Dynamically add properties to a class

Question

I would like to be able to dynamically set properties to an existing class (object), during the runtime.

For example, having the interface MyProperty.java, and other specific properties implementing that interface. Then having an object that can dynamically receive some instance of MyProperty and automatically have a getter for it.

MyPropery.java [interface representing some Object property]

public interface MyProperty {
    String getValue();
}

MyPropertyAge.java [some specific Object property]

public class MyPropertyAge implements MyProperty {

    private final String age;

    public MyPropertyAge(String age) {
        this.age = age;
    }

    @Override
    public String getValue() {
        return age;
    }
}

MyObject.java [Object that should have a getter for MyProperty dynamically]

public class MyObject {

    public MyObject(Set<MyProperty> properties) {
        // receive list of properties,
        // and have getters for them
    }

    // For example, if I passed in a Set with 1 instance of "MyPropertyAge", I want to have this

    public MyPropertyAge getMyPropertyAge() {
        // implementation
    }

}

So the thing is to have properties added to a class dynamically, based on the set of properties it receives in a constructor. Something similar to dynamic keyword in C#: Dynamically add properties to a existing object

Is such thing possible in Java, or some hack that would do the same?

Have a look at [this](http://stackoverflow.com/questions/12832554/how-to-add-attributes-dynamically-for-java-object) Q/A, I think this is what you're looking for :) — Clementine C., Mar 04 '17 at 17:13

score 6 · Answer 1 · answered Mar 04 '17 at 17:24

There is no such thing as the dynamic keyword in Java.

However, you could use an internal Map to store properties. For example:

MyObject.java

public class MyObject {
    private HashMap<String, Object> properties;

    //Create object with properties
    public MyObject(HashMap<String, Object> properties) {
        this.properties = properties;
    }

    //Set properties
    public Object setProperty(String key, Object value) {
        return this.properties.put(key, value); //Returns old value if existing
    }

    //Get properties
    public Object getProperty(String key) {
        return this.properties.getOrDefault(key, null);
    }
}

Example: Person

public static void main(String[] args) {
    //Create properties
    HashMap<String, Object> properties = new HashMap<>();

    //Add name and age
    properties.put("name", "John Doe");
    properties.put("age", 25);

    //Create person
    MyObject person = new MyObject(properties);

    //Get properties
    System.out.println(person.getProperty("age")); //Result: 25
    System.out.println(person.getProperty("name")); //Result: John Doe
}

There exist various ways to implement this, but this is probably the most simple way to do it.

Dynamically add properties to a class

1 Answers1