自定义形状类

package com.jdk7.chapter2.shape;

/**
 * 抽象类不能声明为实例对象
 * 抽象类中的方法也为抽象方法，不予实现，必须由子类实现
 * @author celineluo
 *
 */
public abstract class MyShape {
    protected String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    
    public abstract double area();
    public abstract double girth();

    @Override
    public String toString() {
        return "MyShape [name=" + name + "]";
    }
    
}

子类，一个子类只能继承一个父类，但是可以同时实现多个接口

package com.jdk7.chapter2.shape;

public class Triangle extends MyShape {
    private double sideA;
    private double sideB;
    private double sideC;
    private String lengthErr = "三角形边长不能小于0";
    private String sumSize = "两边之和不能小于第三边";
    
    //默认构造函数
    public Triangle(){
        init();
    }
    
    //自定义构造函数
    public Triangle(double a,double b,double c){
        if(isTriangle(a,b,c)){
            this.sideA = a;
            this.sideB = b;
            this.sideC = c;
        }else{
            init();
        }
    }
    
    //判断是否符合三角形条件
    public boolean isTriangle(double a,double b,double c){
        if(a<=0 || b<=0 || c<=0){
            System.err.println("Error: "+lengthErr);
            return false;
        }else if((a+b)<=c ||(a+c)<=b ||(c+b)<=a){
            System.err.println("Error: "+sumSize);
            return false;
        }else{
            return true;
        }
        
    }
    public void init(){
        this.sideA = 3;
        this.sideB = 4;
        this.sideC = 5;
    }

    @Override
    public double area() {
        double result;
        double a = (this.sideA + this.sideB + this.sideC)/2;
        result = a*(a-this.sideA)*(a-this.sideB)*(a-this.sideC);
        return result;
    }

    @Override
    public double girth() {
        double result;
        result  = this.sideA + this.sideB + this.sideC;
        return result;
    }

    public double getSideA() {
        return sideA;
    }
    
    //在使用对象类型时，如果没有声明一个实例对象则无法调用对象类型的变量和方法，
    //声明实例对象时，会进行初始化
    //调用对象类型变量的Setter方法，则会将设置值替换初始值进行校验
    public void setSideA(double sideA) {
        if(isTriangle(sideA, this.sideB, this.sideC)){
            this.sideA = sideA;
        }
    }

    public double getSideB() {
        return sideB;
    }

    public void setSideB(double sideB) {
        if(isTriangle(this.sideA,sideB,this.sideC)){
            this.sideB = sideB;
        }
    }

    public double getSideC() {
        return sideC;
    }

    public void setSideC(double sideC) {
        if(isTriangle(this.sideA,this.sideB,sideC)){
            this.sideC = sideC;
        }
    }
    
    @Override
    public String toString() {
        return "三角形名称是: " + name +", 三角形第一条边是: " + sideA + ", 三角形第二条边是: " + sideB + ", 三角形第三条边是: "
                + sideC + ", 三角形面积是: "+ area() + ", 三角形周长是: " + girth();
    }

    public static void main(String[] args) {
//        Triangle t = new Triangle();
//        System.out.println(t.girth());
//        System.out.println(t.area());
//        t.setSideA(0);
//        t.setSideA(1);
//        t.setSideB(5);
//        t.setSideC(6);
//        System.out.println(t.girth());
//        System.out.println(t.area());
//        Triangle t1 = new Triangle(1,5,6);
//        Triangle t1 = new Triangle(0,5,6);
        Triangle t1 = new Triangle(3.0,5.0,6.0);
        t1.setName("三角形");
        System.out.println(t1.girth());
        System.out.println(t1.area());
        System.out.println(t1.toString());
        
    }

}
执行结果：
14.0
56.0
三角形名称是: 三角形, 三角形第一条边是: 3.0, 三角形第二条边是: 5.0, 三角形第三条边是: 6.0, 三角形面积是: 56.0, 三角形周长是: 14.0

package com.jdk7.chapter2.shape;

public class Rectangle extends MyShape {
    private double sideA;
    private double sideB;
    private String lengthErr = "边长不能小于0.";
    
    //默认构造函数
    public Rectangle(){
        init();
    }
    
    //自定义构造函数
    public Rectangle(double a,double b){
        if(isRectangle(a,b)){
            this.sideA = a;
            this.sideB = b;
        }
    }
    
    //初始化矩形
    public void init(){
        this.sideA = 4;
        this.sideB = 5;
    }
    
    //判断是否符合矩形条件
    public boolean isRectangle(double a,double b){
        if(a<=0 || b<=0){
            System.err.println(lengthErr);
            return false;
        }else{
            return true;
        }
    }

    @Override
    public double area() {
        double result;
        result = this.sideA * this.sideB;
        return result;
    }

    @Override
    public double girth() {
        double result;
        result = (this.sideA + this.sideB)*2;
        return result;
    }
    public double getSideA() {
        return sideA;
    }
    
    //在使用对象类型时，如果没有声明一个实例对象则无法调用对象类型的变量和方法，
    //声明实例对象时，会进行初始化
    //调用对象类型变量的Setter方法，则会将设置值替换初始值进行校验
    public void setSideA(double sideA) {
        if(isRectangle(sideA,this.sideB)){
            this.sideA = sideA;
        }
    }
    public double getSideB() {
        return sideB;
    }
    public void setSideB(double sideB) {
        if(isRectangle(this.sideA,sideB)){
            this.sideB = sideB;
        }
    }
    
    @Override
    public String toString() {
        return "矩形名称是: " + name + ", 矩形长是: " + sideA + ", 矩形宽是: " + sideB + ", 矩形面积是: "+ area() + ", 矩形周长是: " + girth();
    }

    public static void main(String[] args) {
        Rectangle r = new Rectangle();
        System.out.println(r.girth());
        System.out.println(r.area());
//        r.setSideA(0);
        r.setSideA(3);
        System.out.println(r.girth());
        System.out.println(r.area());
        
        Rectangle r1 = new Rectangle(6,8);
        System.out.println(r1.girth());
        System.out.println(r1.area());
        r1.setName("矩形");
        r1.setSideA(5);
        System.out.println(r1.girth());
        System.out.println(r1.area());
        System.out.println(r1.toString());
    }

}
执行结果：
18.0
20.0
16.0
15.0
28.0
48.0
26.0
40.0
矩形名称是: 矩形, 矩形长是: 5.0, 矩形宽是: 8.0, 矩形面积是: 40.0, 矩形周长是: 26.0