Download public class

Der Java-Experten-Kurs
Constants, Inlining, Classloaders
Karl Pauls
Final is not Final anymore
private static void change(Book p, String name, Object value)
throws NoSuchFieldException, IllegalAccessException {
Field field = Book.class.getDeclaredField(name);
field.setAccessible(true);
field.set(p, value);
}
public class Book {
private final String title;
private final String author;
private final int year;
private final int numberOfPages = 500;
private final Object publisher = "Addison Wesley";
Final is not Final anymore
public static void main(String[] args) {
Book book = new Book("Core Java(TM) 2",
”Gary Cornell", 2000);
change(book, "title", "Beginning C++");
change(book, "author", "Michael Dawson");
change(book, "year", new Integer(2005));
change(book, "publisher", "Prentice Hall");
change(book, "numberOfPages", new Integer(1000));
System.out.println(book);
java.lang.IllegalAccessException: Field is final
Book title: Core Java(TM) 2
Author: Gary Cornell
Year: 2000 Publisher: Addison We
Number of pages: 500
Final is not Final anymore
public static void main(String[] args) {
Book book = new Book("Core Java(TM) 2",
”Gary Cornell", 2000);
change(book, "title", "Beginning C++");
change(book, "author", "Michael Dawson");
change(book, "year", new Integer(2005));
change(book, "publisher", "Prentice Hall");
change(book, "numberOfPages", new Integer(1000));
System.out.println(book);
Jdk 1.5
Book title: Beginning C++
Author: Michael Dawson
Year: 2005 Publisher: Prentice Hall
Number of pages: 500
Constants
public class Main implements InterfaceA {
public static void main (String [] args)
{
System.out.println (A);
}
}
public interface InterfaceA{
public static final int A = 1;
}
Method void main(java.lang.String[]) 0 getstatic #23 <Field java.io.PrintStream out>
3 iconst_1_ 4 invokevirtual #29 <Method void println(int)>
7 return
Constants
public interface InterfaceA{
public static final int A =
new java.util.Random ().nextInt ();
}
Method void main(java.lang.String[])
0 getstatic #27 <Field java.io.PrintStream out>
3 getstatic #31 <Field int A>
6 invokevirtual #37 <Method void println(int)>
9 return
Inlined
Constants
public interface StaticFinalTest {
String LITERAL = "Literal";
String LITERAL_PLUS = "Literal" + "Plus";
String LITERAL_NEW = new String("LiteralNew");
String LITERAL_CONCAT = "LiteralConcat".concat("");
}
public class StaticFinalTestClient {
public static void main(String[] args) {
System.out.println(StaticFinalTest.LITERAL);
System.out.println(StaticFinalTest.LITERAL_PLUS);
System.out.println(StaticFinalTest.LITERAL_NEW);
System.out.println(StaticFinalTest.LITERAL_CONCAT);
}
}
Literal
LiteralPlus
LiteralNew
LiteralConcat
Constants
public interface StaticFinalTest {
String LITERAL = "LiteralXXX";
String LITERAL_PLUS = "Literal" + "PlusXXX";
String LITERAL_NEW = new String("LiteralNewXXX");
String LITERAL_CONCAT = "LiteralConcat".concat("XXX");
}
Was passiert wenn man nur StaticFinalTest aber nicht
StaticFinalTestClient neu übersetzt?
Constants
Literal
LiteralPlus
LiteralNewXXX
LiteralConcatXXX
public interface StaticFinalTest {
String LITERAL = "LiteralXXX";
String LITERAL_PLUS = "Literal" + "PlusXXX";
String LITERAL_NEW = new String("LiteralNewXXX");
String LITERAL_CONCAT = "LiteralConcat".concat("XXX");
}
-> Immer alles !
Constants
public interface InterfaceA{
public static final int A = 2 * InterfaceB.B;
}
public interface InterfaceB{
public static final int B = InterfaceC.C + 1;
}
public interface InterfaceC extends InterfaceA{
public static final int C = A + 1;
}
public class Main implements InterfaceA,
InterfaceB, InterfaceC {
public static void main (String [] args){
System.out.println (A + B + C);
7
}
}
Constants
public interface InterfaceA{
public static final int A = 2 * InterfaceB.B;
}
public interface InterfaceB{
public static final int B = InterfaceC.C + 1;
}
public interface InterfaceC extends InterfaceA{
public static final int C = A + 1;
}
public class Main implements InterfaceA,
InterfaceB, InterfaceC{
public static void main (String [] args) {
System.out.println (C + B + A);
6
}
}
Namen von Klassen
//: a1/A.java
public class A {
public String toString() {
return "This is the first class";
}
}
//: a2/A.java
public class A {
public String toString() {
return "This is the second class";
}
}
public class NormalTest {
public static void main(String[] args) {
System.out.println(new A());
}
}
Classpath
> javac -classpath .;a1 NormalTest.java
> java -classpath .;a1 NormalTest
This is the first class
> java -classpath .;a2 NormalTest
This is the second class
Kann man auch beides haben?
