. - . , .
.
class A {
int x;
...
}
class B extends A {
int y;
...
}
B b = new B();
A a = b; // : B => A
upcasting downcasting. (upcasting) ( ) (). . (. ). , .
(downcasting) . . -, , ,
B b1 = (B)a;
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, , . , , , , B, B A, - A, , , B.
. A , - . , A.
, , B A. , A, B A (upcasting), A B ( ).
- A, B (downcasting). , , .
Java instanceof. (downcasting) . , , .
if (a instanceof B)
b1 = (B)a;
, , instanceof.
(Issue , Book , Newspaper - , Journal ).
|
|
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2.
Issue Book . :
public class Issue {
String name;
public Issue(String name) {
this.name = name;
}
public void printName(PrintStream out) {
out.println(":");
out.println(name);
}
...
}
public class Book extends Issue {
String authors;
public Book(String name, String authors) {
super(name);
this.authors = authors;
}
public void printAuthors(PrintStream out) {
out.println(":");
out.println(authors);
}
...
}
-
Issue[] catalog = new Issue[] {
new Journal("Play Boy"),
new Newspaper(" "),
new Book(" ", "."), };
...
for(int i = 0; i < catalog.length; i++) {
if (catalog[i] instanceof Book)
((Book) catalog[i]).printAuthors(System.out);
catalog[i].printName(System.out);
}
. ( ), , . (upcasting). . , . instanceof , Book. , , .. printAuthors(...) Book, Issue.
12)
.
- , .
, , : (overloading) (overriding).
.
overloading , , . . . . , , . . . .
class X {
int f() {
...
}
void f(int k) {
...
}
...
}
X f(...), . , , , , . .
class Base {
int f(int k) {
...
}
...
}
class Derived extends Base {
|
|
|
int f(String s, int k) {
...
}
...
}
. Derived f(...). Base, Derived.
overloading overriding (, , ). (overriding) . , , .
class A {
int x;
int f(int a) {
return a+x;
}
...
}
class B extends A {
int y;
int f(int s) {
return s*x;
}
...
}
B b = new B();
A a = b; // : B => A
int c = a.f(10); //??? f(...) ???
. "a" A, B. , f() . B.f().
Java ( - ) , , , . .. , .
. (dynamic binding, late binding, run-time binding). C++ .
, . Issue Book .
public class Issue {
String name;
public Issue(String name) {
this.name = name;
}
public void print(PrintStream out) {
out.println(":");
out.println(name);
}
...
}
public class Book extends Issue {
String authors;
public Book(String name, String authors) {
super(name);
this.authors = authors;
}
public void print(PrintStream out) {
out.println(":");
out.println(authors);
super.print(out); //
}
...
}
, .
Issue[] catalog = new Issue[] {
new Journal("Play Boy"),
new Newspaper(" "),
new Book(" ", "."), };
...
for(int i = 0; i < catalog.length; i++) {
catalog[i].print(System.out);
}
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- print(...) Book super. .
Book. print(...) Book .
final ()
Java final, , , .
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final double pi = 3.14;
final static , .. , . pi .
static final double pi = 3.14;
(overriding) , final.
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13)
- . , , (static final - ).
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public interface XXX {
...
int f(String s);
}
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public class R implements Serializable, XXX {
...
}
R Serializable XXX.
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public class R implements Serializable, XXX {
...
public int f(String s) {
...
}
...
}
, f(...) public, R public. , public, . R .
, , .
- . , , - . , , . , , . - . (deployment) -, ( , , ).
, Java.
14)
