Question

I’m using this class as my key to Hashmap with overriden hasCode() and equals()

public class Design {
private double[] factors = null;

public double[] getFactors() {
    return factors;
}

public void setFactors(double[] factors) {
    this.factors = factors;
}

public Design(double[] factors) {
    this.factors = factors;
}

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + Arrays.hashCode(factors);
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (!(obj instanceof Design))
        return false;
    Design other = (Design) obj;
    if (!Arrays.equals(factors, other.factors))
        return false;
    return true;
}

I added values to map using a loop

 public static void main(String[] args) {

    Map<Design, Double> map = new HashMap<Design, Double>();

    double[] dbl = new double[2];

    for(int i=0; i<5; i++){
        for(int j=0; j<2; j++){
            System.out.println(j+i);
            dbl[j] = j+i;
        }
        Design des = new Design(dbl);
        map.put(des, Double.valueOf(i));
    }

    for(Design d: map.keySet()){
        System.out.println(d.getFactors()[0] + "\t" + d.getFactors()[1]);
    }

    for(double d: map.values()){
        System.out.println(d);
    }
}

The problem is in the key values. It displayed the last key added.

4.0 5.0
4.0 5.0
4.0 5.0
4.0 5.0
4.0 5.0

Where am I getting wrong?

Answer 1

If you would move the declaration of your array into the for loop all would go as expected. The problem is now that all your Design instances have the same array.

for(int i=0; i<5; i++){
    double[] dbl = new double[2];
    for(int j=0; j<2; j++){
        System.out.println(j+i);
        dbl[j] = j+i;
    }
    Design des = new Design(dbl);
    map.put(des, Double.valueOf(i));
}

Furthermore, your equals method will yield to incorrect results when you have a subclass of Design. Instead of using instanceof, compare the classes. So change

if (!(obj instanceof Design))
        return false;

to

if (!(obj.getClass() == getClass() ) )
  return false;

This is however unrelated to your problem