Classloader
ClassLoader loader = new NetworkClassLoader(
host, port);
Object main =
loader.loadClass("Main", true).newInstance();
class NetworkClassLoader extends ClassLoader {
String host;
int port;
public Class findClass(String name) {
byte[] b = loadClassData(name);
return defineClass(name, b, 0, b.length);
}
private byte[] loadClassData(String name) {
// load the class data from the connection
}
}
Classloaders
import java.net.*;
public class Loader {
public static void main(String[] args) throws Exception {
ClassLoader a1 = new URLClassLoader(new URL[] {
new URL("file:a1/")}, null);
ClassLoader a2 = new URLClassLoader(new URL[] {
new URL("file:a2/")}, null);
Class c1 = a1.loadClass("A");
Class c2 = a2.loadClass("A");
System.out.println("c1.toString(): " + c1);
System.out.println("c2.toString(): " + c2);
System.out.println("c1.equals(c2): " + c1.equals(c2));
System.out.println("c1.newInstance(): " + c1.newInstance());
System.out.println("c2.newInstance(): " + c2.newInstance()); }}
Classloaders cont.
c1.toString(): class A
c2.toString(): class A
c1.equals(c2): false
c1.newInstance(): This is the first class
c2.newInstance(): This is the second class
Parent Classloader
public class Parent {
public String toString() {
return "Thanks for caring... but what do you want??? ";
}}
//: a1/A.java
public class A extends Parent {
public String toString() {
return super.toString() + "This is the first class";
}}
//: a2/A.java
public class A extends Parent {
public String toString() {
return super.toString() + "This is the second class"; }}
Parent Classloader cont.
import java.net.*;
public class Loader {
public static void main(String[] args) throws Exception {
ClassLoader parent = new URLClassLoader(new URL[] {
new URL("file:./")}, null);
ClassLoader a1 = new URLClassLoader(new URL[] {
new URL("file:a1/")}, parent);
ClassLoader a2 = new URLClassLoader(new URL[] {
new URL("file:a2/")}, parent);
Class c1 = a1.loadClass("A");
Class c2 = a2.loadClass("A");
System.out.println(c1.newInstance());
System.out.println(c2.newInstance());
System.out.println(c1.getSuperclass().equals(c2.getSuperclass()));
System.out.println(Class.forName("Parent").equals(c1.getSuperclass()));
try { Parent p = (Parent)c1.newInstance(); } catch(ClassCastException ex) {
ex.printStackTrace(); }
System.out.println("We expected to get ClassCastException");
}}
Parent Classloader cont.
Thanks for caring... but what do you want??? This is the first class
Thanks for caring... but what do you want??? This is the second class
True
False
java.lang.ClassCastException: A
at Loader.main(Loader.java:18)
We expected to get ClassCastException
Parent
import java.net.*;
public class Loader {
public static void main(String[] args) throws Exception {
// ClassLoader parent = new URLClassLoader(new URL[] {
// new URL("file:./")}, null);
ClassLoader a1 = new URLClassLoader(new URL[] {
new URL("file:a1/")}, Loader.class.getClassLoader());// parent);
ClassLoader a2 = new URLClassLoader(new URL[] {
new URL("file:a2/")}, Loader.class.getClassLoader()); //parent);
Class c1 = a1.loadClass("A");
Class c2 = a2.loadClass("A");
System.out.println(c1.newInstance());
System.out.println(c2.newInstance());
System.out.println(c1.getSuperclass().equals(c2.getSuperclass()));
System.out.println(Class.forName("Parent").equals(c1.getSuperclass()));
try { Parent p = (Parent)c1.newInstance(); } catch(ClassCastException ex) {
ex.printStackTrace(); }
System.out.println("We did not expected to get ClassCastException");
}}
Parent
Thanks for caring... but what do you want??? This is the first class
Thanks for caring... but what do you want??? This is the second class
true
true
We did not expected to get ClassCastException
Nailgun
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– Beliebige Programme
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