Retrieve a static variable using its name dynamically using reflection

Question

How to retrieve a static variable using its name dynamically using Java reflection?

If I have class containing some variables:

public class myClass { 
     final public static string [][] cfg1= {{"01"},{"02"},{"81"},{"82"}}; 
     final public static string [][]cfg2=  {{"c01"},{"c02"},{"c81"},{"c82"}}; 
     final public static string [][] cfg3=  {{"d01"},{"d02"},{"d81"}{"d82"}}; 
     final public static int cfg11 = 5; 
     final public static int cfg22 = 10; 
     final public static int cfg33 = 15; 
 }

And in another class I want variable name is input from user:

class test { 
   Scanner in = new Scanner(System.in); 
   String userInput = in.nextLine();  
    // get variable from class myClass that  has the same name as userInput
   System.out.println("variable name " + // correct variable from class)
}

Using reflection. Any help please?

Rakesh Bute · Accepted Answer · 2015-10-05 06:37:46Z

You need to make use of java reflect. Here is a sample code. For example, I accessed 'cfg1' variable using java reflection, and then printed it into the console. Look into the main method carefully. I have handled no exceptions for simplification. The key line here is:

(String[][]) MyClass.class.getField("cfg1").get(MyClass.class);

__ ^typecast__ ^accessingFeild______________ ^accessFromClassDefinition

public class MyClass {
    final public static String[][] cfg1 = { { "01" }, { "02" }, { "81" },
            { "82" } };
    final public static String[][] cfg2 = { { "c01" }, { "c02" }, { "c81" },
            { "c82" } };
    final public static String[][] cfg3 = { { "d01" }, { "d02" }, { "d81" },
            { "d82" } };
    final public static int cfg11 = 5;
    final public static int cfg22 = 10;
    final public static int cfg33 = 15;

    public static void main(String[] args) throws IllegalArgumentException,
            IllegalAccessException, NoSuchFieldException, SecurityException {

        String[][] str = (String[][]) MyClass.class.getField("cfg1").get(
                MyClass.class);

        for (String[] strings : str) {
            for (String string : strings) {
                System.out.println(string);
            }
        }

    }
}

sp00m · Accepted Answer · 2013-07-01 12:02:38Z

1

If I well understood your needs, this could suit them:

// user input, hardcoded for the example
String fieldName = "cfg22";

MyClass blank = new MyClass();
Object value = null;
try {
    value = MyClass.class.getDeclaredField(fieldName).get(blank);
} catch (IllegalArgumentException e) {
    // if the specified object is not an instance of the class or
    // interface declaring the underlying field (or a subclass or
    // implementor thereof)
    e.printStackTrace();
} catch (SecurityException e) {
    // if a security manager, s, is present [and restricts the access to
    // the field]
    e.printStackTrace();
} catch (IllegalAccessException e) {
    // if the underlying field is inaccessible
    e.printStackTrace();
} catch (NoSuchFieldException e) {
    // if a field with the specified name is not found
    e.printStackTrace();
}

System.out.println(value);

Prints 10.

edited Jul 1, 2013 at 12:02

answered Jul 1, 2013 at 10:03

sp00m

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8 Comments

user2538438 Over a year ago

thanks for your help but this doesn't work with me note : my variables is final public static is that the reason that it doesn't work ??

user207421 Over a year ago

This code doesn't work for static fields, becasue the argument to Field.get() is incorrect for statics. Also no loop is required here. -1 @user2538438 There are no final public statics in the code you posted.

user2538438 Over a year ago

ok , sorry for my mistake but I want them final public static so , any help please ??

user207421 Over a year ago

@user2538438 Have you considered looking up the Javadoc for Field.get() to see what the argument should be?

user2538438 Over a year ago

ok , It works now after I did casting Thanks alot for your help

|

Rick Emanuel Rozo Sevilla · Accepted Answer · 2017-05-15 11:52:11Z

I merge the two above solutions and get:

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.Arrays;

public class MyClass
{

final public static String[][] cfg1 = {{"01"}, {"02"}, {"81"},
{"82"}};
final public static String[][] cfg2 = {{"c01"}, {"c02"}, {"c81"},
{"c82"}};
final public static String[][] cfg3 = {{"d01"}, {"d02"}, {"d81"},
{"d82"}};
final public static int cfg11 = 5;
final public static int cfg22 = 10;
final public static int cfg33 = 15;

public static void main(String[] args) throws IllegalArgumentException,
        IllegalAccessException, NoSuchFieldException, SecurityException
{

    for (Field field : MyClass.class.getDeclaredFields()) {
        if (!Modifier.isStatic(field.getModifiers())) {
            System.out.println("Non-static field: " + field.getName());
        }
        else {
            System.out.println("Static field: " + field.getName());
            Object obj = MyClass.class.getField(field.getName()).get(MyClass.class);
            if (obj instanceof String[][]) {
                String[][] cad = (String[][]) obj;
                for (String[] strings : cad) {
                    System.out.println("Values:: " + Arrays.toString(strings));
                }
            }
            else {
                System.out.println("  " + obj.toString());
            }
        }
    }

}
}

AllTooSir · Accepted Answer · 2013-07-01 09:47:30Z

0

You can try something like this :

for (Field field : myClass.class.getDeclaredFields()) {
    if (!Modifier.isStatic(field.getModifiers())) {
        System.out.println("Non-static field: " + field.getName());              
    }
    else {
        System.out.println("Static field: " + field.getName());
    }
}

Use Field#get(Object obj) to get the value .

Note: Please follow Java naming conventions.

edited Jul 1, 2013 at 9:47

answered Jul 1, 2013 at 9:41

AllTooSir

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2 Comments

user2538438 Over a year ago

I want the value of modifier not the name (i.e. I want the value of array name entered by user and access the element [0][0] in it )

user2538438 Over a year ago

sorry for bothering but I try this code and give me an exception Field f = s.getClass().getDeclaredField(VariableName); String ss[][]=null; f.get(ss); System.out.println(f.getName() + " = " + ss[0][0]); I want to retrieve the value of a 2D array in myClass s

user207421 · Accepted Answer · 2013-07-01 12:21:43Z

0

Just call Class.getField() or Class.getDeclaredField(), then call Field.getValue() on the result, providing null (or the class itself) as the parameter in the case of a static variable, or an instance of the class in the case of an instance variable.

edited Jul 1, 2013 at 12:21

answered Jul 1, 2013 at 10:15

user207421

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1 Comment

sp00m Over a year ago

You should rather give null as parameter if you want to access a static field. The parameter is simply ignored.

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