Java/Class/Static
Содержание
Explicit static initialization with the static clause
// : c04:ExplicitStatic.java
// Explicit static initialization with the "static" clause.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
class Cup {
Cup(int marker) {
System.out.println("Cup(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Cups {
static Cup c1;
static Cup c2;
static {
c1 = new Cup(1);
c2 = new Cup(2);
}
Cups() {
System.out.println("Cups()");
}
}
public class ExplicitStatic {
public static void main(String[] args) {
System.out.println("Inside main()");
Cups.c1.f(99); // (1)
}
// static Cups x = new Cups(); // (2)
// static Cups y = new Cups(); // (2)
} ///:~
Java static member variable example
public class Main {
public static void main(String[] args) {
Counter object1 = new Counter();
System.out.println(object1.getNumberOfObjects());
Counter object2 = new Counter();
System.out.println(object2.getNumberOfObjects());
}
}
class Counter {
static int counter = 0;
public Counter() {
counter++;
}
public int getNumberOfObjects() {
return counter;
}
}
Java static method
public class Main {
public static void main(String[] args) {
int result = add(1, 2);
System.out.println("(1+2) is : " + result);
}
public static int add(int first, int second) {
return first + second;
}
}
//(1+2) is : 3
Show that you can"t have static variables in a method
/*
* Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
* All rights reserved. Software written by Ian F. Darwin and others.
* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
* Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
* cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
* pioneering role in inventing and promulgating (and standardizing) the Java
* language and environment is gratefully acknowledged.
*
* The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
* inventing predecessor languages C and C++ is also gratefully acknowledged.
*/
/**
* Show that you can"t have static variables in a method, unlike C/C++
*/
public class NoLocalStatics {
public static void main(String[] argv) {
NoLocalStatics t = new NoLocalStatics();
t.process();
}
void process() {
static int a = 42; // EXPECT COMPILE ERROR
System.out.println("Process: " + a);
}
}
Show that you do inherit static fields
/*
* Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
* All rights reserved. Software written by Ian F. Darwin and others.
* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
* Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
* cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
* pioneering role in inventing and promulgating (and standardizing) the Java
* language and environment is gratefully acknowledged.
*
* The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
* inventing predecessor languages C and C++ is also gratefully acknowledged.
*/
// Show that you do inherit static fields.
/** This is the class that we extend. */
class AnOtherClass {
AnOtherClass() {
System.out.println("In AnOtherClass::<init>");
}
static int foo = 24;
static int bar() {
return 42;
}
}
/** This is the subclass */
public class InheritStatics extends AnOtherClass {
public static void main(String[] c) {
new InheritStatics().run();
}
public void run() {
System.out.println("In InheritStatics::run");
System.out.println("Foo is " + foo);
System.out.println("Bar is " + bar());
}
}
Static field, constructor and exception
public class Main {
static Bar bar;
static {
try {
bar = new Bar();
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] argv) {
System.out.println(bar);
}
}
class Bar {
public Bar() throws Exception {
}
public String toString() {
return "Bar";
}
}
Static Init Demo
/*
* Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
* All rights reserved. Software written by Ian F. Darwin and others.
* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
* Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
* cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
* pioneering role in inventing and promulgating (and standardizing) the Java
* language and environment is gratefully acknowledged.
*
* The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
* inventing predecessor languages C and C++ is also gratefully acknowledged.
*/
/**
* Show static initializers. These are run when the class is loaded.
* If you run the main program here, you will see the static
* initializer before the "About to load class", since the class
* is loaded before main gets run. If you copy the guts of main into
* another class, the messages will come out in the "expected" order.
*
* @author Ian F. Darwin, http://www.darwinsys.ru/
* @version $Id: StaticIniDemo.java,v 1.5 2004/02/09 03:33:54 ian Exp $
*/
public class StaticIniDemo {
/** A static initializer - called when class is loaded. */
static {
System.out.println("In static initializer");
}
/** A constructor -- called when object is instantiated. */
public StaticIniDemo() {
System.out.println("In Constructor");
}
public static void main(String[] a) {
try {
System.err.println("About to load class");
Class c = Class.forName("StaticIniDemo");
System.err.println("About to construct instance");
Object sd = c.newInstance();
System.err.println("Object is " + sd);
} catch (Exception e) {
e.printStackTrace();
}
}
}
Using Static Variables
/*
In this example, the Box class contains a static variable, numBoxes, which is incremented
each time a Box object is created. The main() method of the TestStaticVar class creates
two Box objects, then prints out the value of the static variable.
*/
class Box {
double width;
public static int numBoxes = 0; // static variable is declared and initialized
public Box() {
width = 5.0;
numBoxes++; // numBoxes is incremented to count number of objects.
}
}
public class TestStaticVar {
public static void main (String args[]) {
Box box1 = new Box();
Box box2 = new Box();
System.out.println("Number of objects = " + Box.numBoxes);
}
}
