( 显示三条消息）编写程序，显示Welcome to ]ava、Welcome to Andrew Programming 和 Programming is fun！

public class E1_01 {
 
  public static void main(String[] args) {
    System.out.println("Welcome to Java");
    System.out.println("Welcome to Computer Science");
    System.out.println("Programming is fun");
  }
}

(显示五条消息）编写程序，显示Welcome to Java 五次您的答案

public class E1_02 {
 public static void main(String[] args) {
  System.out.println("Welcome to Java.");
  System.out.println("Welcome to Java.");
  System.out.println("Welcome to Java.");
  System.out.println("Welcome to Java.");
  System.out.println("Welcome to Java.");
 }
}

(数列求和）编写程序，显示1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 的结果。

public class E1_03 {
  public static void main(String[] args) {
    int result = 0;
    for (int i = 1; i <= 9; i++) {
      result += i;
    }
    System.out.println(result);
  }
}

(圆的面积和周长）编写程序，使用以下公式计算并显示半径为 5.5 的圆的面积和周长。

周长=2 x 半径 x π
面积= 半径 x 半径 x π

public class E1_04 {
 
  public static void main(String[] args) {
    double perimeter = perimeter(5.5);
    double area = area(5.5);
    System.out.println("perimeter: " + perimeter);
    System.out.println("area: " + area);
  }
 
  private static double perimeter(double radius) {
    return 2 * radius * Math.PI;
  }
 
  private static double area(double radius) {
    return radius * radius * Math.PI;
  }
}

( 矩形的面积和周长）编写程序，使用以下公式计算并显示宽度为 4.5 、高度为7.9 的矩形的面积和周长。

面积= 宽x高

public class E1_05 {
 
  public static void main(String[] args) {
    System.out.println("area: " + area(4.5, 7.9));
    System.out.println("perimeter: " + perimeter(4.5, 7.9));
  }
 
  private static double area(double width, double height) {
    return width * height;
  }
 
  private static double perimeter(double width, double height) {
    return 2 * (width + height);
  }
}

(人口估算）

美国人口调査局基于以下假设进行人口估算：

• 每7 秒有一个人诞生
• 每13 秒有一个人死亡
• 每45 秒有一个移民迁人

编写一个程序，显示未来 5 年的每年的人口数。假设当前的人口是312 032 486, 每年有 365 天。提示：Java 中，两个整数相除，结果还是整数，小数部分被去掉。例如，5/4 等于1 (而不是1.25 ), 10/4 等于2(而不是2.5 )。如果想得到有小数部分的精确结果，进行除法运算的两个值之一必须是一个具有小数点的数值。例如，5.0/4 等于1.25, 10/4.0 等于2.5。

public class E6_11 {
 
  public static void main(String[] args) {
    System.out.println("Population projection");
    System.out.println("Current population: 312032486");
    System.out.println("One birth every 7 seconds.");
    System.out.println("One death every 13 seconds.");
    System.out.println("One new immigrant every 45 seconds.");
    System.out.println("Yearly Population projection formula: ");
    System.out.println("Current population + ((births per year)");
    System.out.println("                   - (deaths per year)");
    System.out.println("                   + (new immigrants per year))");
    System.out.println("                   * year.");
    System.out.println("Year 1 projection: ");
    System.out.println(312032486 + (((31536000 / 7)
        - (31536000 / 13)
        + (31536000 / 45)) * 1));
    System.out.println("Year 2 projection: ");
    System.out.println(312032486 + (((31536000 / 7)
        - (31536000 / 13)
        + (31536000 / 45)) * 2));
    System.out.println("Year 3 projection: ");
    System.out.println(312032486 + (((31536000 / 7)
        - (31536000 / 13)
        + (31536000 / 45)) * 3));
    System.out.println("Year 4 projection: ");
    System.out.println(312032486 + (((31536000 / 7)
        - (31536000 / 13)
        + (31536000 / 45)) * 4));
    System.out.println("Year 5 projection: ");
    System.out.println(312032486 + (((31536000 / 7)
        - (31536000 / 13)
        + (31536000 / 45)) * 5));
  }
}