无法将对象变量传递给 Java 中另一个类的构造函数

Question

我正在从核心Java第1卷的书中学习Java，并且对java非常陌生.很抱歉标题令人困惑，但我不知道发生了什么。我写了一个程序，可以在屏幕上打印有关一个人的信息。所以我创建了一个 Man 类的对象数组：

这是我的男人课：-

package com.smit.man;
public class Man
{
    private int age;
    private long id;
    private String name;
    private double weight;
    public Man(int aage,String aname,double aweight,long aid)
    {
        age =aage;
        name = aname;
        weight = aweight;
        id = aid;
    }
   private Add add = new Add((name + "'s Shellter;"),(name + "'s hous's front road;"),(name + " colony;"),id+422);

    public int getAge()
    {
        return age;
    }

    public long getId()
    {
        return id;
    }
        public String getName()
    {
        return name;
    }

    /**
     * @return the weight
     */
    public double getWeight()
    {
        return weight;
    }
    public void pall()
    {
        System.out.println
        (
                "Name = " +this.getName() +
                " Age = "+this.getAge() +
                " ID = " +this.getId() +
                " Weight = "+this.getWeight()
        );
        add.pall();
        System.out.println("nnn");
    }
}

这个 Man 类使用 Add 类，它是这样的：-

package com.smit.man;
/**
 * @author smit
 *
 */
public class Add
{
    private String shellterN,Rd,colony;
    private long pin;
    Add(String a,String b,String c,long d)
    {
        this.shellterN = a;
        this.Rd = b;
        this.colony =c;
        this.pin = d;
    }
    public String getShellterN()
    {
        return shellterN;
    }
    public String getRd()
    {
        return Rd;
    }
    public String getColony()
    {
        return colony;
    }
    public long getPin()
    {
        return pin;
    }
    public void pall()
    {
        System.out.println("ShellterName = " + this.getShellterN() + 
                " Road = "+this.getRd()+ 
                " Colony= "+ this.getColony()+
                " PIN = " + this.getPin());
    }

}

这是主要方法类：-

package com.smit.main;
import com.smit.man.*;
public class Mtest
{
    public static void main(String[] args)
    {
        Man man [] = new Man[3];
        man[0]= new Man(17,"Smit",55,34);
        man[1] = new Man(100,"Master",60,400);
        man[2] = new Man(200,"Guru",10,39);
        for(Man a:man)
            a.pall();
    }
}

每当我在日食中运行它时，我都会得到以下输出

Name = Smit Age = 17 ID = 34 Weight = 55.0
ShellterName = null's Shellter; Road = null's hous's front road; Colony= null colony; PIN = 422


Name = Master Age = 100 ID = 400 Weight = 60.0
ShellterName = null's Shellter; Road = null's hous's front road; Colony= null colony; PIN = 422


Name = Guru Age = 200 ID = 39 Weight = 10.0
ShellterName = null's Shellter; Road = null's hous's front road; Colony= null colony; PIN = 422

我不想要这个空？？为什么我得到这个空？？

Answer 1

private Add add = new Add((name + "'s Shellter;"),(name + "'s hous's front road;"),(name + " colony;"),id+422);

此行位于构造函数之外，因此在创建对象时（甚至在构造函数之前）会立即调用它。您必须像所有其他变量一样添加到私有变量，并在构造函数的末尾对其进行初始化。

当调用此行时，该人的名字尚未初始化，因此为空。这就是造成问题的原因。

Answer 2

因为在构造函数执行之前，您要在Man类中创建private add类级成员。 只要类存在，就会执行类级实例化。 然后构造函数运行。 因此，name成员在当时具有其默认值。

在构造函数中实例化name值存在后的值：

private Add;
public Man(int aage,String aname,double aweight,long aid)
{
    age =aage;
    name = aname;
    weight = aweight;
    id = aid;
    add = new Add((name + "'s Shellter;"),(name + "'s hous's front road;"),(name + " colony;"),id+422);
}