Java核心技术-卷1-基础知识-学习笔记第四章-对象与类

1. 学习 LocalDate

    public static void main(String\u005B\u005D args) throws IOException {LocalDate date = LocalDate.now();
        int month = date.getMonthValue();
        int today = date.getDayOfMonth();

        date = date.minusDays(today - 1); // Set to start of month，minusDays(int n)方法生成当前日期之后或之前 N 天的日期
        DayOfWeek weekday = date.getDayOfWeek();
        int value = weekday.getValue(); // 1 = Monday, ... 7 = Sunday

        System.out.println("Mon Tue Wed Thu Fri Sat Sun");
        for (int i = 1; i < value; i++)
            System.out.print(" ");
        while (date.getMonthValue() == month) {System.out.printf("%3d", date.getDayOfMonth());
            if (date.getDayOfMonth() == today)
                System.out.print("*");
            else
                System.out.print(" ");
            date = date.plusDays(1);
            if (date.getDayOfWeek().getValue() == 1)
                System.out.println();}
        if (date.getDayOfWeek().getValue() != 1)
            System.out.println();}

输出结果

2.Java 文件名必须与 public 类的名字相匹配，在一个源文件中，只能有一个公共类。
3. 如果需要返回一个可变对象的引用，应该首先对他进行克隆（clone）；
4. 一个方法不能修改一个基本数据类型的参数（即数值型或布尔型）；
一个方法可以改变一个对象参数的状态；
一个方法不能让对象参数引用一个新的对象；

public class ParamTest
{public static void main(String[] args)
   {
      /*
       * Test 1: Methods can't modify numeric parameters
       */
      System.out.println("Testing tripleValue:");
      double percent = 10;
      System.out.println("Before: percent=" + percent);
      tripleValue(percent);
      System.out.println("After: percent=" + percent);

      /*
       * Test 2: Methods can change the state of object parameters
       */
      System.out.println("\nTesting tripleSalary:");
      Employee harry = new Employee("Harry", 50000);
      System.out.println("Before: salary=" + harry.getSalary());
      tripleSalary(harry);
      System.out.println("After: salary=" + harry.getSalary());

      /*
       * Test 3: Methods can't attach new objects to object parameters
       */
      System.out.println("\nTesting swap:");
      Employee a = new Employee("Alice", 70000);
      Employee b = new Employee("Bob", 60000);
      System.out.println("Before: a=" + a.getName());
      System.out.println("Before: b=" + b.getName());
      swap(a, b);
      System.out.println("After: a=" + a.getName());
      System.out.println("After: b=" + b.getName());
   }

   public static void tripleValue(double x) // doesn't work
   {
      x = 3 * x;
      System.out.println("End of method: x=" + x);
   }

   public static void tripleSalary(Employee x) // works
   {x.raiseSalary(200);
      System.out.println("End of method: salary=" + x.getSalary());
   }

   public static void swap(Employee x, Employee y)
   {
      Employee temp = x;
      x = y;
      y = temp;
      System.out.println("End of method: x=" + x.getName());
      System.out.println("End of method: y=" + y.getName());
   }
}

class Employee // simplified Employee class
{
   private String name;
   private double salary;

   public Employee(String n, double s)
   {
      name = n;
      salary = s;
   }

   public String getName()
   {return name;}

   public double getSalary()
   {return salary;}

   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }
}

5. 如果没有指定 private 或者 public，这一部分可以被同一个包中的所有方法访问；