Use of hashcode() and equals()


Tech Read..

Use of hashCode() and equals(). 

Object class provides two methods hashcode() and equals() to represent the identity of an object. It is a common convention that if one method is overridden then other should also be implemented.

Before explaining why, let see what the contract these two methods hold. As per the Java API documentation:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashcode() method must consistently return the same integer, provided no information used in equals() comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
  • If two objects are equal according to the equals(object) method, then calling the hashCode() method on each of the two objects must produce the same integer result.
  • It is NOT required that if two objects are…

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Ignore transitive dependency in Maven


Maven helps to manage the dependency of JARS . Most of the time it may happen that same JAR file with different versions is being loaded because some other jar files have it as dependency and thus causing the conflicts .

Typical example is have log4j or slf4j jar with different version in spring-xx.jar and junit-xxx.jar . So when you load you classes an conflict may arise and because of incompatible version of same jar file . For e.g.

<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-core</artifactId>
	<version>3.2.1.RELEASE</version>
</dependency>

The above jar uses log4j 1.2.17 (you can check the dependency using mvn dependency:tree).

Consider this log4j jar file is conflicting with some other higher version of log4j which is used by other jar. To resolve this issue we would have to remove the conflicting log4j jar from org.springframework.
it can be done as follows :

<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-core</artifactId>
	<version>3.2.1.RELEASE</version>
         <exclusions>
                <exclusion>
                    <groupId>log4j</groupId>
                    <artifactId>log4j</artifactId>
                </exclusion>
            </exclusions>
</dependency>

Cheers..

Convert JSON to Map or Map to Json in Java


One of the common uses cases while working with json is to convert it to a java object or convert java object to json. There is an open source library available in java which helps in processing json objects.

Add the Jackson dependency in pom.xml file

<repositories>
	<repository>
		<id>codehaus</id>
		<url>http://repository.codehaus.org/org/codehaus</url>
	</repository>
  </repositories>

  <dependencies>
	<dependency>
		<groupId>org.codehaus.jackson</groupId>
		<artifactId>jackson-mapper-asl</artifactId>
		<version>1.9.12</version>
	</dependency>
  </dependencies>

The Java class :

public static void main(String args[]) {
        ObjectMapper mapper = new ObjectMapper();
        Map m = new HashMap();
        Map personMap = new HashMap<>();
        Map personDetail = new HashMap();
        personDetail.put("firstname", "Bob");
        personDetail.put("lastname", "jackson");
        personDetail.put("age", "12");
        personDetail.put("city", "Berlin");

        personMap.put("person", personDetail);
        //convert Map to json string
        try {
            System.out.println(mapper.writeValueAsString(personMap));
        } catch (IOException e) {
            e.printStackTrace();
        }

        // convert json to Map
        String json = "{\"person\":{\"age\":\"12\",\"lastname\":\"jackson\""
                + ",\"firstname\":\"Bob\",\"city\":\"Berlin\"}}"
        try {
            Map map = mapper.readValue(json, Map.class);
            System.out.println("Map is " + map);
        } catch (Exception e) {
            e.printStackTrace();
        }
 }

Output is

Output: {“person”:{“age”:”12″,”lastname”:”jackson”,”firstname”:”Bob”,”city”:”Berlin”}}

Map is {person={age=12, lastname=jackson, firstname=Bob, city=Berlin}}

ObjectMapper mapper = new ObjectMapper();

Creating an object of ObjectMapper is a expensive operation and as best practice it should be created as a singleton object in a class.

Skip maven test while building a project .


Though I would encourage people to skip tests while building a project but sometimes its helpful to go down this path.
Skip Unit Test
To skip the entire unit test, uses argument “-Dmaven.test.skip=true“.

 mvn install -Dmaven.test.skip=true
 mvn package -Dmaven.test.skip=true

Or define skipTests in maven-surefire-plugin.

pom.xml
   <plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-surefire-plugin</artifactId>
	<version>2.12.4</version>
	<configuration>
		<skipTests>true</skipTests>
	</configuration>
    </plugin>

Now, build the project again, the entire unit tests will be ignored.