Java Hashtable.contains（）错误

Question

I am using the latest version of Java for x64.

java version "1.6.0_24"
Java(TM) SE Runtime Environment (build 1.6.0_24-b07)
Java HotSpot(TM) 64-Bit Server VM (build 19.1-b02, mixed mode)

I found out that the Hashtable has undesired behavior. Here is a snipped code example:

public class Test {
    public static void main(String[] args) {
        Hashtable<MyObject, MyObject> table = 
            new Hashtable<MyObject, MyObject>();

        MyObject myObj = new MyObject();
        System.out.println(myObj.hashCode());
        System.out.println(myObj.equals(myObj));

        if (!table.contains(myObj)) {
            System.out.println("OK");
            table.put(myObj, myObj);
        }

        if (!table.contains(myObj)) {
            System.out.println("ERROR");
            System.out.println(table);
        }

    }
}

Here is the output:

1500
true
OK
ERROR
{"myObject"="myObject"}

Any clue how come it behaves that way? Can someone point to the problem? By the way, when I am doing the same using HashSet(), I do not get the undesired effect.

Answer 1

Hashtable works just fine but is undesirable was replaced by HashMap in Java 1.2 (1998) I suggest you not use it unless you have to.

public static void main(String... args) {
    Hashtable<MyObject, MyObject> table =
            new Hashtable<MyObject, MyObject>();

    MyObject myObj = new MyObject();
    System.out.println(myObj.hashCode());
    System.out.println(myObj.equals(myObj));

    if (!table.contains(myObj)) {
        System.out.println("OK");
        table.put(myObj, myObj);
    }

    if (!table.contains(myObj)) {
        System.out.println("ERROR");
        System.out.println(table);
    }
}

static class MyObject { }

prints

1584673689
true
OK

however a better solution would be to use a Map like HashMap.