How to check if an object reference is not null?

By Wayan in Core API, Util Package 0 Comment

Usually, if not always, we use the if statement combined with == or != operators to check if an object reference is null or not. We do this to validate arguments passed to constructors or methods doesn’t contain a null value. These null check can be seen as clutter in our code.

The solution is to use the java.util.Objects class. This static utility class provides methods like requireNonNull(T) and requireNonNull(T, String) to check if the specified object reference is not null. If null, these methods will throw a NullPointerException. Using the second method variant we can customise the exception message.

The example below shows how we use these methods.

package org.kodejava.util;

import java.util.Objects;

public class ObjectsNullCheckDemo {
    private String firstName;
    private String lastName;

    /**
     * Validate constructor arguments. The firstName and lastName
     * arguments can't be null. A NullPointerException with the
     * specified message will be thrown.
     */
    public ObjectsNullCheckDemo(String firstName, String lastName) {
        this.firstName = Objects.requireNonNull(firstName,
                "First name can't be null.");
        this.lastName = Objects.requireNonNull(lastName,
                "Last name can't be null.");
    }

    public static void main(String[] args) {
        // This line is fine.
        ObjectsNullCheckDemo demo = new ObjectsNullCheckDemo("John", "Doe");
        System.out.println("demo = " + demo);

        try {
            // This line produce a NullPointerException
            ObjectsNullCheckDemo demo1 = new ObjectsNullCheckDemo("Alice", null);
        } catch (Exception e) {
            e.printStackTrace();
        }

        String name = null;
        try {
            // The line below will throw java.lang.NullPointerException.
            Objects.requireNonNull(name);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void setFirstName(String firstName) {
        // First name can't be null.
        this.firstName = Objects.requireNonNull(firstName,
                "First name can't be null.");
    }

    public void setLastName(String lastName) {
        // Last name can't be null.
        this.lastName = Objects.requireNonNull(lastName,
                "Last name can't be null.");
    }

    @Override
    public String toString() {
        return "ObjectsNullCheckDemo{" +
                "firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                '}';
    }
}

Running the code above will print the following result:

demo = ObjectsNullCheckDemo{firstName='John', lastName='Doe'}
java.lang.NullPointerException: Last name can't be null.
    at java.base/java.util.Objects.requireNonNull(Objects.java:233)
    at org.kodejava.util.ObjectsNullCheckDemo.<init>(ObjectsNullCheckDemo.java:17)
    at org.kodejava.util.ObjectsNullCheckDemo.main(ObjectsNullCheckDemo.java:28)
java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:208)
    at org.kodejava.util.ObjectsNullCheckDemo.main(ObjectsNullCheckDemo.java:36)

How do I validate XML against XSD in Java?

By Wayan in XML 25 Comments

The javax.xml.validation package provides an API for XML documents validation. The validation process verify that an XML document is an instance of a specified XML schema file or XSD file. In this example we are going to validate if the records.xml file below ins an instance of the records.xsd schema. First we will create the following XML file and an XSD file it should follow.

The XML file:

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <records>
        <record>
            <title>Brand New Eyes</title>
            <artist>Paramore</artist>
            <genre>Punk Rock</genre>
            <year>2011</year>
        </record>
        <record>
            <artist>Various Artist</artist>
            <genre>Rock</genre>
            <year/>
        </record>
    </records>
</root>

The XSD file:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" attributeFormDefault="unqualified"
           elementFormDefault="qualified">
    <xs:element name="root" type="rootType">
    </xs:element>

    <xs:complexType name="rootType">
        <xs:sequence>
            <xs:element name="records" type="recordsType"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="recordsType">
        <xs:sequence>
            <xs:element name="record" type="recordType" maxOccurs="unbounded" minOccurs="0"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="recordType">
        <xs:sequence>
            <xs:element type="xs:string" name="title"/>
            <xs:element type="xs:string" name="artist"/>
            <xs:element type="xs:string" name="genre"/>
            <xs:element type="xs:short" name="year"/>
        </xs:sequence>
    </xs:complexType>
</xs:schema>

The code snippet below will handle the validation process in the following steps. In the main() method we create the XMLValidator instance and call the validate() method and pass the XML file and the XSD file. Our validate() method start by creating an instance of SchemaFactory. The SchemaFactory.newInstance() method return an instance of SchemaFactory. In this example we are creating a W3C XML Schema.

