用不同的参数传递方式调用构造函数的差异

public Time2()
{
    this(0,0,0)
}

和

public Time2()
{
}

如果 Time2(int hour,int minute,int seconds) 构造函数只将传递给它的值分配给3个实例变量(小时、分钟和秒),因为这些实例变量将获得 0 默认值。

另一方面,如果 时间2(整数小时、整数分钟、整数秒) 构造函数包含一些额外的逻辑(例如计算其他实例变量的值),这两个无参数构造函数的实现将具有不同的行为。

必须提供重载/参数化构造函数才能在默认构造函数中使用它:

public class TimeTwo {
    private int hour;
    private int minute;
    private int seconds;

    public TimeTwo() {
        /*
         * the overloaded constructor is used here,
         * you have to write this(...) because
         * you want to initialize the current object
         */
        this(0, 0, 0);
    }

    public TimeTwo(int hour, int minute, int second) {
        this.hour = hour;
        this.minute = minute;
        this.seconds = second;
    }
}

另一种设置默认值的方法如下:

public class TimeTwo {
    private int hour;
    private int minute;
    private int seconds;

    public TimeTwo() {
        this.hour = 0;
        this.minute = 0;
        this.seconds = 0;
    }
}

甚至还有第三种设置默认值的可能性:

public class TimeTwo {
    private int hour = 0;
    private int minute = 0;
    private int seconds = 0;

    public TimeTwo() {}
}