How to get attributes for HashMap value?

Question

"my" code:

public void iterateHashmap2() {
    HashMap hashmap = this.media;
    Iterator it = hashmap.keySet().iterator();
    while (it.hasNext())
    {
        Object key = it.next();
        Object val = hashmap.get(key);
            // doesn't work..
        System.out.println(val.getAttribute);
    }
}

So my question is, how do I get the attributes that the value contains. The value is a class I made myself, which contains 4 Strings and another class I made, as attributes. (What I want to do is iterate through a hashmap and compare the String of an attribute that is stored in the value with input data.. so I need to be able to access the attributes in the values of the hashmap..hope that makes sense..)

Answer 1

You access values in a map by providing a key:

// create a map
Map<String, MyObject> map = new HashMap<String, MyObject>();
// store a value
map.put("key", someObject);
// retrieve a value
MyObject someObject2 = map.get("key");

You can also use

• map.keySet() to retrieve all keys
• map.values() to retrieve all values, without keys
• map.entrySet() to retrieve all Mappings. Each Entry represents one key mapped to one value.

a) In my code I use Java Generics, because that has been the standard way to do it for at least 5 years now.
b) You should consider reading The Map Interface from the Java Tutorial.

Answer 2

a): If you are using JDK1.5 or higher, please use generic and enhanced loop. The code will much simpler and safer:

 for(Map.Entry<Key, YourObj> entry: this.media.entrySet()){
    System.out.println(entry.getValue().getAttribute());
 }

b): If you have to use JDK1.4 or lower, you have to cast your object: ((YourObj)val).getAttribute();

Answer 3

Object val = hashmap.get(key);
            // doesn't work..
System.out.println(val.getAttribute());

The problem here is that Java is statically typed, which means you would need to cast val in order to access non-Object methods. You need to cast val as the type actually in the hashmap, as you're not using generics.

MyClass val = (MyClass) hashmap.get(key);

Answer 4

Try changing this line:

Object val = hashmap.get(key);

to this line:

YourClass val = (YourClass) hashmap.get(key);

Since you are getting an Object type you cannot just call your class's method without first casting it correctly.

Btw you should seriously consider using Java Generics

Answer 5

You will need a cast to a your target object's type. Like:

YourObject val = (YourObject) hashmap.get(key); 

But ...

• Your Map should be generic like Map<KeyClass, ValueClass>
• There is better way to iterate over a Map:

Example:

for (Map.Entry<KeyClass, ValueClass> entry : map.entrySet()) {
    KeyClass key = entry.getKey();
    ValueClass val = entry.getValue());
} 

Answer 6

If your question is how to cast the value mapped to the key in the HashMap why not use generics to achieve that:

public void iterateHashmap2() {
    HashMap<Object,YourClass> hashmap = this.media;
    Iterator<YourClass> it = hashmap.keySet().iterator();
    while (it.hasNext())
    {
        Object key = it.next();
        YourClass val = hashmap.get(key);
            // if YourClass has a getAttribute() method then this will work
        System.out.println(val.getAttribute());
    }
}

Also note that it's always good practice to work at interface level -- so this line:

    HashMap<Object,YourClass> hashmap = this.media;

should be

    Map<Object,YourClass> hashmap = this.media;