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；