How do I get a class reference from a String in Java?

Question

If I need to return a Class instanced based on a String (e.g., creating logic from JSON), how should that be written?

My initial thought was a switch, but in the Android framework's version of Java, it looks like switch statements don't allow Strings.

Next thought was a HashMap:

private HashMap<String, Class> classMap = new HashMap<String, Class>();
classMap.put("someClass", SomeClass.class)?
// ... etc
String someString = jsonObject.getString("someProperty");
Class classRef = classMap.get(someString);
SomeSuperClass classInstance = new classRef();

This doesn't work. I'd rather not if/else down a lengthy list of possibilities.

No version of Java allows switch statements with strings (except Java 7, but everyone knows that's just a myth to keep us quiet). — Tharwen, May 07 '12 at 18:41

score 6 · Accepted Answer · answered May 07 '12 at 18:43

6

Just use Class<?> clazz = Class.forName("com.yourpackage.YourClass") Then to instantiate it: clazz.newInstance();

answered May 07 '12 at 18:43

Thomas Dignan

7,052
3
40
48

This will return the reference of the Object class not the reference of the Class class. – Bhavik Ambani May 07 '12 at 18:48
4

@Bhavik the first line Class.forName("...") returns a class reference. The second line instantiates it and creates an object. – Thomas Dignan May 07 '12 at 18:50

score 1 · Answer 2 · answered May 07 '12 at 18:53

1

You can also load the class with having the constructors parameters.

E.g.

public static void load(String p)
    throws ClassNotFoundException, InstantiationException,
    IllegalAccessException{
        Class cl = classLoader.loadClass(p);
        Constructor c = cl.getConstructor(String.class);
        Plugin plug = (Plugin) c.newInstance("myArgument");
    }

answered May 07 '12 at 18:53

Bhavik Ambani

6,557
14
55
86

Shouldn't you return the created Plugin instance? – Natix May 07 '12 at 18:56
We can return the created `Plugin` instance but my intention was to display example that you can create instance of the Object though it has constructor with arguments. – Bhavik Ambani May 07 '12 at 19:00

score 0 · Answer 3 · answered May 07 '12 at 18:44

0

Your approach using a map is what I would do (and you'll see it in Factory Pattern implementations). You're just not instantiating the class correctly:

Class classRef = classMap.get(someString);
SomeSuperClass classInstance = classRef.newInstance();

answered May 07 '12 at 18:44

Brian Roach

76,169
12
136
161

How do I get a class reference from a String in Java?

3 Answers3

Linked