How to Use Reflection to Get Information about a Class in Java?

Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine. The class java.lang.Class and the package java.lang.reflect (classes Array, Constructor, Field, Method, Modifier) implement Java's reflection. The methods of java.lang.Class are used to get information about a class. The methods of the classes in java.lang.reflect provide further information about a class and allow methods to be invoked.

The following example shows how to get information about java.lang.String a class:

public class Program {

    public static void main(String[] args) {
        try {

            Class clz = Class.forName("java.lang.String");

            Field[] flds = clz.getDeclaredFields();

            System.out.println("\nVariables...");
            for ( int k = 0; k < flds.length; k++ ) {
                String name = flds[k].getName();
                Class type = flds[k].getType();
                int modifiers = flds[k].getModifiers();
                System.out.print(Modifier.toString( modifiers) + " ");
                if ( type.isArray() )
                    System.out.print(type.getComponentType().getName()
                             + "[] ");
                else 
                    System.out.print(type.getName() + " ");
                System.out.println(name);
            }  
            
            Constructor[] cnsts = clz.getDeclaredConstructors();
            System.out.println( "\nConstructors..." );
            for ( int k = 0; k < cnsts.length; k++ ) {
                String name = cnsts[k].getName();
                Class params[] = cnsts[k].getParameterTypes();
                int modifiers = cnsts[k].getModifiers();
                System.out.print(Modifier.toString( modifiers) +" " 
                        + name + "(");
                for (int i = 0; params != null && i < params.length; i++) {
                    if (i != 0)
                        System.out.print(", ");

                    Class type = params[i];
                    if ( type.isArray() )
                        System.out.print(type.getComponentType().getName()
                               +"[]");
                    else
                        System.out.print(type.getName() );
                }
                System.out.println(")");
            }

            Method[] mms = clz.getDeclaredMethods();
            System.out.println( "\nMethods..." );
            for (int i = 0; i < mms.length; i++) {
                int md = mms[i].getModifiers();
                Class retType = mms[i].getReturnType();
                System.out.print(Modifier.toString(md)+" ");
                if (retType.isArray())
                    System.out.print(retType.getComponentType().getName()
                               +"[] ");
                else
                    System.out.print(retType.getName() + " ");
                System.out.print(mms[i].getName());
                Class cx[] = mms[i].getParameterTypes();
                System.out.print("( ");
                if (cx.length > 0) {
                    for (int j = 0; j < cx.length; j++) {
                        if (cx[j].isArray())
                            System.out.print(
                                 cx[j].getComponentType().getName() +"[]");
                        else
                            System.out.print(cx[j].getName());
                        
                        if (j < (cx.length - 1)) System.out.print(", ");
                    }
                }
                System.out.print(") ");
                System.out.println("{ }");
            }

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

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