6 Control Can reuse for-loop index variables. For example: for(int i=0;i<5;i++){ for(int i=0;i<20;i++){ 9/7/ String literals String is NOT a primitive type Constants written as literals a string literal In output statements: System.out.println ( first part of string + last part of string ); 9/7/ Arithmetic Integer arithmetic never overflows or underflows If value exceeds range, it will be wrapped modulo the range For division by 0, an ArithmeticException is thrown Floating point arithmetic errors never throw exceptions see p.83 9/7/

7 Pointers vs Object References Java always uses pointers. They re called references An object variable is a reference to an object i.e., a value that describes the location of the object. Java C++ C++ Bank b; Bank *b; (dynamic) Bank b; (static) b = new Bank(); b = new Bank(); In Java, only the dot operator is used for dereference (no arrows or *s). Java C++ C++ b.setbalance(100); b->setbalance(100); b.setbalance(100); (*b).setbalance(100); 9/7/ Object References Multiple variables can store references to the same object. For example, after the following statements: Bank b = new Bank(); Bank anotherbank = b; both object variables refer to the same object 9/7/ Java References Pointers in Java are safe You cannot create invalid pointers The garbage collector automatically reclaims unused objects -- you can't have a memory leak. 9/7/

8 Object reference: this this refers to the current object on which the method was invoked. Assume class Bank has the method public void setname (String name) { this.name = name; then bank.setname( First National ) sets the name field of the Bank object to the value of the explicit parameter which is also called name. Use of this resolves ambiguity between the name field and name parameter 9/7/ this It's commonly used to pass the object itself as a parameter alist.insert(this); It can also be used to access hidden variables: public class Point { public int x, y; public Point(int x, int y) { this.x = x; this.y = y; 9/7/ Java super Method keyword super refers to the superclass or parent class of a class. super is a reference you can use to call a method in the parent class. public void paintcomponent(graphics g) { super.paintcomponent(g); 9/7/

9 Java super Method The reference super is also used to call a specific constructor in a parent class. public class TypeSafeVector extends Vector { This Class s Constructor TypeSafeVector(int size, Object elementtype) { super(size);... Vector class constructor 9/7/ Java vs C++ Java objects don't have destructors. Java doesn't have a pre-processor. Java has no typedefs, #defines or structs 9/7/ Java vs C++ Java has no arithmetic operator overloading. Instead, declare a class with appropriate instance variables, and appropriate methods to manipulate those variables. 9/7/

10 Java vs C++ : Exceptions Exception handling is mandatory in Java. If you do something that causes ( throws ) an exception, then the program must handle ( catch ) it. 9/7/ Exception Handling When an exception occurs, the runtime system takes control and looks up the call stack for an exception handler. If it finds one, it transfers control to the exception handler. If it doesn't find one, the default behavior is to terminate the program. 9/7/ Java vs C++ No multiple inheritance. Java uses interfaces to implement multiple inheritance. An interface is a definition of a set of methods that one or more classes will implement. An important issue of interfaces is that they declare only methods and constants. Variables may not be defined in interfaces. (Sun) 9/7/

11 String 9/7/ String class Strings are Java programming language objects A length() accessor method is provided to obtain the number of characters in the string. The + operator indicates string concatenation. 9/7/ String class two kinds of string objects: 1. The String class is for read-only (immutable) objects. 2. The StringBuffer class is for string objects you wish to modify (mutable string objects). Can create a string in two ways: String stra = stringa ; //preferred method String stra = new String( stringa ); 9/7/

12 String comparison s1 == s2 tests equality of two references If true, then s1 and s2 point to the same memory location holding a string object s1.equals(s2) If true, then the string values referenced by s1 and s2 are the same values 9/7/ String methods Many listed on p.119 of Jia text tostring() method of a class defines a string representation of the instances of the class Programmer defined If not defined, returns the OID for the class instance 9/7/ The tostring() Method The tostring() method converts objects to strings. public class Point { public int x, y;... String tostring() { return "(" + x + "," + y + ")"; Then, you can do Point p = new Point(10,20); System.out.println("A point at " + p); // Output: A point at (10,20) 9/7/

13 Collections (ver. 5.0) ArrayList LinkedList Array Vector 9/7/ Enhancements in 5.0 (Sun) Generics - adds compile-time type safety to the collections framework and eliminates the need to cast when reading elements from collections. Enhanced for loop - eliminates the need for explicit iterators when iterating over collections. Autoboxing/unboxing - automatically converts primitives (such as int) to wrapper classes (such as Integer) when inserting them into collections, and converts wrapper class instances to primitives when reading from collections. 9/7/ java.util.arraylist lets you collect a sequence of objects of any type add method inserts new objects at the end of the array list ArrayList<String> namelist = new ArrayList<String>(); namelist.add( Hong ); namelist.add( Juan ); namelist.add( Gracie ); Hong Juan Gracie /7/

