How to check if an object reference is not null?

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 method will throw a NullPointerException. Using the second method variant we can customize the exception message.

The example below shows how we use these methods.

package org.kodejava.example.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 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.");
    }

    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();
        }
    }

    @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.util.Objects.requireNonNull(Objects.java:228)
    at org.kodejava.example.util.ObjectsNullCheckDemo.(ObjectsNullCheckDemo.java:14)
    at org.kodejava.example.util.ObjectsNullCheckDemo.main(ObjectsNullCheckDemo.java:34)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.lang.NullPointerException
    at java.util.Objects.requireNonNull(Objects.java:203)
    at org.kodejava.example.util.ObjectsNullCheckDemo.main(ObjectsNullCheckDemo.java:42)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

How do I validate XML against XSD in Java?

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 create 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 the 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.example.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();
    }
}