Get HashMap with Reflection

user_12451309 0
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public class First {
    public final static Map<String, String> MAP = new HashMap<>();
    static {
        MAP.put("A", "1");
        MAP.put("B", "2");
    }
}

public class Second {
    public static void main(String[] args) {
        Class<?> clazz = Class.forName("First");
        Field field = clazz.getField("MAP");
        Map<String, String> newMap = (HashMap<String, String>) field.get(null); // Obviously doesn't work
    }
}

Pretty much it. I have no trouble getting for example values of String variables, but I'm stuck with this one. Tryed to google it, failed. Also, if possible I'd like to get this Map without instantiating its class.

hashmap reflection java
2 Answers

The only thing you are missing is to handle the exceptions for:

  • Class.forName("First");
  • clazz.getField("MAP");
  • field.get(null);

The code below get the static map field from First class. Here I'm just throwing/propagating the exceptions in the main method but you should handle the exceptions in a try/catch block accordingly.

class First {
  public final static Map<String, String> MAP = new HashMap<>();
  static {
    MAP.put("A", "1");
    MAP.put("B", "2");
  }
}


public class Second {
  public static void main(String[] args) throws IllegalArgumentException, IllegalAccessException,
      ClassNotFoundException, NoSuchFieldException, SecurityException {
    Class<?> clazz = Class.forName("First");
    Field field = clazz.getField("MAP");
    Map<String, String> newMap = (HashMap<String, String>) field.get(null); // Obviously doesn't work
    System.out.println(newMap); //Prints {A=1, B=2}
  }
}
user_12532500

Here the same example with a non static class:

package at.noe.szb;
import static org.junit.Assert.assertEquals;
import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;
import org.junit.Test;

public class ReflectionTest {
    private class First {
        public Map<String, String> MAP = new HashMap<>();
        First(){
            MAP.put("A", "1");
            MAP.put("B", "2");
        }
    }
    @Test
    public void testMap() throws Exception {
        Class<?> clazz = Class.forName("at.noe.szb.First");
        Field field = clazz.getField("MAP");
        Map<String, String> newMap = (HashMap<String, String>) field.get(clazz);
        assertEquals("{A=1, B=2}", newMap.toString());
    }
}
user_28661610
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