java對象屬性的初始值

當我們創建一個實例時，如果沒有在構造器中初始化非靜態屬性，編譯器會將數值類型數據設為0，布爾類型數據設為false，對象類型數據設為null。

也可以通過一下方式初始化：

（1）在構造方法中初始化

（2）直接給屬性默認值或調靜態方法初始化。雖然PHP不能調靜態方法初始化非靜態屬性，但是可以在構造方法中調靜態方法初始化非靜態屬性

public class Employee {
    protected String name;
    protected double salary;
    protected int id = assignId();
    protected static int nextId = 1;

    protected LocalDate hireDay;

    public Employee(String name, double salary, String hireDay) {
        this.name = name;
        this.salary = salary;
        this.hireDay = LocalDate.parse(hireDay);
    }

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public String getHireDay() {
        return hireDay.format(DateTimeFormatter.ofPattern("y-MM-dd"));
    }

    public String getHireDay(String DayFormatter) {
        return hireDay.format(DateTimeFormatter.ofPattern(DayFormatter));
    }

    public String getName() {
        return name;
    }

    public int getId()
    {
        return id;
    }

    protected int assignId()
    {
        return nextId++;
    }
}