如何使用构造函数在Java中初始化不同类的对象？

Question

public class Location { 

    final public static int INITIAL = 0;
    final public static int PRISON = 1;
    final public static int DEATH = 2;
    final public static int SQUARE = 3;

    private String name;
    private int type;
    private int section;
    private int damage;

    private Square square ; 


    // this constructor construct a Location from a name and a type.

    public Location(String name, int type) { 
        this.name = name;
        this.type = type;       

    }   

    // this constructor constructs a Location of type SQUARE from a name, section, and damage.
    public Location(String name, int section, int damage) {
        this.name = name;
        this.section = section;
        this.damage = damage;   
        this.square = new Square(name,section,damage);
    }   

    // Get the square associated with this Location.

    public Square getSquare() {
        return square;
    }
}

我想我误解了第二个构造函数在做什么，因为当前该构造函数对实例变量平方没有任何作用。

Answer 1

在第二个构造函数中，您只是在初始化属性-根据提供给构造函数的参数， Location类的private Square square 。

初始化对象/非原始类型（例如-这里的square ）在Java中是完全有效的，就像初始化其他原始类型（例如-此处的type ， section等）一样

Answer 2

在第二个构造函数中，您只需要使用Location.SQUARE初始化type ：

// this constructor constructs a Location 
// of type SQUARE from a name, section, and damage.
public Location(String name, int section, int damage) {
    this.name = name;
    this.section = section;
    this.damage = damage;   
    this.type = Location.SQUARE;
    this.square = new Square(name,section,damage);
}  

现在您班级的属性将保持一致。

关于square属性的初始化，对我来说似乎还可以。 您可以在构造函数中创建其他类的实例，并根据需要将其分配给属性。

Answer 3

您有一个问题：您的第一个构造函数对Square引用不执行任何操作，因此它为null。

像这样尝试：

/**
 * @link http://stackoverflow.com/questions/31523704/how-to-use-a-constructor-to-initialize-an-different-classs-object-in-java
 * User: mduffy
 * Date: 7/20/2015
 * Time: 3:07 PM
 */
public class Location {

    final public static int INITIAL = 0;
    final public static int PRISON = 1;
    final public static int DEATH = 2;
    final public static int SQUARE = 3;

    private String name;
    private int type;
    private int section;
    private int damage;

    private Square square;

    // No idea what damage ought to be.  Is that your type?
    public Location(String name, int type) {
        this(name, Location.SQUARE, type);
    }

    public Location(String name, int section, int damage) {
        this.name = name;
        this.section = section;
        this.damage = damage;
        this.square = new Square(name, section, damage);
    }

    public Square getSquare() {
        return square;
    }
}