Update Java object with a Hashmap

Question

I have a model with some properties e.g.

public class Example
{
long id;
String a, b;
int c, d;
boolean e;
}

Now I want to create a method like this

public void update(long id, Map<String, Object> properties)
{
....
}

in this properties map I want to have sth like

properties.put("a","Test"); properties.put("c", 8);

I'm not exactly sure on how to achieve this.

at the end I want to do sth like this:

Example e = new Example(....);
.....
e.update(5L,properties);

can some1 point me to the correct path? I cant figure out a searchterm that doesnt lead me to the Properties or HashMap entries.

thanks in advance

You need to look at reflection. Although in many cases that's not actually a good idea... — Jon Skeet
– Jon Skeet, Commented Aug 18, 2014 at 15:18

Seelenvirtuose · Accepted Answer · 2014-08-19 05:48:05Z

4

You are searching for the keyword reflection. With reflective access you would write your update method like that:

public void update(long id, Map<String, Object> properties) {
    Object obj = getObjectById(id); // you have to implement that method
    for (String property : properties.keySet()) {
        Field field = obj.getClass().getField(property);
        field.set(obj, properties.get(property));
    }
}

Note, that I did not declare or handle any exceptions that come along with reflection.

A completely other issue: Why do you want to do it this way? Using reflection to update fields of an object smells like a real design issue. You really should consider another model.

edited Aug 19, 2014 at 5:48

answered Aug 18, 2014 at 15:26

Seelenvirtuose

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6 Comments

SebastianD Over a year ago

Maybe I am having a design issue. My actual case is that I use Ebean to map a model into a database and populate it via REST. Now I want to enable the update properties method for any object (identified by Id) I get a JSON object with the updated properties and than have to edit it. Would there be a better way to achieve that? Sorry that I didnt state this in the initial question, wanted to keep it simple

Seelenvirtuose Over a year ago

Seems like a legal usage of reflection. In fact, many frameworks that map classes to database tables writing objects into them simply use reflection for that.

SebastianD Over a year ago

Thank you, I accepted your answer and will use it (there is already a getObjectById method provided by the Ebean Framwork).

SebastianD Over a year ago

just a note for your solution in line 4 it has to be obj.getClass().getField(..... right? maybe you can fix that so others will see a working solution.

Seelenvirtuose Over a year ago

@SebastianD Thanks. I bugfixed it.

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