How do I check if a class represent an interface type?

You can use the isInterface() method call of the java.lang.Class to identify if a class objects represent an interface type.

package org.kodejava.example.lang.reflect;

import java.io.Serializable;

public class IsInterfaceDemo {
    public static void main(String[] args) {
        IsInterfaceDemo.get(Serializable.class);
        IsInterfaceDemo.get(Long.class);
    }

    private static void get(Class clazz) {
        if (clazz.isInterface()) {
            System.out.println(clazz.getName() +
                    " is an interface type.");
        } else {
            System.out.println(clazz.getName() +
                    " is not an interface type.");
        }
    }
}

Here is the result of the program:

java.io.Serializable is an interface type.
java.lang.Long is not an interface type.

How do I get modifiers of a class object?

package org.kodejava.example.reflect;

import java.lang.reflect.Modifier;

public class ClassModifier {
    public static void main(String[] args) {
        getClassModifier(String.class);        
        getClassModifier(TestA.class);
        getClassModifier(TestB.class);
    }

    private static void getClassModifier(Class clazz) {
        int modifier = clazz.getModifiers();

        //
        // Return true if the integer argument includes the public modifier,
        // false otherwise.
        //
        if (Modifier.isPublic(modifier)) {
            System.out.println(clazz.getName() + " class modifier is public");
        }

        //
        // Return true if the integer argument includes the protected modifier,
        // false otherwise.
        //
        if (Modifier.isProtected(modifier)) {
            System.out.println(clazz.getName() + " class modifier is protected");
        }

        //
        // Return true if the integer argument includes the private modifier,
        // false otherwise.
        //
        if (Modifier.isPrivate(modifier)) {
            System.out.println(clazz.getName() + " class modifier is private");
        }

        //
        // Return true if the integer argument includes the static modifier,
        // false otherwise.
        //
        if (Modifier.isStatic(modifier)) {
            System.out.println(clazz.getName() + " class modifier is static");
        }

        //
        // Return true if the integer argument includes the final modifier,
        // false otherwise.
        //
        if (Modifier.isFinal(modifier)) {
            System.out.println(clazz.getName() + " class modifier is final");
        }

        //
        // Return true if the integer argument includes the abstract modifier,
        // false otherwise.
        //
        if (Modifier.isAbstract(modifier)) {
            System.out.println(clazz.getName() + " class modifier is abstract");
        }
    }

    protected static final class TestA {
    }

    private abstract class TestB {
    }
}

How do I create object using Constructor object?

The example below using a constructor reflection to create a string object by calling String(String) and String(StringBuilder) constructors.

package org.kodejava.example.reflect;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class CreateObjectDemo {
    public static void main(String[] args) {
        Class clazz = String.class;

        try {
            Constructor constructor = clazz.getConstructor(new Class[] {String.class});

            String object = (String) constructor.newInstance(new Object[] {"Hello World!"});
            System.out.println("String = " + object);

            constructor = clazz.getConstructor(new Class[] {StringBuilder.class});
            object = (String) constructor.newInstance(new Object[] {new StringBuilder("Hello Universe!")});
            System.out.println("String = " + object);
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}

How do I get constructors of a class object?

Below is an example that showing you how to get constructors of a class object. In the code below we get the constructors by calling the Class.getDeclaredConstructors() or the Class.getConstructor(Class[]) method.

package org.kodejava.example.reflect;

import java.lang.reflect.Constructor;

public class GetConstructors {
    public static void main(String[] args) {
        Class clazz = String.class;

        //
        // Get all declared contructors and iterate the constructors to get their
        // name and parameter types.
        //
        Constructor[] constructors = clazz.getDeclaredConstructors();
        for (Constructor constructor : constructors) {
            String name = constructor.getName();
            System.out.println("Constructor name= " + name);
            
            Class[] paramterTypes = constructor.getParameterTypes();
            for (Class c : paramterTypes) {
                System.out.println("Param type name = " + c.getName());
            }
            System.out.println("----------------------------------------");
        }

        //
        // Getting a specific constructor of the java.lang.String
        //
        try {
            Constructor constructor = String.class.getConstructor(new Class[] {String.class});
            System.out.println("Constructor     = " + constructor.getName());
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }
}

How do I get field of a class object and set or get its value?

A refection demo to get fields of a class object and set or get their value.

package org.kodejava.example.lang;

import java.util.Date;
import java.lang.reflect.Field;

public class GetSetFieldDemo {
    public static Date now;
    public Long id;
    public String name;

    public static void main(String[] args) {
        GetSetFieldDemo demo = new GetSetFieldDemo();
        Class clazz = demo.getClass();

        try {
            //
            // Get field id, set it value and read it back
            //
            Field field = clazz.getField("id");
            field.set(demo, new Long(10));
            Object value = field.get(demo);
            System.out.println("Value = " + value);

            //
            // Get static field date, set it value and read it back
            //
            field = clazz.getField("now");
            field.set(null, new Date());
            value = field.get(null);
            System.out.println("Value = " + value);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

    }
}