Java 程序设计 Java 程序设计 Java Programming Spring, 2013.

Java Java ProgrammingSpring, 2013

Chapter 4: Classes & ObjectsClass Declaration Class Modifiers Class Members Class Fields Object Creation Fields Initilazation Methods Package

Class Declaration ()The class is the fundamental programming unit of the Java programming language.Classes provide the structure for objects and the mechanisms to manufacture objects from a class definition.

Each object is an instance of a class.A program is defined by using one or more classes.

Class Declaration ()Example:

class Body {public long idNum;public String name;public Body orbits;

public static long nextID = 0;}

Class DeclarationSyntax():

class is a keyword for class declaration. When the compiler sees the word class, it understands that the word after class is the name for the class;The first letter of the name for the class must be a Unicode letter or underline _ . class ClassName { Declarations of class members }

Class Modifier ()Modifiers are reserved words to specify the properties of the data, methods, and classes and how they can be used.

Class modifier:public a public class is publicly accessible. Without this modifier a class is only accessible within its own package.abstract used to declare an abstract class. final a final class cannot be subclassed.strictfp Strict floating point , a strictfp class has all floating-point arithmetic, defined within the class, evaluated strictly.

Class Modifier ()A class declaration can be modified by several modifiers, and modifiers are allowed in any order.

A class cannot be both final and abstract!(Why?)

Class Members ()Fields the data variables associated with a class and its objects, which hold the state of the class or object;Methods contains the executable code of a class and define the behavior of objects.Nested classes & nested interfaces ()classes or interfaces declared within the declaration of another class or interface.

Fields ()Syntax(): ; = ;

class Body {public long idNum;public String name;public Body orbits;

public static long nextID = 0;}

Field Types () boolean char byte, short, int, long float, double

Modifier of FieldsAccess modifier ()private accessible only in the class in which it is declared;package members with no access modifier are accessible in classes in the same package; protected accessible in subclasses of the class and in the class itself. public accessible anywhere the class is accessible.

Modifier of Fieldsstatic to declare class variable shared by all objects of a class;final a final variable cannot be changed after it has been initialized;transient transient(, ) variable cannot be serialized;volatile a read of volatile variable by different threads always returns the most recent value.

A field cannot be both final & volatile!

final Fields()final The value of a final variable cannot be changed after it has been initialized.used to define an immutable() property which doesnt change for the lifetime of a class or object.

Fields ()static

()static

Field Initialization ()When a field is declared, it can be initialized by assigning it a value of the corresponding right type.Initializer() can be:a constant, another field, a method invocation, or an expression.

Field Initialization ()Example:

double zero = 0.0; //constantdouble sum = 4.5 + 3.; //constant expressiondouble zeroCopy = zero; //fielddouble rootTwo = Math.sqrt(2); //method invocationdouble someVal = sum + 2*Math.sqrt(rootTwo); //mixed

Field Initialization ()Default() InitializationIf a field is not initialized, a default initial value is assigned to it depending on its type.

//Student.java

class Student{ String name = zhang;// String stuNumber; name=zhang; // void setBase(String s1, String s2){ name=s1; stuNumber=s2; System.out.println(name+" "+stuNumber); }

}

Initialization Blocks ()A block of statement that appears within the class declaration, outside of anything else of the class;To perform complex initialization of fields;It is executed at the beginning of every constructor in the class;Example: {idNum = nextID++;}

Initialization Blocks ()class Body{ public long idNum; public String name=; public Body orbits=null; public static long nextID=0;

//Initialization Blocks () {idNum=nextID++; }

Body(String name){ this.name=name; }}

Class Variables/Static Fields/A static field shared by all objects of a class;only one copy of the field exists.Declaration: static type

public class Circle {static double PI = 3.14159; //Static fieldint radius;}

/radius CircleOne copy for each instanceOne copy for class Circle

Static Initialization Block()Only be used to initialize static variables;A static initialization block is declared static in the form of a non-static initialization block;The static initializers are executed when the class is loaded;

Static Initialization Block() class Evens{ static int[] evens;

static{ //Static Initialization block () evens = new int[4];

evens[0]=0; for(int i=1;i

Class Variables/Static Fields/Access():Within its own class a static field can be referred to directly;When accessed externally it must be accessed via the class name.

System.out.println(Circle.PI); not recommendedCircle circle=new Circle();System.out.println(circle.PI);

//static:Stu.java, Student.javaclass Student{ String name, stuNumber; double score1, score2, score3;static int count= 0;//

void setBase(String s1,String s2) { name=s1; stuNumber=s2; System.out.println(name+" " + stuNumber); }}public class Stu{ public static void main(String args[]){Student s1=new Student( ); //s1.count++; //System.out.println(s1.count);Student s2=new Student( );s2.count++; //System.out.println(Student.count); // }}12

Constructors ()a class can have more than one constructor;have the same name as the class;take zero or more arguments;can be used to initialize an object;like method with no return type.

Example:class Point{int x,y;Point() //no-arg constructor {}Point(int a, int b){ //Constructor with arguments x=a; y=b;}}

ConstructorsDefault() constructorconstructor with no arguments are called no-arg constructors (); Java system automatically provides a default no-arg constructor that does nothing if the programmer doesnt provide any constructors;If both a no-arg constructor and one or more constructors with arguments are wanted in a class, the programmer must explicitly provide a no-arg constructor.

Exampleclass Point{ int x, y;}

Point p = new Point( );How to create an object of Point?

//class Point {int x,y;

Point(int a,int b){x=a;y=b;}}public class A {public static void main(String args[ ]) {Point p1,p2;p1=new Point(10,10);p2=new Point();}}// ?

