Java genenic Fan Form

One reason to use the paradigm:

For example: Now you want to set up a point-and-click class with X-coordinates and y-coordinates. The two properties of this point require X, y to hold an int, float, and string type. Then you can do the following design, the type of x, Y is set to object to realize the arbitrary parameter

classPoint {PrivateObject x; PrivateObject y;  PublicObject GetX () {return  This. x; }         PublicObject GetY () {return  This. Y; }         Public voidSetX (Object x) { This. x =x; }         Public voidsety (Object y) { This. y =y; }}

1. representing coordinates in integers

 Public classTest { Public Static voidMain (string[] args) {point P1=NewPoint (); P1.setx (3);//automatic boxing operation int integer ObjectP1.sety (5);//automatic boxing operation int integer Object//remove x, y coordinates         intx = (Integer) p1.getx ();//Automatic unpacking operation Interger int        inty = (Integer) p1.gety ();//Automatic unpacking operation Interger intSystem.out.println (x);    System.out.println (y); }        }/*Error: 1. Integer i is not capitalized, note that the first letter of each word in the class name is uppercase. */

Output: 3

5

2. Using strings to represent coordinates

 Public class Test {    publicstaticvoid  main (string[] args) {        new Point ();        P1.setx ("Longitude 3");        P1.sety ("Latitude 5");                 // remove x,    y coordinates         string x = (String) p1.getx ();         = (String) p1.gety ();                SYSTEM.OUT.PRINTLN (x);        System.out.println (y);    }        }

Output: Longitude 3

Latitude 5

3. If there is a transition error like the following

 Public class Test {    publicstaticvoid  main (string[] args) {        new Point ();        P1.setx ("3");        P1.sety ("Latitude 5");                 // remove x,    y coordinates         int x = (Integer) p1.getx ();         int y = (Integer) p1.gety ();                SYSTEM.OUT.PRINTLN (x);        System.out.println (y);    }        }

The syntax of the program is not a problem and can be compiled correctly. An error occurs while the program is running

Output: java.lang.ClassCastException:java.lang.String cannot is cast to Java.lang.Integer

Because the properties in the point class are received using the object class, there is a security issue, a generic reference to solve the problem.

Use of two generic types

The principle of definition: primarily when a class declaration identifies an attribute type in a class or the return type of a method by an identity, the form parameter type

1. Generic parameters

classPerson<i,n>{I age;    N name;        String Skincolor; Person (I age,n name,string skincolor) { This. Age =Age ;  This. Name =name;  This. Skincolor =Skincolor; }             PublicString toString () {return  This. age.tostring () + "" + This. name.tostring () + "" + This. skincolor.tostring (); }} Public classTest { Public Static voidMain (string[] args) { person<Integer,String> P1 =NewPerson<integer,string> (25, "Lei Feng", "Black");        System.out.println (P1.tostring ()); }}/*Error: 1. Person<int,string> It's supposed to be typed as an integer.*/

Output: 25 Lei Feng Black

The person class above is a generic class because it is used in the constructor method of person.

2. Generic interface

Application One: Specify the specific type directly in the interface

Comparable interface and CompareTo are defined as follows

Interface comparable<t>
int CompareTo (T o)

classPersonImplementsComparable<person> {    intAge ;    String name; Person (intage,string name) {         This. Age =Age ;  This. Name =name; }             PublicString toString () {return  This. Age + "" + This. Name; }         Public intcompareTo (person p) {if( This. Age >p.age) {return1; }    Else if( This. Age <p.age) {return-1; }Else {            return  This. Name.compareto (P.name); }    }} Public classTest { Public Static voidMain (string[] args) {person P1=NewPerson (21, "Zhang San"); Person P2=NewPerson (22, "John Doe");        System.out.println (P1.compareto (p2)); }}/*error: 1. this.age.toString () + "" + this.name; Error: cannot dereference int*/

Output: –1

Java genenic Fan Form