第1关 编写一个代表三角形的类

package step1;

import java.util.Scanner;

public class Triangle {
	// 自行设计类的实现
	/********** Begin **********/
    private double a;
    private double b;
    private double c;

    public Triangle(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public double getArea() {
        double p = (a + b + c) / 2;
        return Math.sqrt(p * (p - a) * (p - b) * (p - c));
    }

    public double getPerimeter() {
        return a + b + c;
    }

    //@Override
    public String toString() {
        return String.format("三角形(%.1f,%.1f,%.1f)", a, b, c);
    }
	/********** End **********/

	public static void main(String[] args) {
		/********** Begin **********/
		Scanner sc = new Scanner(System.in);
		double a,b,c; //定义三条边
		a = sc.nextDouble();
		b = sc.nextDouble();
		c = sc.nextDouble();
		Triangle t1 = new Triangle(a, b, c);
		//输出面积
		System.out.println(t1 + "的面积为:" + String.format("%.2f", t1.getArea()));
		//输出周长
		System.out.println(t1 + "的周长为:" +  String.format("%.2f", t1.getPerimeter()));	
		/********** End **********/
	}
}

第2关 编写一个圆环类

package step2;

public class Ring {
	/********** Begin **********/
	//自行设计类的实现  
	  private double innerRadius; //内半径
    private double outerRadius; //外半径
    private String color; //颜色

    //构造方法
    public Ring(double innerRadius, double outerRadius, String color){
        this.innerRadius = innerRadius;
        this.outerRadius = outerRadius;
        this.color = color;
    }

    //计算圆环面积
    public double area() {
        return Math.PI * (outerRadius * outerRadius - innerRadius * innerRadius);
    }

    //计算圆环外圆周长
    public double perimeter() {
        return 2 * Math.PI * outerRadius;
    }

    //计算圆环内圆周长
    public double innerPerimeter() {
        return 2 * Math.PI * innerRadius;
    }

    //getter和setter方法
    public double getInnerRadius() {
        return innerRadius;
    }

    public void setInnerRadius(double innerRadius) {
        this.innerRadius = innerRadius;
    }

    public double getOuterRadius() {
        return outerRadius;
    }

    public void setOuterRadius(double outerRadius) {
        this.outerRadius = outerRadius;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }
	/********** End **********/
                                                     
	public static void main(String[] args) {
		Ring ring = new Ring(5, 8, "red");
		System.out.printf("圆环的内半径: %.1f\n", ring.getInnerRadius());
		System.out.printf("圆环的外半径: %.1f\n", ring.getOuterRadius());
		System.out.println("圆环的颜色: " + ring.getColor());
		System.out.printf("圆环的面积: %.2f\n", ring.area());
		System.out.printf("圆环的外圆周长: %.2f\n", ring.perimeter());
		System.out.printf("圆环的内圆周长: %.2f\n", ring.innerPerimeter());
		System.out.println();

		ring.setInnerRadius(4);
		ring.setOuterRadius(6);
		ring.setColor("blue");

		System.out.printf("圆环的内半径: %.1f\n", ring.getInnerRadius());
		System.out.printf("圆环的外半径: %.1f\n", ring.getOuterRadius());
		System.out.println("圆环的颜色: " + ring.getColor());
		System.out.printf("圆环的面积: %.2f\n", ring.area());
		System.out.printf("圆环的外圆周长: %.2f\n", ring.perimeter());
		System.out.printf("圆环的内圆周长: %.2f\n", ring.innerPerimeter());
	}
}

第3关 编写一个学生类

package step3;

public class Student {
	/********** Begin **********/
	// 自行设计类的实现
    // 成员变量
    private int id;     // 学号
    private int age;    // 年龄
    private String name;    // 姓名

    // 构造方法
    public Student(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    // getter方法
    public int getId() {
        return id;
    }

    public int getAge() {
        return age;
    }

    public String getName() {
        return name;
    }

    // toString方法，返回学生的字符串表示
    public String toString() {
        return "学号:" + id + ",姓名:" + name + ",年龄:" + age;
    }
	/********** End **********/
                                                     
	public static void main(String args[]) {
		Student s1 = new Student(1, 18, "小明");
		Student s2 = new Student(2, 20, "小张");
		Student s3 = new Student(3, 19, "小李");
		Student s4 = new Student(4, 18, "小王");
		Student s5 = new Student(5, 20, "小赵");

		Student s[] = { s1, s2, s3, s4, s5 }; // 给对象数组赋值
		System.out.println("班级学生名单如下：");
		output(s); // 第1次调用output方法输出数组
		/********** Begin **********/
		// 将所有学生年龄加 1
        for (int i = 0; i < s.length; i++) {
            s[i].age++;
        }
		/********** End **********/                             
		System.out.println("所有学生年龄加 1 后...");
		output(s); /// 第2次调用output方法输出数组
		int count = 0;
		/********** Begin **********/
		//统计大于20岁的学生个数
        for (int i = 0; i < s.length; i++) {
            if (s[i].age > 20) {
                count++;
            }
        }
		/********** End **********/                    
		System.out.println("大于 20 岁人数是：" + count);
	}

	/* 以下方法输岀学生数组的所有元素 */
	private static void output(Student s[]) {
		for (int i = 0; i < s.length; i++)
			System.out.println(s[i]);
	}
}