The next step is to create a Schema object by calling the schemaFactory.newSchema() and pass the schema / XSD file. The Schema object will allow us to create an instance of javax.xml.validation.Validator by calling the schema.newValidator() method. And finally to validate if the XML is valid we call validator.validate() method and pass the XML file to be validated. If the XML is not valid, this validate() method will throw exceptions.

The Java code:

package org.kodejava.xml;

import org.xml.sax.SAXException;

import javax.xml.XMLConstants;
import javax.xml.transform.stream.StreamSource;
import javax.xml.validation.Schema;
import javax.xml.validation.SchemaFactory;
import javax.xml.validation.Validator;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.net.URL;
import java.util.Objects;

public class XMLValidator {
    public static final String XML_FILE = "records.xml";
    public static final String SCHEMA_FILE = "records.xsd";

    public static void main(String[] args) {
        XMLValidator XMLValidator = new XMLValidator();
        boolean valid = XMLValidator.validate(XML_FILE, SCHEMA_FILE);

        System.out.printf("%s validation = %b.", XML_FILE, valid);
    }

    private boolean validate(String xmlFile, String schemaFile) {
        SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
        try {
            Schema schema = schemaFactory.newSchema(new File(getResource(schemaFile)));

            Validator validator = schema.newValidator();
            validator.validate(new StreamSource(new File(getResource(xmlFile))));
            return true;
        } catch (SAXException | IOException e) {
            e.printStackTrace();
            return false;
        }
    }

    private String getResource(String filename) throws FileNotFoundException {
        URL resource = getClass().getClassLoader().getResource(filename);
        Objects.requireNonNull(resource);

        return resource.getFile();
    }
}

How do I validate input when using Scanner?

By Wayan in Core API, Util Package 13 Comments

This example show you how to validate input when using java.util.Scanner. To validate input the Scanner class provides some hasNextXXX() method that can be use to validate input. For example if we want to check whether the input is a valid integer we can use the hasNextInt() method.

In the code snippet below will demonstrate how to validate whether the user provide a positive integer number. The program will repeat until the correct input is supplied.

package org.kodejava.util;

import java.util.Scanner;

public class ScannerValidateInput {
    public static void main(String[] args) {
        ScannerValidateInput demo = new ScannerValidateInput();
        demo.validatePositiveNumber();
    }

    private void validatePositiveNumber() {
        Scanner scanner = new Scanner(System.in);

        int number;
        do {
            System.out.print("Please enter a positive number: ");
            while (!scanner.hasNextInt()) {
                String input = scanner.next();
                System.out.printf("\"%s\" is not a valid number.%n", input);
            }
            number = scanner.nextInt();
        } while (number < 0);

        System.out.printf("You have entered a positive number %d.%n", number);
    }
}

The output produce by the snippet:

Please enter a positive number: qwerty
"qwerty" is not a valid number.
@@@
"@@@" is not a valid number.
-100
Please enter a positive number: 99
You have entered a positive number 99.

Another example is to validate if user correctly input letters to guest a secret word. In the code snippet below if the user does not enter a letter the code will keep asking for a valid letter. It loops until the length of the inputted letters equals to the length of secret word.

package org.kodejava.util;

import java.util.Scanner;

public class ScannerValidateLetter {
    public static void main(String[] args) {
        ScannerValidateLetter demo = new ScannerValidateLetter();
        demo.validateLetter();
    }

    private void validateLetter() {
        String secretWord = "Hello";
        Scanner scanner = new Scanner(System.in);

        int length = 0;
        StringBuilder guess = new StringBuilder();
        do {
            System.out.print("Enter a letter to guess: ");
            char letter = scanner.next().charAt(0);
            if (Character.isLetter(letter)) {
                guess.append(letter);
                length++;
            }
        } while (length < secretWord.length());

        if (secretWord.equalsIgnoreCase(guess.toString())) {
            System.out.println("You are correct!");
        } else {
            System.out.println("Please try again!");
        }
    }
}

Enter a letter to guess: 1
Enter a letter to guess: 2
Enter a letter to guess: H
Enter a letter to guess: e
Enter a letter to guess: l
Enter a letter to guess: l
Enter a letter to guess: o
You are correct!