14 ArrayList Methods size : returns number elements in array list get : returns element at i th position as anobject for (int i=0; i<namelist.size(); i++) { String name = (String) namelist.get(i); Must cast returned object set : lets you overwrite an existing element namelist.set(2, George ); If a nonexistent position is accessed, an IndexOutOfBounds Exception is thrown 9/7/ ArrayList Methods remove : takes out an element from middle of array list namelist.remove(1); add(i,object) : adds element in middle of array list namelist.add(1, Olivia ); These operations are not efficient, since they require that remaining elements be moved up or down Hong Olivia Juan Gracie /7/ LinkedList class Doubly-linked list implementation of the List interface List<String> namelist = new LinkedList<String>(); supports insertion and removal of objects efficiently from any location can be used as stack, queue, or double-ended queue (deque) add method inserts elements at end of linked list remove takes element from end of linked list 9/7/

15 class Iterator Accessing elements in the middle of the list is done with an iterator Iterator objects can access a position anywhere in the list ListIterator<type> iterator = namelist.iterator(); next() advances iterator to next position of the list and returns element the iterator just passed hasnext() tests if iterator has already passed the last element in the list 9/7/ LinkedList class while (iterator.hasnext()) { String name = iterator.next(); To traverse the list again, after reaching the end of the list, define another iterator 9/7/ For-each loop replaces iterator Pre-java 5: public static void ShowAll(List list) { for (Iterator it = list.iterator(); it.hasnext();) { System.out.println (it.next()); Enhanced for: public static void ShowAllAndNice(List list) { for (Object obj : list) { System.out.println (obj); Example from 5#autoboxing 9/7/

16 Arrays Can hold primitive types (int,float ) Arrays are first-class objects; that is, they have an internal runtime representation in memory. An exception is generated if the index is outside the bounds of the array Elements are initialized to zero, false or null 9/7/ Declaring Arrays int[ ] test = new int[100]; String[ ] name = new String[15]; BankAccount[ ] customer = new BankAccount[100]; public static void main(string [ ] argv); 9/7/ Arrays initializing without using new int numbers = {2,4,6,8,10; The length variable returns the capacity of an array. System.out.println(numbers.length); //returns 5 If size is exceeded, you must construct a new array and move elements from old array to new one 9/7/

17 Arrays as parameters passed like they are in C++ public static void main (String[ ] args) args is an array that holds input strings typed at the keyboard when a program is launched from the command line. For example: java ComputeResults in.data 9/7/ Vector class Alternative to arrays automatic resizing To use, must import java.util.* or java.util.vector Instantiating the Vector class: Vector<Object) coursestaken = new Vector<Object>(); Adding courses to the vector: Course c1 = new Course ( CS101 ); Course c2 = new Course ( CS191 ); coursestaken.add(c1); coursestaken.add(c2); How many have been added: int courses = coursestaken.size(); 9/7/ Vectors and Casting Vectors hold Object references Object class (in the java.lang package) is the parent class to all other classes. It is the root of the entire Java inheritance hierarchy. When a vector is accessed, it returns a generic Object reference. The programmer must cast the result to the appropriate type of data(i.e. class) that is stored in the vector. 9/7/

18 Example:Vectors and Casting Assume that we have added 5 Course objects to the vector coursestaken. To retrieve those courses, we can use a forloop: for (int i=0; i<coursestaken.size(); i++) { Course c = (Course) coursestaken.elementat(i); System.out.println(c.getName()); 9/7/ Print using Enumeration for (Enumeration e = coursevector.elements() ; e.hasmoreelements() ;) { System.out.println(e.nextElement()); 9/7/ Some(not all) Vector Methods add(object): adds object reference to end of vector add(int, Object) : adds object reference to position in vector indicated by integer argument set(int,object) : replaces nth object reference with specified object reference elementat(int) : retrieves nth object reference as type Object (requires cast) removeelementat(int) : takes out nth reference 9/7/

19 Some(not all) Vector Methods remove(object) :removes first occurrence of Object reference indexof(object) : returns an integer indicating the first position of the object reference in the vector,if found contains(object) : returns true if object reference found in the vector; false otherwise isempty() : returns true if vector is empty; false otherwise clear() : empties the vector 9/7/