java - 使用不同的参数传递方式调用构造函数的差异

Question

我正在研究构造函数，并且通过了这个示例

public class Time2
{
    private int hour;
    private int minute;
    private int seconds;
    public Time2()
    {
        this(0, 0, 0)
    }
}

第二种方法是

public class Time2
{
    private int hour;
    private int minute;
    private int seconds;
    public Time2()
    {
    }
}

我在第二种方法中没有使用this(0,0,0)。以这种方式使用它们有什么区别？

如果我想在其中添加参数怎么办？

public Time2(int hour)
{
    this(hour, 0, 0);
}

为什么我们需要添加null或0而不是不添加或根本不添加任何内容？

Answer 1

您必须提供一个重载/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;
    }
}

设置默认值的另一种方法可以像这样完成:

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() {}
}