Java Tutorial/Class Definition/Class Fields
Содержание
A Class that Demonstrates Shadowing
<source lang="java">
public class MainClass {
static int x;
public static void main(String[] args) {
x = 5;
System.out.println("x = " + x);
int x;
x = 10;
System.out.println("x = " + x);
System.out.println("ShadowApp.x = " + ShadowApp.x);
}
}</source>
Dynamically changing the behavior of an object via composition (the "State" design pattern)
<source lang="java">
abstract class Actor {
public abstract void act();
} class HappyActor extends Actor {
public void act() {
System.out.println("HappyActor");
}
} class SadActor extends Actor {
public void act() {
System.out.println("SadActor");
}
} class Stage {
private Actor actor = new HappyActor();
public void change() {
actor = new SadActor();
}
public void performPlay() {
actor.act();
}
} public class MainClass {
public static void main(String[] args) {
Stage stage = new Stage();
stage.performPlay();
stage.change();
stage.performPlay();
}
}</source>
HappyActor SadActor
Scope class demonstrates field and local variable scopes
<source lang="java">
public class MainClass {
public static void main( String args[] )
{
Scope testScope = new Scope();
testScope.begin();
}
} class Scope {
private int x = 1;
public void begin()
{
int x = 5; // method"s local variable x shadows field x
System.out.printf( "local x in method begin is %d\n", x );
useLocalVariable();
useField();
useLocalVariable();
useField();
System.out.printf( "\nlocal x in method begin is %d\n", x );
}
public void useLocalVariable()
{
int x = 25; // initialized each time useLocalVariable is called
System.out.printf("\nlocal x on entering method useLocalVariable is %d\n", x );
++x; // modifies this method"s local variable x
System.out.printf(
"local x before exiting method useLocalVariable is %d\n", x );
}
// modify class Scope"s field x during each call
public void useField()
{
System.out.printf("\nfield x on entering method useField is %d\n", x );
x *= 10; // modifies class Scope"s field x
System.out.printf("field x before exiting method useField is %d\n", x );
}
}</source>
local x in method begin is 5 local x on entering method useLocalVariable is 25 local x before exiting method useLocalVariable is 26 field x on entering method useField is 1 field x before exiting method useField is 10 local x on entering method useLocalVariable is 25 local x before exiting method useLocalVariable is 26 field x on entering method useField is 10 field x before exiting method useField is 100 local x in method begin is 5
Scope for Class and Local Variables
<source lang="java">
public class MainClass {
static int x;
public static void main(String[] args) {
x = 5;
System.out.println("main: x = " + x);
myMethod();
}
public static void myMethod() {
int y;
y = 10;
if (y == x + 5) {
int z;
z = 15;
System.out.println("myMethod: z = " + z);
}
System.out.println("myMethod: x = " + x);
System.out.println("myMethod: y = " + y);
}
}</source>