How do I validate email address using Java Mail API?

By Wayan in Mail 0 Comment

This code snippet shows you how to validate an email address using the javax.mail.internet.InternetAddress class. The validate() method throws a javax.mail.internet.AddressException when the email address passed to the constructor is not a valid email address.

Here is the complete code snippet:

package org.kodejava.mail;

import javax.mail.internet.AddressException;
import javax.mail.internet.InternetAddress;

public class ValidateEmail {
    public static void main(String[] args) {
        ValidateEmail demo = new ValidateEmail();

        String email = "kodejava@example.com";
        boolean isValid = demo.validateEmail(email);
        demo.printStatus(email, isValid);

        email = "kodejava.example";
        isValid = demo.validateEmail(email);
        demo.printStatus(email, isValid);
    }

    private boolean validateEmail(String email) {
        boolean isValid = false;
        try {
            // Create InternetAddress object and validated the supplied
            // address which is this case is an email address.
            InternetAddress internetAddress = new InternetAddress(email);
            internetAddress.validate();
            isValid = true;
        } catch (AddressException e) {
            e.printStackTrace();
        }
        return isValid;
    }

    private void printStatus(String email, boolean valid) {
        System.out.println(email + " is " + (valid ? "a" : "not a") +
                " valid email address");
    }
}

When running the program you will get the following message printed on the screen.

kodejava@example.com is a valid email address
kodejava.example is not a valid email address
javax.mail.internet.AddressException: Missing final '@domain' in string ``kodejava.example''
    at javax.mail.internet.InternetAddress.checkAddress(InternetAddress.java:1279)
    at javax.mail.internet.InternetAddress.validate(InternetAddress.java:1154)
    at org.kodejava.mail.ValidateEmail.validateEmail(ValidateEmail.java:25)
    at org.kodejava.mail.ValidateEmail.main(ValidateEmail.java:15)

Maven Dependencies

<dependencies>
    <dependency>
        <groupId>javax.mail</groupId>
        <artifactId>javax.mail-api</artifactId>
        <version>1.5.6</version>
    </dependency>
    <dependency>
        <groupId>javax.mail</groupId>
        <artifactId>mail</artifactId>
        <version>1.4.7</version>
    </dependency>
</dependencies>

Maven Central Maven Central

How do I validate email address using regular expression?

By Wayan in Core API, Lang Package 0 Comment

In this example we use the String‘s class matches() methods to match a string to be a valid email address based on the given regex.

This example also demonstrate the power of regular expression to validate an email address. Using regular expression makes it easier to validate data such as email address. After the code you’ll see the meaning of the regular expression used in the code below.

package org.kodejava.lang;

public class EmailAddressValidation {
    private static final String EMAIL_REGEX =
            "^[\\w-_\\.+]*[\\w-_\\.]\\@([\\w]+\\.)+[\\w]+[\\w]$";

    public static void main(String[] args) {
        EmailAddressValidation validator = new EmailAddressValidation();

        System.out.println("isValid = "
                + validator.isValidEmailAddress("user@domain.com"));
        System.out.println("isValid = "
                + validator.isValidEmailAddress("user.name@domain.com"));
        System.out.println("isValid = "
                + validator.isValidEmailAddress("user.name@domain.co.id"));
        System.out.println("isValid = "
                + validator.isValidEmailAddress("user.domain.co.id"));
    }

    /**
     * Validates email address against email regular expression.
     *
     * @param email an email address to check
     * @return true if email address is valid otherwise return false.
     */
    private boolean isValidEmailAddress(String email) {
        return email.matches(EMAIL_REGEX);
    }
}

The first ^[\\w-_\\.+]. The ^ symbols means check the first character. This the regex processor that the email address should start with a word character formed of alphanumeric value (a-z 0-9) or it can also be a hyphen, underscore, dot or a plus symbol.

The second part, *[\\w-_\\.]. The * symbol means match the preceding zero or more times. As the first part, this tell the regex processor to check for another zero or more characters, and it can also contain hyphen, underscore and a dot.

The third part, \\@([\\w]+\\.)+. This check that email address should contain the @ symbol followed by one or more word separated by the dot symbol.

The last part is, [\\w]+[\\w]$, this check that after the last period there should be another word for the domain suffix such as the co.uk or co.id. And the $ ask that the email address should end by a word character.