我收到此错误,这令我感到困惑。我不知道如何解决它。我知道那里还有其他一些错误,但我现在不太担心这些错误。
以下是代码:
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() + ")";
} }
以下是类文件:
// 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
答案 0 :(得分:6)
我同意user1490835的诊断,但我认为解释这个错误的原因可能会很好,以防您不确定。
抽象方法是对编译器的承诺,即所有子类(使用抽象方法扩展类的子类)将能够执行该方法。实现该承诺的部分原因是子类必须使用完全相同的头来实现抽象方法。因此,在Java运行时,您可以在任何子类的实例上调用抽象方法,并知道您将使用正确的参数等等。
为什么呢?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(它会给出错误你得到) 。
例如,假设一个抽象类A
具有抽象方法f()
和两个具体子类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();
我们不知道运行时avar
是B
还是C
,因为它取决于if语句的结果。但我们知道avar
将是A
的子类。由于A
的所有子类都必须使用正确的标头实现f
(否则程序员会得到你正在获得的编译错误),我们知道这段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。
答案 1 :(得分:1)
我能找到的第一个问题是你已经声明了一个没有参数的Scale抽象方法,但是当你扩展抽象类时,你正在使用参数实现Scale方法。这就是为什么在所有子类中抛出此错误的原因:
Circle不是抽象的,并且不会覆盖Shape
中的抽象方法scale()还有以下错误:
./Shape.java:46: ';' expected
return "Position: (" + getX() + ", " + getY() + ") Color: (" +
getRed() + getBlue() + ", " getGreen() + ")";
你忘了在getGreen函数之前添加+符号,Java希望将其作为行尾,这就是为什么错误';'
getRed符号错误是因为你没有名为getRed的函数?等等!
答案 2 :(得分:0)
在Java中使用类之前,必须首先导入该类。
在ShapeTest.java中,您需要添加:
import Circle;
import Rectangle;
import java.awt.Color;
在Circle.java和Rectangle.java中,您需要添加:
import Shape;
此外,您还应确保文件位于包中。见http://en.wikipedia.org/wiki/Java_package
答案 3 :(得分:0)
嗯,所以它告诉你的原因是,你继承了“Shape”这个类。该类定义方法scale()。
因此,要么在“Circle”和“Rectangle”中实现方法scale(),要么将它们定义为asbtract。我建议你只实现scale()方法,不要把它们变成抽象,因为我认为那是你打算做的。