追问大神---向上转型和向下转型的实现。
package com.syq.test;

public class Shape {

public void draw() {
System.out.println("绘制图形…………");
}

public double getPerimter() {
return 0;
}

public double getArea() {
return 0;
}

public void print() {
System.out.println("这是父类的print方法");
}

}



package com.syq.test;

public class Triangle extends Shape {
private double sid1;
private double sid2;
private double sid3;

public Triangle() {
}

public Triangle(double sid1, double sid2, double sid3) {
this.setSid1(sid1);
this.setSid2(sid2);
this.setSid3(sid3);
}

// 求三边中最长的边
public double sidMax() {
return (sid1 > sid2) ? sid1 : (sid2 > sid3) ? sid2 : sid1;
}

// 重写toString方法，用于描述对象
public String toString() {
return "三角形周长：" + this.getPerimter() + "\n三角形面积：" + this.getArea()
+ "\n最长的边长："+this.sidMax();
}

public void draw() {
System.out.println("绘制三角形…………三边长："+this.getSid1()+" "+this.getSid2()+" "+this.getSid3());
}

// 绘制三角形，返回是否符合三角形的定义
public void draw(Triangle triangle) {
if (this.sid1 + sid2 > sid3 & sid3 - sid1 < sid2) {
this.setSid1(sid1);
this.setSid2(sid2);
this.setSid3(sid3);
System.out.println("成功绘制三角形!");
}else {
System.out.println("不符合绘制三角形的条件!");
}
}
// 计算周长
public double getPerimter() {
return this.sid1 + this.sid2 + this.sid3;
}

// 计算三角形面积
public double getArea() {
double p = this.getPerimter() / 2;
double area = Math.sqrt(p * (p - sid1) * (p - sid2) * (p - sid3));
return p;
}

public double getSid1() {
return sid1;
}

public void setSid1(double sid1) {
this.sid1 = sid1;
}

public double getSid2() {
return sid2;
}

public void setSid2(double sid2) {
this.sid2 = sid2;
}

public double getSid3() {
return sid3;
}

public void setSid3(double sid3) {
this.sid3 = sid3;
}

}



package com.syq.test;

public class Circle extends Shape {
private double r;

public Circle() {
}

public Circle(double r) {
this.setR(r);
}

// 计算圆的直径
public double diameter() {
return 2 * r;
}

public void draw() {
System.out.println("绘制圆…………半径："+this.getR());
}

// 绘制圆
public void draw(Circle circle) {
if (this.r > 0) {

this.setR(r);
System.out.println("成功绘制圆!");
}else {
System.out.println("绘制圆条件不符合!");
}
}

// 重写toString方法
public String toString() {
return "圆的半径: " + this.r + "\n圆的周长：" + this.getPerimter() + "\n圆的面积："
+ this.getArea();
}

// 计算圆的周长
public double getPerimter() {
return 2 * 3.1415926 * r;
}

// 计算圆的面积
public double getArea() {
return r * r * 3.1415926;
}

public double getR() {
return r;
}

public void setR(double r) {
this.r = r;
}

}



package com.syq.test;

public class Rectangle extends Shape {

private double length;
private double wide;

public Rectangle() {
}

public Rectangle(double length, double wide) {
this.setLength(length);
this.setWide(wide);
}

// 计算对角线
public double diagonal() {
return Math.sqrt(length * length + wide * wide);
}

public void draw() {
System.out.println("绘制矩形…………长："+this.getLength()+" 宽："+this.getWide());
}

// 绘制矩形
public void draw(Rectangle rectangle) {
if (this.length > 0 && this.wide > 0) {
this.setLength(length);
this.setWide(wide);
System.out.println("成功绘制矩形!");
} else {
System.out.println("绘制矩形条件不符合!");
}
}

// 计算周长
public double getPerimter() {
return 2 * (this.length + this.wide);
}

// 计算面积
public double getArea() {
return this.wide * this.length;
}

// 重写toString方法
public String toString() {
return "矩形的周长: " + this.getPerimter() + "\n矩形的面积: " + this.getArea()
+ "\n矩形的对角线：" + this.diagonal();
}

public double getLength() {
return length;
}

public void setLength(double length) {
this.length = length;
}

public double getWide() {
return wide;
}

public void setWide(double wide) {
this.wide = wide;
}

}



package com.syq.test;

import java.util.Iterator;

public class TestShape {
public static void main(String[] args) {
Shape[] shapes = new Shape[3];

shapes[0] = new Circle(1);
shapes[1] = new Rectangle(1, 1);
shapes[2] = new Triangle(1, 2, 1);

for (int i = 0; i < shapes.length; i++) {

// 到这步已经可以实现向上转型：父类 父类引用变量 = 子类对象
Shape shape = shapes[i];
shape.draw();

System.out.println("面积：" + shape.getArea());
System.out.println("周长：" + shape.getPerimter());

//System.out.println( shape.toString );
}
}
}

-------------------------------------------

在测试类中的for循环里面（红色字体），怎么才能调用到子类的方法呢？比如调用矩形Rectangle类中的diagonal()方法。麻烦大神帮我想想指点思路。在线等。

[解决办法]

Java code

if(shape instanceof Rectangle){    Rectangle r = (Rectangle)shape;    r.diagonal();}