How do I create a class in Java?

By Wayan in Basic, Core API 0 Comment

A class is a specification or blueprint from which individual objects are created. A class contains fields that represent the object’s states and methods that defines the operations that are possible on the objects of the class.

The file name that contains the definition of a class is always the same as the public class name and the extension is .java to identify that the file contains a Java source code.

A class has constructors, a special method that is used to create an instance or object of the class. When no constructor define a default constructor will be used. The constructor method have the same name with the class name without a return value. The constructors can have parameters that will be used to initialize object’s states.

Here is a Person.java file that defines the Person class.

package org.kodejava.example.fundamental;

public class Person {
    private String name;
    private String title;
    private String address;

    /**
     * Constructor to create Person object
     */
    public Person() {

    }

    /**
     * Constructor with parameter
     *
     * @param name
     */
    public Person(String name) {
        this.name = name;
    }

    /**
     * Method to get the name of person
     *
     * @return name
     */
    public String getName() {
        return name;
    }

    /**
     * Method to set the name of person
     *
     * @param name
     */
    public void setName(String name) {
        this.name = name;
    }

    /**
     * Method to get the title of person
     *
     * @return title
     */
    public String getTitle() {
        return title;
    }

    /**
     * Method to set the title of person
     *
     * @param title
     */
    public void setTitle(String title) {
        this.title = title;
    }

    /**
     * Method to get address of person
     *
     * @return address
     */
    public String getAddress() {
        return address;
    }

    /**
     * Method to set the address of person
     *
     * @param address
     */
    public void setAddress(String address) {
        this.address = address;
    }

    /**
     * Method to get name with title of person
     *
     * @return nameTitle
     */
    public String getNameWithTitle() {
        String nameTitle;
        if (title != null) {
            nameTitle = name + ", " + title;
        } else {
            nameTitle = name;
        }
        return nameTitle;
    }

    /**
     * Method used to print the information of person
     */
    @Override
    public String toString() {
        return "Info [" +
                "name='" + name + ''' +
                ", title='" + title + ''' +
                ", address='" + address + ''' +
                ']';
    }
}

Here is a ClassExample.java file that defines the ClassExample class that use the Person class.

package org.kodejava.example.fundamental;

public class ClassExample {
    public static void main(String[] args) {
        Person person = new Person();
        person.setName("Andy");
        person.setTitle("MBA");
        person.setAddress("NY City");
        System.out.println(person);

        String nameTitle1 = person.getNameWithTitle();
        System.out.println("Name with title: " + nameTitle1);

        Person person2 = new Person("Sarah");
        String nameTitle2 = person2.getNameWithTitle();
        System.out.println("Name with title 2: " + nameTitle2);
    }
}

How do I annotate a class or method?

By Wayan in Core API, Lang Package 0 Comment

This example show you how to use the HelloAnnotation annotation on the previous example code, How do I create a simple annotation?. We add the HelloAnnotation annotation to our class and its methods.

package org.kodejava.lang.annotation;

@HelloAnnotation(value = "Good Morning", greetTo = "Universe")
public class HelloAnnotationExample {
    public static void main(String[] args) {
        HelloAnnotationExample hello = new HelloAnnotationExample();
        hello.sayHi();
        hello.sayHello();
    }

    @HelloAnnotation(value = "Hi there", greetTo = "Alice")
    private void sayHi() {
    }

    @HelloAnnotation(value = "Hello there", greetTo = "Bob")
    private void sayHello() {
    }
}

How do I get constructors of a class object?

By Wayan in Reflection 0 Comment

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.lang.reflect;

import java.lang.reflect.Constructor;

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

        // Get all declared constructors 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<?>[] parameterTypes = constructor.getParameterTypes();
            for (Class<?> c : parameterTypes) {
                System.out.println("Param type name = " + c.getName());
            }
            System.out.println("----------------------------------------");
        }

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

How do I get fields of a class object?

By Wayan in Reflection 0 Comment

The example below using reflection to obtain the fields of a class object. We’ll get the field names and their corresponding type. There are three ways shown below which can be used to get an object fields:

  • Class.getDeclaredFields()
  • Class.getFields()
  • Class.getField(String)
package org.kodejava.lang.reflect;

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

public class GetFields {
    public Long id;
    protected String name;
    private Date birthDate;
    Double weight;

    public static void main(String[] args) {
        GetFields object = new GetFields();
        Class<? extends GetFields> clazz = object.getClass();

        // Get all object fields including public, protected, package and private
        // access fields.
        Field[] fields = clazz.getDeclaredFields();
        System.out.println("Number of fields = " + fields.length);
        for (Field field : fields) {
            System.out.println("Field name = " + field.getName());
            System.out.println("Field type = " + field.getType().getName());
        }

        System.out.println("\n----------------------------------------\n");

        // Get all object accessible public fields.
        fields = clazz.getFields();
        System.out.println("Number of fields = " + fields.length);
        for (Field field : fields) {
            System.out.println("Field name = " + field.getName());
            System.out.println("Field type = " + field.getType().getName());
        }

        System.out.println("\n----------------------------------------\n");

        try {
            // Get field name id with public access modifier
            Field field = clazz.getField("id");
            System.out.println("Field name = " + field.getName());
            System.out.println("Field type = " + field.getType().getName());
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
    }
}

The output of the code snippet above are:

Number of fields = 4
Field name = id
Field type = java.lang.Long
Field name = name
Field type = java.lang.String
Field name = birthDate
Field type = java.util.Date
Field name = weight
Field type = java.lang.Double

----------------------------------------

Number of fields = 1
Field name = id
Field type = java.lang.Long

----------------------------------------

Field name = id
Field type = java.lang.Long

How do I invoke a method using Method class?

By Wayan in Reflection 0 Comment

This example demonstrate using reflection to invoke methods of a class object. Using reflection we can call method of an object using the given string name of the method. When using this method we need to catch for the NoSuchMethodException, IllegalAccessException and InvocationTargetException.

package org.kodejava.lang.reflect;

import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

public class InvokingMethod {
    public static void main(String[] args) {
        InvokingMethod object = new InvokingMethod();
        Class<? extends InvokingMethod> clazz = object.getClass();

        try {
            // Invoking the add(int, int) method
            Method method = clazz.getMethod("add", int.class, int.class);
            Object result = method.invoke(object, 10, 10);
            System.out.println("Result = " + result);

            // Invoking the multiply(int, int) method
            method = clazz.getMethod("multiply", int.class, int.class);
            result = method.invoke(object, 10, 10);
            System.out.println("Result = " + result);

        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
            e.printStackTrace();
        }
    }

    public int add(int numberA, int numberB) {
        return numberA + numberB;
    }

    public int multiply(int numberA, int numberB) {
        return numberA * numberB;
    }
}