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?

C. K. Young · Accepted Answer · 2008-11-09 22:22:54Z

37

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);
}

answered Nov 9, 2008 at 22:22

C. K. Young

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Comments

Mat Kelly · Accepted Answer · 2015-04-23 16:54:29Z

5

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.

edited Apr 23, 2015 at 16:54

Mat Kelly

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answered Nov 9, 2008 at 22:17

Greg Hewgill

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2 Comments

Vlad Over a year ago

Link seems to be broken ;(

Mat Kelly Over a year ago

Re-fixed the link to point to archive.org's capture of the resource.

toolkit · Accepted Answer · 2008-11-09 22:21:26Z

4

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

answered Nov 9, 2008 at 22:21

toolkit

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Comments

Brian Risk · Accepted Answer · 2015-05-18 18:16:39Z

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);
    }

}

PhiLho · Accepted Answer · 2008-11-10 09:43:49Z

1

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

answered Nov 10, 2008 at 9:43

PhiLho

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Comments

Frank Krueger · Accepted Answer · 2008-11-09 23:16:14Z

-1

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);
    }
}

answered Nov 9, 2008 at 23:16

Frank Krueger

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1 Comment

C. K. Young Over a year ago

Why bother when the reflection layer already does this caching? Read the source code for java.lang.Class and see for yourself; it even handles invalidating the cache when a class gets reloaded.

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