ConstructorsExplicit constructor invocation () : this() invokes() another constructor from the same class as its first executable statement;

The type and number of arguments are used to determine which constructor gets invoked.class Body{ . Body(){ idNum=nextID++; }

Body(String name){ this( ); //must be at the first line this.name=name; }}

this :

class Point{int x,y,z;Point(){ }Point(int a,int b){x=a;y=b;}

Point(int a,int b,int c){//this(a,b); x=a;y=b; z=c;}}thisPoint(int a,int b,int c){ this(a,b); //must be at the first line z=c;}

//=new ([])//

=new([])

Class Declaration ()Example:

class Body {public long idNum;public String name;public Body orbits;

public static long nextID = 0;}

Creating ObjectsEg.: Body earth;earth = new Body();

create an object by invoking a constructor and its arguments if there is any;the runtime system allocates enough space to store the fields of the object;initialize the objects variables;

//Point.javaclass Point{int x,y;

Point(int a,int b){ // x=a; y=b;}}//A.javapublic class A{public static void main(String args[]){Point p1, p2, p3;p1=new Point(10,10);p2=new Point(5,6);p3=p1;}}

()1)Point p1;2)p1 = new Point(10, 10);x, yp1p1p1null0xAB12

3)p1=new Point(10, 10);p2=new Point(5, 6);

Methods()A method consists of two parts: the method header ;the method body;

The method header consists of a set of modifiers, the method return type, signature and a throws clause for exceptions;The method signature consists of the method name and the parameter type list enclosed in ( ); The method body consists of statements enclosed between { and }. () { /*method body*/ }

Example:

class Body {public long idNum;public String name;public Body orbits;public static long nextID = 0;

Body(String s){this.Name = s;}}

Methods ()class BodyPrint{ public static void main(String[] args) { Body sun=new Body(Sun); Body earth=new Body(Earth); earth.orbits=sun; System.out.println(Body +earth.name+ orbits +earth.orbits.name) }}

Methods ()Methods can have a return type, either a primitive type or a reference type;If a method does not return any value, the place where a return type would go is filled with a void.

Method Modifiers()access modifiers: same as modifiers of variables;abstractstaticfinalsynchronizednativestrictfp: strict floating point

An abstract method cannot be static, final, synchronized, native, or strict.

Static Methods/Class Methods/A static method is invoked on behalf of an entire class, not on a specific object.A static method might perform a general task for all objects of the class;

A static method can access only static fields and other static methods of the class;

()(not recommended)

()class A {int a;static int b;

void f(int x,int y) {a=x;b=y;}

static void g(int z){///b=23;a=z; //legal or not?}}//

Method Invocation()How to invoke a method:

reference. method(arguments)

When a method is invoked, the caller must provide an argument of the appropriate type for each of the parameters declared by the method.

Method Execution() & ReturnWhen a method is invoked, the statements of the method are executed;A method completes execution and returns to the caller when:a return statement is executed;the end of the method;an uncaught exception is thrown.

A method can only return a single result if a method returns any result.

Overload()int add(int i,int j){ return i+j; }

float add(float i,float j){ return i+j; }

int add(int i,int j,int k){ return i+j+k; }

Parameter() ValuesParameter Passing()Pass by value()For all premitive types, values of parameter variables in a method copies of the values of arguments passed.Pass object reference by value()When the parameter is an object reference, the object reference() is passed by value;the object referred to by the reference can be changed.

//Pass by value()class PassByValue{ public static void main(String[] args){ double one=1.0; System.out.println(before: one=+one); halveIt(one); System.out.println(after: one=+one); }

public static void halveIt(double arg){ arg=arg/2.0; System.out.println(havled: arg=+arg); }}before one=1.0halved arg=0.5after: one=1.0

Example:

class Body {public long idNum;public String name;public Body orbits;public static long nextID = 0;

Body(String s){this.name = s;}}

//Pass object reference by value()class PassRef{

public static void main(String[] args){ Body sirius=new Body("Sirius"); // System.out.println("before: "+sirius.name); commonName(sirius); System.out.println("after: "+sirius.name); }

public static void commonName(Body bodyRef){ bodyRef.name="Dog Star";// bodyRef=null; }}before: Siriusafter: Dog Star

AssignmentsComplete exercises as follows:P51 Exercise 2.9 P54 Exercise 2.10 P59 Exercise 2.12P59 Exercise 2.14 P61 Exercise 2.16 P62 Exercise 2.17

class Xiyoujirenwu{float height,weight;String head,ear,hand,foot,mouth;void speak(String s){head="";System.out.println(s);}}class E_monkey{public static void main(String args[]){Xiyoujirenwu zhubajie,sunwukong;zhubajie=new Xiyoujirenwu();sunwukong=new Xiyoujirenwu();zhubajie.height=1.80f;zhubajie.weight=160f;zhubajie.hand="";zhubajie.foot="";zhubajie.head="";zhubajie.ear="";zhubajie.mouth="";sunwukong.height=1.62f;sunwukong.weight=1000f;sunwukong.hand="";sunwukong.foot="";sunwukong.head="";sunwukong.ear="";sunwukong.mouth="";System.out.println("zhubajie"+zhubajie.height);System.out.println("zhubajie"+zhubajie.head);System.out.println("sunwukong"+sunwukong.weight);System.out.println("sunwukong"+sunwukong.head);zhubajie.speak("");System.out.println("zhubajie"+zhubajie.head);sunwukong.speak("1000");System.out.println("sunwukong"+sunwukong.head);}}