How to add attributes Dynamically for java object?

Question

Having Student Class.

Class Student{
    String _name;
    ....
    ....

    public Student(){
    }
}

is there any possibility to add dynamic attributes to Student Object? without extending the student class.

Answer 1

In short, yes it is possible to modify bytecode at runtime, but it can be extremely messy and it (most likely) isn't the approach you want. However, if you decide to take this approach, I recommend a byte code manipulation library such as ASM.

The better approach would be to use a Map<String, String> for "dynamic" getters and setters, and a Map<String, Callable<Object>> for anything that isn't a getter or setter. Yet, the best approach may be to reconsider why you need dynamic classes altogether.

public class Student {

    private Map<String, String> properties = new HashMap<String, String>();
    private Map<String, Callable<Object>> callables = new HashMap<String, Callable<Object>>();
    ....
    ....
    public String getProperty(String key) {
        return properties.get(key);
    }

    public void setProperty(String key, String value) {
        properties.put(key, value);
    }

    public Object call(String key) {
        Callable<Object> callable = callables.get(key);
        if (callable != null) {
            return callable.call();
        }
        return null;
    }

    public void define(String key, Callable<Object> callable) {
        callables.put(key, callable);
    }
}

As a note, you can define void methods with this concept by using Callable and returning null in it.

Answer 2

You could get into bytecode manipulation but that way madness lies (especially for the programmer who has to maintain the code).

Store attributes in a Map<String,String> instead.