"不是抽象的,不会覆盖抽象方法"错误

Question

我收到这个错误,这让我感到困惑.我不知道如何解决它.我知道那里还有其他一些错误,但我现在并不太担心.

这是代码:

import java.awt.*;

public abstract class Shape {
// Instance variables
private int x;
private int y;
private Color color;

// Constructor
protected Shape(int x, int y, Color color) {
this.x = x;
this.y = y;
this.color = color;
}

// Abstract methods
 public abstract void draw(Graphics g);
public abstract int getHeight();
public abstract int getWidth();
public abstract void scale();
public abstract double area();

// Other instance methods
public Color getColor() {
return color;
}

public int getX() {
return x;
}

public int getY() {
return y;
}

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

public void setColor(Color color) {
this.color = color;
}

public String toString() {
return "Position: (" + getX() + ", " + getY() + ") Color: (" + getRed() + ", " + getBlue() + ", " getGreen() + ")";

}}

这是类文件:

// Represents a circle that can be displayed in a graphics
// context

import java.awt.*;

public class Circle extends Shape {
// Instance variables
private int diameter;
private double scaleFactor = 2;
private double area;

// Constructor
public Circle(int x, int y, Color color, int diameter) {
super(x, y, color);
this.diameter = diameter;
}

 // Instance methods
public void draw(Graphics g) {
g.setColor(getColor());
g.fillOval(getX(), getY(), diameter, diameter);
}

public int getHeight() {
return diameter;
}

public int getWidth() {
return diameter;
}

public void scale(double scaleFactor) {
(double) diameter *= scaleFactor;
}

public double area() {
area = (3.14 * (diameter * diameter)) / 4;
return area;
}

public String toString() {
return "Diameter: " + diameter + "\nWidth: " + getWidth() + "Height: " + getHeight();
}
}

import java.awt.*;

public class Rectangle extends Shape {
// Instance variables
private int width;
private int height;
private double scaleFactor = 2;
private double diameter = width * height;
private double area;

// Constructor
public Rectangle(int x, int y, Color color,
               int width, int height) {
super(x, y, color);
this.width = width;
this.height = height;
}

// Instance methods
public void draw(Graphics g) {
g.setColor(getColor());
g.fillRect(getX(), getY(), width, height);
}

public int getHeight() {
return height;
}

public int getWidth() {
return width;
}

public void scale(double scaleFactor){
width *= scaleFactor;
height *= scaleFactor;
}

public double area() {
    area = (3.14 * (diameter * diameter)) / 4;
    return area;
}

public String toString() {
    return "Diameter: " + diameter + "\nWidth: " + getWidth() + "Height: " +
getHeight();
}
}

这是测试文件:

public class ShapeTest
{
public static void main(String[] args)
{
    Color myC = new Color(10, 30, 20);
    Circle myCircle = new Circle(0, 0, myC, 5);
    Rectangle myRectangle = new Rectangle(0, 0, myC, 2, 3);

    System.out.println(myCircle.toString());
    System.out.println(myRectangle.toString()); 


}
}

以下是错误:

javac ShapeTest.java
ShapeTest.java:5: cannot find symbol
symbol  : class Color
location: class ShapeTest
    Color myC = new Color(10, 30, 20);
    ^
ShapeTest.java:5: cannot find symbol
symbol  : class Color
location: class ShapeTest
    Color myC = new Color(10, 30, 20);
                    ^
./Shape.java:46: ';' expected
return "Position: (" + getX() + ", " + getY() + ") Color: (" + getRed() + ", " +   getBlue() + ", " getGreen() + ")";
                                                                                                   ^
./Shape.java:46: not a statement
return "Position: (" + getX() + ", " + getY() + ") Color: (" + getRed() + ", " + g etBlue() + ", " getGreen() + ")";
                                                                                                             ^
./Shape.java:46: cannot find symbol
symbol  : method getRed()
location: class Shape
return "Position: (" + getX() + ", " + getY() + ") Color: (" + getRed() + ", " + getBlue() + ", " getGreen() + ")";
                                                               ^
./Shape.java:46: cannot find symbol
symbol  : method getBlue()
location: class Shape
return "Position: (" + getX() + ", " + getY() + ") Color: (" + getRed() + ", " + getBlue() + ", " getGreen() + ")";
                                                                                 ^
./Shape.java:46: cannot find symbol
symbol  : method getGreen()
location: class Shape
return "Position: (" + getX() + ", " + getY() + ") Color: (" + getRed() + ", " + getBlue() + ", " getGreen() + ")";
                                                                                                  ^
./Circle.java:16: Circle is not abstract and does not override abstract method scale() in     Shape
public class Circle extends Shape {
   ^
./Circle.java:43: unexpected type
required: variable
found   : value
(double) diameter *= scaleFactor;
^
 ./Rectangle.java:3: Rectangle is not abstract and does not override abstract method scale() in Shape
 public class Rectangle extends Shape {
   ^
10 errors

Answer 1

我同意user1490835的诊断,但我认为解释这个错误的原因可能是好的,以防你不确定.

抽象方法是对编译器的一种承诺,即所有子类(使用抽象方法扩展类的子类)都能够执行该方法.实现该承诺的一部分是子类必须使用完全相同的头实现抽象方法.所以在Java运行时,你可以调用的一个实例的抽象方法的任何子类的,你却知道已经使用了正确的参数等等.

为什么？因为如果子类为该方法使用稍微不同的头(导致我们试图避免的运行时错误),代码将不会编译(它会给出你得到的错误).

例如,想象一个抽象类A,它有一个抽象方法f()和两个具体的子类B和C.在程序的某个地方我们有:

    A avar;
    if (somevar < othervar) {
        avar = new B();
    } else {
        avar = new C();
    }
    avar.f();

我们不知道阉avar将是B或C在运行时,因为它依赖于if语句的结果.但我们知道这avar将是一个子类A.由于所有子类A必须f使用正确的头实现(否则程序员会得到你得到的编译错误),我们知道这个代码在运行时不会中断!

在您的代码中,抽象方法头与其实现之间的区别是:

public abstract void scale(); // no parameter
public void scale(double scaleFactor) { // one parameter

要修复它,请将double参数添加到抽象方法中.