Setting variables by name in Java

Question

I'm looking to implement something in Java along the lines of:

class Foo{
 private int lorem; //
 private int ipsum;      

 public setAttribute(String attr, int val){
  //sets attribute based on name
 }

 public static void main(String [] args){
  Foo f = new Foo();
  f.setAttribute("lorem",1);
  f.setAttribute("ipsum",2);
 }

 public Foo(){}
}

...where a variable is set based on the variable name without the variable names hard-coded and without using any other data structures. Is this possible?

Answer 1

Here's how you might implement setAttribute using reflection (I've renamed the function; there are different reflection functions for different field types):

public void setIntField(String fieldName, int value)
        throws NoSuchFieldException, IllegalAccessException {
    Field field = getClass().getDeclaredField(fieldName);
    field.setInt(this, value);
}

Answer 2

In general, you want to use Reflection. Here is a good introduction to the topic with examples

In particular, the "Changing Values of Fields" section describes how to do what you'd like to do.

I note that the author says, "This feature is extremely powerful and has no equivalent in other conventional languages." Of course, in the last ten years (the article was written in 1998) we have seen great strides made in dynamic languages. The above is fairly easily done in Perl, Python, PHP, Ruby, and so on. I suspect this is the direction you might have come from based on the "eval" tag.

Answer 3

Also, take a look at BeanUtils which can hide some of the complexity of using reflection from you.

Answer 4

The question is specific to ints, which is helpful, however here is something a bit more general. This type of method is useful if you are loading in String representations of field name / field value pairs.

import java.lang.reflect.Field;

public class FieldTest {

    static boolean isValid = false;
    static int count = 5;

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        FieldTest test = new FieldTest();
        test.setProperty("count", "24");
        System.out.println(count);
        test.setProperty("isValid", "true");
        System.out.println(isValid);
    }

    public void setProperty(String fieldName, String value) throws NoSuchFieldException, IllegalAccessException {
        Field field = this.getClass().getDeclaredField(fieldName);
        if (field.getType() == Character.TYPE) {field.set(getClass(), value.charAt(0)); return;}
        if (field.getType() == Short.TYPE) {field.set(getClass(), Short.parseShort(value)); return;}
        if (field.getType() == Integer.TYPE) {field.set(getClass(), Integer.parseInt(value)); return;}
        if (field.getType() == Long.TYPE) {field.set(getClass(), Long.parseLong(value)); return;}
        if (field.getType() == Float.TYPE) {field.set(getClass(), Float.parseFloat(value)); return;}
        if (field.getType() == Double.TYPE) {field.set(getClass(), Double.parseDouble(value)); return;}
        if (field.getType() == Byte.TYPE) {field.set(getClass(), Byte.parseByte(value)); return;}
        if (field.getType() == Boolean.TYPE) {field.set(getClass(), Boolean.parseBoolean(value)); return;}
        field.set(getClass(), value);
    }

}

Answer 5

Depending on the usage, you can use reflection as advised above, or perhaps a HashMap would be better suited...

Answer 6

You might want to cache some of the reflection data while you're at it:

import java.lang.reflect.Field;
import java.util.HashMap;

class Foo {
    private HashMap<String, Field> fields = new HashMap<String, Field>();

    private void setAttribute(Field field, Object value) {
        field.set(this, value);
    }

    public void setAttribute(String fieldName, Object value) {
        if (!fields.containsKey(fieldName)) {
            fields.put(fieldName, value);
        }
        setAttribute(fields.get(fieldName), value);
    }
}