Java 超类继承

Question

我有一个超类(SimpleGeometricObject(，它扩展到两个子类(CircleFromSimpleGeometricObject和RectangleFromSimpleGeometricObject(，以及一个调用CircleFromSimpleGeometricObject

和RectangleFromSimpleGeometricObject的类，称为TestCircleRectangle。在调试器之后，对于子类 CircleFromSumpleGeometricObject，这行代码：

public CircleFromSimpleGeometricObject(double radius){
    this.radius = radius;
}

以某种方式调用超类 SimpleGeometricObject：

    /** Construct a default geometric object */
public SimpleGeometricObject() {
    dateCreated = new java.util.Date();
}

我对这种情况是如何发生的以及为什么感到有点困惑，有人可以帮助我理解为什么会发生这种情况吗？以下是所有课程的代码。

public class SimpleGeometricObject {
private String color = "white";
private boolean filled;
private java.util.Date dateCreated;
/** Construct a default geometric object */
public SimpleGeometricObject() {
    dateCreated = new java.util.Date();
}
/** Construct a geometric object with the specified color
 * and filled value */
public SimpleGeometricObject(String color, boolean filled) {
    dateCreated = new java.util.Date();
    this.color = color;
    this.filled = filled;
}
/** Return color */
public String getColor() {
    return color;
}
/** Set a new color */
public void setColor(String color) {
    this.color = color;
}
/** Return filled. Since filled is boolean, 
    its get method is named isFilled */
public boolean isFilled() {
    return filled;
}
/** Set a new filled */
public void setFilled(boolean filled) {
    this.filled = filled;
}
/** Get dateCreated */
public java.util.Date getDateCreated() {
    return dateCreated;
}
/** Return a string representation of this object */
public String toString() {
    return "created on " + dateCreated + "ncolor: " + color +
        " and filled: " + filled;
}
}

public class CircleFromSimpleGeometricObject 
extends SimpleGeometricObject {
private double radius;
public CircleFromSimpleGeometricObject() {
}
public CircleFromSimpleGeometricObject(double radius){
    this.radius = radius;
}
public CircleFromSimpleGeometricObject(double radius,
        String color, boolean filled) {
    this.radius = radius;
    setColor(color);
    setFilled(filled);
}
/** Return radius */
public double getRadius() {
    return radius;
}
/** Set a new radius */
public void setRadius(double radius) {
    this.radius = radius;
}
/** Return area */
public double getArea() {
    return radius * radius * Math.PI;
}
/** Return diameter */
public double getDiameter() {
    return 2 * radius;
}
/** Return perimeter */
public double getPerimeter() {
    return 2 * radius * Math.PI;
}
/** Print the circle info */
public void printCircle() {
    System.out.println("The circle is created " + getDateCreated() +
            " and the radius is " + radius);
}
}

public class RectangleFromSimpleGeometricObject 
extends SimpleGeometricObject {
private double width;
private double height;
public RectangleFromSimpleGeometricObject() {
}
public RectangleFromSimpleGeometricObject(
        double width, double height) {
    this.width = width;
    this.height = height;
}
public RectangleFromSimpleGeometricObject(
        double width, double height, String color, boolean filled) {
    this.width = width;
    this.height = height; 
    setColor(color);
    setFilled(filled);
}
/** Return width */
public double getWidth() {
    return width;
}
/** Set a new width */
public void setWidth(double width) {
    this.width = width;
}
/** Return height */
public double getHeight() {
    return height;
}
/** Set a new height */
public void setHeight(double height) {
    this.height = height;
}
/** Return area */
public double getArea() {
    return width * height;
}
/** Return perimeter */
public double getPerimeter() {
    return 2 * (width * height);
}
}

public class TestCircleRectangle {
public static void main(String[] args) {
    CircleFromSimpleGeometricObject circle = 
            new CircleFromSimpleGeometricObject(1);
    System.out.println("A circle " + circle.toString());
    System.out.println("The color is " + circle.getColor());
    System.out.println("The radius is " + circle.getRadius());
    System.out.println("The area is " + circle.getArea());
    System.out.println("The diamter is " + circle.getDiameter());
    RectangleFromSimpleGeometricObject rectangle = 
            new RectangleFromSimpleGeometricObject(2, 4);
    System.out.println("nA rectangle " + rectangle.toString());
    System.out.println("The area is " + rectangle.getArea());
    System.out.println("The perimeter is " + 
        rectangle.getPerimeter());
}
}

Answer 1

像public CircleFromSimpleGeometricObject(double radius)这样的构造函数总是必须包含对其超类构造函数的调用作为第一行;如果你不显式地这样做，编译器将无形地插入对超类的无参数构造函数的调用(如果有的话(。这就是这里发生的事情;构造函数自动调用public SimpleGeometricObject() 。

构造

函数可以像这样调用超类构造函数：

super();

如果需要，可以包含参数。

附言正如评论者所提到的，您的类名真的很奇怪且没有必要; Circle和Rectangle就足够了。

Answer 2

任何构造函数都调用超类构造函数(依此类推，直到调用Object构造函数(。如果未显式调用 super() ，编译器将为您插入它。

看到这一点的最明显方法是在你的超类中没有默认构造函数(没有任何参数的构造函数(。在这种情况下，在插入对所需super构造函数的显式调用之前，子类将不会编译。

Answer 3

根据 java 规范，所有对象构造函数都隐式调用其超类的构造函数。想象一下，如果没有：你的dateCreated对象就不会被初始化。

这是我找到的指向java规范的博客：

http://www.dribin.org/dave/blog/archives/2004/11/23/java_constructor/

Answer 4

由于 CircleFormSimpleGeometricObject 扩展了 SimpleGeometricObject

，因此当调用构造函数 CircleFormSimpleGeometricObject(( 时会自动调用构造函数 SimpleGeometricObject((。

这很有用，因为子类需要从超类中获取的任何变量都将被初始化，但如果是这种情况，则调用 super(( 会更安全，它显式调用超类的构造函数。 若要指定要使用的构造函数，可以将特定构造函数调用的变量放入 super(( 中。 例如，此行：

    super(String, boolean);

将在 SimpleGeometricObject 中调用相关的构造函数。