What's Going On in There? Java Reflection for Program Insight

Using Reflection to Retrieve Attributes

A class has other entities besides the ones you have seen so far. What about variables, for example? You can use reflection to get a detailed output of these entities as well. Here is another example using a Field class:

import java.lang.reflect.*;
 
public class DumpFields {
  public static void main(String args[])
  {
     try {
         Class c = Class.forName(args[0]);
         Field[] fields = c.getFields();
         for (int i = 0; i < fields.length; i++)
         System.out.println(fields[i].toString());
     }
     catch (Throwable e) {
        System.err.println(e);
     }
  }
}

When you compile this and execute it with an argument of Employee...

java DumpFields Employee 

...You receive the following output:

public java.lang.String Employee.empNum
public java.lang.String Employee.empName

This result lists all the fields/attributes that belong to the class.

Using Reflection to Retrieve Constructors

This example of a Constructor class will help complete your understanding of the various constructors available, as well as how to use them.

import java.lang.reflect.*;
 
public class DumpConstructors {
  public static void main(String args[])
  {
     try {
         Class c = Class.forName(args[0]);
         Constructor[] constructor = c.getConstructors();
         for (int i = 0; i < constructor.length; i++)
         {
             System.out.println(constructor[i].toString());
            //To print the parameter types
            Class pvec[] = constructor[i].getParameterTypes();
            for (int j = 0; j < pvec.length; j++)
                System.out.println("param #" + j + " " 
                  + pvec[j]);
        }
 
     }
     catch (Throwable e) {
        System.err.println(e);
     }
  }
}

When you compile this and execute it with an argument of Employee...

javac DumpConstructors Employee

...You receive the following result:

public Employee(java.lang.String,java.lang.String)
param #0 class java.lang.String
param #1 class java.lang.String
public Employee()

Analyzing the result, you will find that the Employee class has two constructors. The first one is a parameterized constructor (you can see the parameter types also). The second does not take any parameters and the result is self-explanatory.

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This article was originally published on May 12, 2009