Java Tutorial/Class Definition/Initialization Block

Содержание

1 A class that contains a static initializer:
2 Demonstrates the pitfalls of depending on the order of static initializers
3 Demonstrates various initializers
4 Explicit static initialization with the static clause
5 Initialization order
6 Initializing Data Members
7 Mixed Initializer
8 The full process of initialization
9 Using Initialization Blocks: A non-static initialization block
10 Using Initialization Blocks: static initialization block

A class that contains a static initializer:

   <source lang="java">

class StaticInit {

   public static int x;
   static
   {
       x = 32;
   }

}</source>

Demonstrates the pitfalls of depending on the order of static initializers

   <source lang="java">

/*

*     file: StaticOrderDemo.java
*  package: oreilly.hcj.review
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

import java.util.Arrays; /**

* Demonstrates the pitfalls of depending on the order of static initializers.
*
* @author 
  * @version $Revision: 1.3 $
  */
 public static class Values {
   /** A value holder */
   public static final String VALUE = "Blue";
   /** A specifier for the value */
   public static final String VALUE_SPECIFIER;
   static {
     System.out.println("static{} method for Values");
     System.out.println(VALUE);
     System.out.println(Ranges.RANGE_BLUE);
     VALUE_SPECIFIER = Ranges.RANGE_BLUE[1];
   }
 }

} /* ########## End of File ########## */</source>

Demonstrates various initializers

   <source lang="java">

/*

*     file: InitializerDemo.java
*  package: oreilly.hcj.review
*
* This software is granted under the terms of the Common Public License,
* CPL, which may be found at the following URL:
* http://www-124.ibm.ru/developerworks/oss/CPLv1.0.htm
*
* Copyright(c) 2003-2005 by the authors indicated in the @author tags.
* All Rights are Reserved by the various authors.
*

1. 1. 1. 1. DO NOT EDIT ABOVE THIS LINE ########## */

import java.util.StringTokenizer; /**

* Demonstrates various initializers.
*
* @author 
* @version $Revision: 1.4 $
*/

public class InitializerDemo {

 /** Simple static initialization. */
 public static final String NAME = "Initializer Demo";
 /** Initialized static on one line. */
 public static final String ARCH = System.getProperty("os.arch");
 /** Static method based initialization. */
 public static final String USER_HOME;
 static {
   USER_HOME = System.getProperty("user.home");
 }
 /** Simple instance member initialization. */
 public String description = "An initialized member";
 /** Method call instance member initialization. */
 public long timestamp = System.currentTimeMillis();
 /** Complex instance member initialization. */
 private String xmlClasspath;
 {
   final StringBuffer buf = new StringBuffer(500);
   final String classPath = System.getProperty("java.class.path");
   StringTokenizer tok =
     new StringTokenizer(classPath, System.getProperty("path.separator"));
   buf.append("<classpath>\n");
   while (tok.hasMoreTokens()) {
     buf.append("  <pathelement location=\"");
     buf.append(tok.nextToken());
     buf.append("\"/>\n");
   }
   buf.append("</classpath>");
   xmlClasspath = buf.toString();
 }
 /** 
  * Creates a new instance of Initalizers
  */
 public InitializerDemo() {
 }
 /** 
  * Main method of the demonstration.
  *
  * @param args Command line arguments (ignored).
  */
 public static final void main(final String[] args) {
   InitializerDemo demo = new InitializerDemo();
   System.out.println("------Dumping Contents-----------");
   System.out.println("---------------------------------");
   System.out.println(InitializerDemo.NAME);
   System.out.println(InitializerDemo.ARCH);
   System.out.println(InitializerDemo.USER_HOME);
   System.out.println(demo.description);
   System.out.println(demo.xmlClasspath);
   System.out.println("---------------------------------");
 }

} /* ########## End of File ########## */</source>

Explicit static initialization with the static clause

   <source lang="java">

class MyClass {

 MyClass(int marker) {
   System.out.println("Cup(" + marker + ")");
 }
 void f(int marker) {
   System.out.println("f(" + marker + ")");
 }

} class MyStatic {

 static MyClass c1;
 static MyClass c2;
 static {
   c1 = new MyClass(1);
   c2 = new MyClass(2);
 }
 MyStatic() {
   System.out.println("Cups()");
 }

} public class MainClass {

 public static void main(String[] args) {
   System.out.println("Inside main()");
   MyStatic.c1.f(99); // (1)
 }

}</source>

Inside main()
Cup(1)
Cup(2)
f(99)

