HashMap的get（）方法

Question

I'm trying to get a value with the key. I'm using get() method. My key is an object composed with int and String. So I make and object

HashMap<Keys,String> test = readFile(fileName2);
Keys myKey = new Keys(2,"SM");
test.get(myKey);

And I received null. When I look at debbuging mode or when I print keySet I received something like this

[Keys@d9c6e2, Keys@10285d8, Keys@c3dd7e]

although my key should be

[1,"GM", 2,"SM", 3"PM"]

why the key look like this Keys@d9c6e2 instead of 2,"SM"? and how to get the value with the key 2,"SM" ?

I override toString methid in Keys . It looks better but still i have null value and im sure there is some value.

    Keys myKey = new Keys(2,"GL-G--T");
    System.out.println(myKey.toString());
    System.out.println(test.get(myKey.toString()));
    Set keyset = test.keySet();
    System.out.println(keyset);


    2,GL-G--T
    null
    [3,PNSN--G, 2,GL-G--T, 1,SM]

Answer 1

You need to override toString method on your Keys object. Otherwise you will get the default toString provided by java.lang.Object .

You could implement the toString method to look something like this:

public class Keys {
    private final Integer i;
    private final String s;

    public Keys(Integer i, String s) {
        this.i = i;
        this.s = s;
    }

    @Override 
    public String toString() {
        return i + "," + s;
    }
}

if you want the quotes to be displayed then you'd need to provide those:

return i + ",\"" + s + "\"";

You'll also need to override the equals and hashCode for this object to be used as a key in a map:

    @Override
    public boolean equals(Object o) {
        if (!(o instanceof Keys)) {
            return false;
        }
        Keys other = (Keys)o;
        return other.s.equals(s) && other.i.equals(i);
    }

    @Override
    public int hashCode() {
        return toString().hashCode();
    }

If you don't override equals and hashcode, then the map uses the default implementations, which results in two Keys objects with the same values being unequal.

Answer 2

You could as you are doing use a special Keys object as the key to your hash map-- you then just need to correctly implement hashCode and equals on that Keys class as others have explained.

Unless you have a specific reason not to, though, you really could just use a String as the key to the hash map. Create some method such as the following:

private static String getHashMapKeyFor(int intKey, String stringKey) {
  return stringKey + "|" + intKey;
}

and declare your hash map as taking a String as the key type. Then, whenever you want to put/find a value in the hash map, call the above method first to get the actual key to use to the hash map.

Using the custom object class may have a superficial air of "correctness" or "engineeredness" to it, but in reality, just using a String will generally perform equally well and if anything may even save slightly on memory.

Answer 3

In your Keys.java object override the toString method. Currently it's using the method defined in java.lang.Object#toString