Java：自定义对象引用数组

Question

I am trying to build an array of custom object references. Here is some simplified code.

import java.util.*;

public class Map {

    private int[] dimension;

    private City[] cities;
    private int nCities;

    public static void main( String[] args ) {
        Map map = new Map( 5, 20, 500, 500 );
    }

    public Map( int nCities, int diameter, int w, int h ) {
        this.dimension = new int[2];

        this.dimension[0] = w;
        this.dimension[1] = h;

        this.create_cities( nCities, diameter );
    }

    private void create_cities( int nCities, int diameter ) {
        // data
        this.cities = new City[nCities];
        this.nCities = nCities;

        // locate cities
        long seed = System.currentTimeMillis();
        Random random = new Random( seed );

        int[] location = new int[2];

        for( int n = 0; n < nCities; n++ ) {
            location[0] = random.nextInt( this.dimension[0] );
            location[1] = random.nextInt( this.dimension[1] );

            this.cities[n] = new City( diameter, location );

            System.out.println( this.cities[n].toString() );
        }
        System.out.println( "\n" + Arrays.toString( this.cities ) );
    }
}

Result:

diameter: 20, location: [311, 324]
diameter: 20, location: [85, 294]
diameter: 20, location: [364, 182]
diameter: 20, location: [269, 412]
diameter: 20, location: [123, 200]

[diameter: 20, location: [123, 200], diameter: 20, location: [123, 200], diameter: 20, location: [123, 200], diameter: 20, location: [123, 200], diameter: 20, location: [123, 200]]

Can anyone unbang my head from the wall? I am storing the same reference 5 times over, correct? How do I not do that? I would like to store a reference to each unique City object.

Answer 1

You're storing the same location array in each city. Make a new array for each one and you should be good to go.

    for( int n = 0; n < nCities; n++ ) {
        int[] location = new int[2];
        location[0] = random.nextInt( this.dimension[0] );

Answer 2

        this.cities[n] = new City( diameter, location );

This is assigning the Reference to same 'location' object to each city, Thus in the end when you are printing all the cities, it shows the locations as latest object refernce..

Move the

       int[] location = new int[2];

inside the loop

     for( int n = 0; n < nCities; n++ ) {

or simply

        this.cities[n] = new City( diameter, new int[](random.nextInt( this.dimension[0] ),random.nextInt( this.dimension[1] ) );

Answer 3

You are passing same diameter and location to the City constructor while creating City object, you need to find a way to pass different diameter and location each time you create a City object.

store the different diameter and location in an array and pass proper values while creating City Object.