Initialization order

   <source lang="java">

class MyClass {

 MyClass(int marker) {
   System.out.println("Tag(" + marker + ")");
 }

} class MyInit {

 MyClass t1 = new MyClass(1); // Before constructor
 MyInit() {
   System.out.println("Card()");
   t3 = new MyClass(33);
 }
 MyClass t2 = new MyClass(2); // After constructor
 void f() {
   System.out.println("f()");
 }
 MyClass t3 = new MyClass(3); // At end

} public class MainClass {

 public static void main(String[] args) {
   MyInit t = new MyInit();
   t.f();
 }

}</source>

Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()

Initializing Data Members

   <source lang="java">

public class MainClass {

 static final double PI = 3.14;          // Class variable that has a fixed value
 static int count = 0;                   // Class variable to count objects

 public void aMethod() {
 }

}</source>

Mixed Initializer

   <source lang="java">

public class MixedInitializer {

 int i1;
 static int i2;
 int i3 = 2;
 static int i4 = 4;
 {
   System.out.println("i1 = " + i1);
   i1 = 6;
   System.out.println("i1 = 6");
 }
 static {
   System.out.println("i2 = " + i2);
   i2 = 8;
   System.out.println("i2 = 8");
 }
 public static void main(String[] args) {
   System.out.println("main() entered");
   MixedInitializer mi = new MixedInitializer();
   System.out.println("mi.i1 = " + mi.i1);
   System.out.println("i2 = " + i2);
   System.out.println("mi.i3 = " + mi.i3);
   System.out.println("i4 = " + i4);
   System.out.println("main() exited");
 }
 {
   i1 += 6;
   System.out.println("6 + i1");
 }
 static {
   i2 -= 3;
   System.out.println("i2 -= 3");
 }

}</source>

The full process of initialization

   <source lang="java">

class Insect {

 private int i = 1;
 protected int j;
 Insect() {
   System.out.println("i = " + i + ", j = " + j);
   j = 1;
 }
 private static int x1 = print("static Insect.x1 initialized");
 static int print(String s) {
   System.out.println(s);
   return 0;
 }

} class Beetle extends Insect {

 private int k = print("Beetle.k initialized");
 public Beetle() {
   System.out.println("k = " + k);
   System.out.println("j = " + j);
 }
 private static int x2 = print("static Beetle.x2 initialized");

} public class MainClass {

 public static void main(String[] args) {
   Beetle b = new Beetle();
 }

}</source>

static Insect.x1 initialized
static Beetle.x2 initialized
i = 1, j = 0
Beetle.k initialized
k = 0
j = 1

Using Initialization Blocks: A non-static initialization block

Executed for each object that is created.
Can initialize instance variables in a class.

   <source lang="java">

public class MainClass {

 static int[] values = new int[10];
 {
   System.out.println("Running initialization block.");
   for (int i = 0; i < values.length; i++) {
     values[i] = (int) (100.0 * Math.random());
   }
 }
 void listValues() {
   for (int value : values) {
     System.out.println(value);
   }
 }
 public static void main(String[] args) {
   MainClass example = new MainClass();
   System.out.println("\nFirst object:");
   example.listValues();
   example = new MainClass();
   System.out.println("\nSecond object:");
   example.listValues();
 }

}</source>

Running initialization block.
First object:
35
17
1
10
42
38
42
71
24
7
Running initialization block.
Second object:
45
78
49
9
11
36
0
18
56
38

Using Initialization Blocks: static initialization block

A block defined using the keyword static.
Executed once when the class is loaded.
Can initialize only static data members of the class.

   <source lang="java">

public class MainClass {

 static int[] values = new int[10];
 static {
   System.out.println("Running initialization block.");
   for (int i = 0; i < values.length; i++) {
     values[i] = (int) (100.0 * Math.random());
   }
 }
 void listValues() {
   for (int value : values) {
     System.out.println(value);
   }
 }
 public static void main(String[] args) {
   MainClass example = new MainClass();
   System.out.println("\nFirst object:");
   example.listValues();
   example = new MainClass();
   System.out.println("\nSecond object:");
   example.listValues();
 }

}</source>

Running initialization block.
First object:
58
22
49
75
1
35
76
19
27
63
Second object:
58
22
49
75
1
35
76
19
27
63

Java Tutorial/Class Definition/Initialization Block

Содержание

A class that contains a static initializer:

Demonstrates the pitfalls of depending on the order of static initializers

Demonstrates various initializers

Explicit static initialization with the static clause

Initialization order

Initializing Data Members

Mixed Initializer

The full process of initialization

Using Initialization Blocks: A non-static initialization block

Using Initialization Blocks: static initialization block

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