8.1.1.1 abstract Classes

1. An abstract class is a class that is incomplete, or to be considered incomplete.
2. abstract class cannot be instantiated.

例子：抽象class不可以被初始化，正常class才可以。但是正常class可以extend 抽象class。

//Example 8.1.1.1-1. Abstract Class Declaration
abstract class Point {
    int x = 1, y = 1;
    void move(int dx, int dy) {
        x += dx;
        y += dy;
        alert();
    }
    abstract void alert();
}

abstract class ColoredPoint extends Point {
    int color;
}

class SimplePoint extends Point {
    void alert() { }
}

//run
jshell> Point p = new Point()
|  Error:
|  Point is abstract; cannot be instantiated
|  Point p = new Point();
|            ^---------^

jshell> Point p = new SimplePoint()
p ==> SimplePoint@4cf777e8

8.1.1.2 final Classes

不可变，只能初始化一次。

8.1.2 Generic Classes and Type Parameters

8.1.2通用类和类型参数
如果一个类声明了一个或多个类型变量（§4.4），则它是通用的。这些类型变量称为类的类型参数。 方式参数部分在类名后面，并由尖括号分隔。

8.1.5超级接口

类声明中的可选implements子句列出了接口的名称这是被声明的类的直接超接口。

注意以下例子，通用class中需要完整声明interface包含的信息。

interface Colorable {
    void setColor(int color);
    int getColor();
}
enum Finish { MATTE, GLOSSY }

interface Paintable extends Colorable {
    void setFinish(Finish finish);
    Finish getFinish();
}
class Point { int x, y; }
class ColoredPoint extends Point implements Colorable {
    int color;
    public void setColor(int color) { this.color = color; }
    public int getColor() { return color; }
}
class PaintedPoint extends ColoredPoint implements Paintable {
    Finish finish;
    public void setFinish(Finish finish) {
        this.finish = finish;
    }
    public Finish getFinish() { return finish; }
}

8.2 Class Members 继承

注意作用范围，如果没有public，package中的class不能被外部class调用，会出现“XX is not public”的错误。class的继承一般用super

package Inheritance

1. 如果可以改package，可以在参数前加个public
2. 不改package，可以用super 继承

//Example 8.2-2. Inheritance of Class Members with Package Access

package points;

public class Point {
    int x,y;

    public void move(int dx, int dy){
        x += dx; y += dy;
    }
}

package points;

public class Point3d extends Point{
    int z;
    public void move(int dx, int dy,int dz){
        x += dx; y += dy; z += dz;
    }
}

//wrong one 
import points.Point3d;

class Point4d extends Point3d{
    int w;
    public void move(int dx, int dy, int dz, int dw){
        x += dx; y += dy; z += dz; w += dw;
    }
}
//

//correct 
import points.Point3d;

class Point4d extends Point3d{
    int w;
    public void move(int dx, int dy, int dz, int dw){
        super.move(dx,dy,dz); w += dw;
    }
}

Inheritance of public and protected Class Members

private变量不可被子集继承，只能作用于private的类。

the public and protected fieldsx, y, useCount, and totalUseCount are inherited in all subclasses of Point.

//Example 8.2-3. Inheritance of public and protected Class Members

package points;

public class Point {
    public int x,y;
    protected int useCount = 0;
    static protected int totalUseCount = 0;

    public void move(int dx, int dy){
        x += dx; y += dy; useCount++; totalUseCount++;
    }

}

class Point4d extends points.Point{
    public void moverBack(int dx, int dy){
        x -= dx; y -= dy;useCount++; totalUseCount++;
    }
}

Inheritance of private Class Members

the class variable totalMoves can be used only within the class Point; it is not inherited by the subclass Point3d. A compile-time error occurs because method move of class Point3d tries to increment totalMoves.

// Example 8.2-4. Inheritance of private Class Members
package points;

class Point {
    int x,y;
    void move(int dx, int dy){ x += dx; y += dy; totalMoves++; }

    private static int totalMoves ;

    void printMoves(){ System.out.println(totalMoves); }

}

package points;

public class Point3d extends Point{
    int z;
    void move(int dx, int dy,int dz){
        super.move(dx,dy); z += dz;
        totalMoves++; // error
    }
}

8.3 Field Declarations

Example 8.3-1. Multiply Inherited Fields

同名的会混乱，class用super继承，其他用 fields.name 继承。

jshell> interface Frob { float v = 2.0f; }
|  已创建 接口 Frob

jshell> class SuperTest { int v = 3; }
|  已创建 类 SuperTest

jshell> class Test extends SuperTest implements Frob {
   ...>  public static void main(String[] args) {
   ...>  new Test().printV();
   ...>  }
   ...>  void printV() { System.out.println(v); }
   ...> }
|  错误:
|  对v的引用不明确
|    SuperTest 中的变量 v 和 Frob 中的变量 v 都匹配
|   void printV() { System.out.println(v); }
|                                      ^

jshell> class Test extends SuperTest implements Frob {
   ...>  public static void main(String[] args) {
   ...>  new Test().printV();
   ...>  }
   ...>  void printV() {
   ...>  System.out.println((super.v + Frob.v)/2);
   ...>  }
   ...> }
|  已创建 类 Test

8.3.1 Field Modifiers

用于修饰field的有以下。

FieldModifier:
(one of)
Annotation public protected private
static final transient volatile

8.3.1.1 static Fields

If a field is declared static, there exists exactly one incarnation of the field, nomatter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized (§12.4).

//Example 8.3.1.1-2. Hiding of Class Variables


class Point {
    static int x = 2;
}
class Test extends Point {
    static double x = 4.7;
    public static void main(String[] args) {
        new Test().printX();
    }
    void printX() {
        System.out.println(x + " " + super.x);
    }
}

//output 
4.7 2

jshell> Test.x
$7 ==> 4.7

jshell> Point.x
$8 ==> 2

8.4 Method Declarations

Example 8.4.2-1. Override-Equivalent Signatures

抽象method只能用在抽象class中。

abstract class Point {
    int x, y;
    public abstract String toString();
}

Example 8.4.8.1-2. Overriding

need to do it later

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进度 297

2018.7.30