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;
    }
}

I think I'm misunderstanding what the second constructor is doing, as currently the constructor isn't doing anything to the instance variable square.

Answer 1

In your second constructor you are simply initializing your property - private Square square of your Location class from the parameters you provided to the constructor.

Initializing an Object/non-primitive (eg.- square here) type is completely valid in java like initializing other primitive type (eg. - type , section etc here)

Answer 2

In your second constructor, you just have to initialize type with Location.SQUARE :

// 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);
}  

And now your class' attributes will be consistent.

Regarding initialization of the square attribute, it seems to be OK to me. You can create instances of other classes inside a constructor and assign them to attributes if you want.

Answer 3

You have a problem: Your first constructor does nothing with the Square reference, so it's null.

Try it like this:

/**
 * @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;
    }
}