通过反射读取R.drawable对应地图片资源id

1.首先R文件是一个java类而drawable是其一个静态内部类其所有的字段都为静态字段 对于静态字段通过反射读取时是不需要指定对象的所以 getInt(null)中的参数置空

protected int getDrawableId(String key) {

      try {
            String name = key;
            Field field = R.drawable.class.getField(name);
            return field.getInt(null);
        } catch (SecurityException e) {
        } catch (NoSuchFieldException e) {
        } catch (IllegalAccessException e) {
        }
        return -1;
    }

2,对于非静态字段我们是要指定对象参数的 like this

import java.lang.reflect.*;

public class ClassDemo {
   public int num = 123;
   public static void main(String[] args) {

     ClassDemo c = new ClassDemo();
     Class cls = c.getClass();

     System.out.println("Field =");

     try {
        // string field
        Field sField = cls.getField("num");
        int num1=0;
        try {

             num1=sField.getInt(c);
        } catch (IllegalArgumentException e) {

            e.printStackTrace();
        } catch (IllegalAccessException e) {

            e.printStackTrace();
        }
        System.out.println("Public field found: " + sField.toString()+"value="+num1);
     }
     catch(NoSuchFieldException e) {
        System.out.println(e.toString());
     }
   }

   public ClassDemo() {

   }

   public ClassDemo(int num) {       
      this.num = num;
   }


}