Contributing Java “Currying” in StackOverflow Documentation

A week or so ago I’ve contributed in the StackOverflow Documentation an entry about Currying  which until now it’s mostly intact and only cosmetic changes have been made to the original. This is how it goes:

Currying is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument.

This can be useful when:

  1. Different arguments of a function are calculated at different times. (see Example 1)
  2. Different arguments of a function are calculated by different tiers of the application. (see Example 2)

This generic utility applies currying on a 2-argument function:

class FunctionUtils {

    public static <A,B,C> Function<A,Function<B,C>> curry(BiFunction<A, B, C> f) {
        return a -> b -> f.apply(a,b);
    }

}

The above returned curried lambda expression can also be viewed/written written as:

a -> ( b -> f.apply(a,b) );

Example 1

Let’s assume that the total yearly income is a function composed by the income and a bonus:

BiFunction<Integer,Integer,Integer> totalYearlyIncome = (income,bonus) -> income + bonus;

Let’s assume that the yearly income portion is known in advance:

Function<Integer,Integer> partialTotalYearlyIncome = FunctionUtils.curry(totalYearlyIncome).apply(10000);

And at some point down the line the bonus is known:

System.out.println(partialTotalYearlyIncome.apply(100));

Example 2

Let’s assume that the car manufacturing involves the application of car wheels and car body:

BiFunction<String,String,String> carManufacturing = (wheels,body) -> wheels.concat(body);

These parts are applied by different factories:

class CarWheelsFactory {
    public Function<String,String> applyCarWheels(BiFunction<String,String,String> carManufacturing) {
        return FunctionUtils.curry(carManufacturing).apply("applied wheels..");
    }
}

class CarBodyFactory {
    public String applyCarBody(Function<String,String> partialCarWithWheels) {
        return partialCarWithWheels.apply("applied car body..");
    }
}

Notice that the CarWheelsFactory above curries the car manufacturing function and only applies the wheels. The car manufacturing process then will take the below form:

CarWheelsFactory carWheelsFactory = new CarWheelsFactory();
CarBodyFactory   carBodyFactory   = new CarBodyFactory();

BiFunction<String,String,String> carManufacturing = (wheels,body) -> wheels.concat(body);

Function<String,String> partialCarWheelsApplied = carWheelsFactory.applyCarWheels(carManufacturing);
String carCompleted = carBodyFactory.applyCarBody(partialCarWheelsApplied);

ConcurrentHashMap computeIfAbsent method in Java 8

The very nifty method computeIfAbsent has been added in the ConcurrentMap interface in Java 8 as part of the atomic operations of the ConcurrentMap interface. It’s more precisely a default method that provides an alternative to what we use to code ourselves:

if (map.get(key) == null) {
   V newValue = mappingFunction.apply(key);
   if (newValue != null)
      return map.putIfAbsent(key, newValue);
   }
}

but this time providing a function as a second argument.

Most often this method will be used in the context of ConcurrentHashMap in which case the method is implemented in a thread-safe synchronised way.

In terms of usage the method is handy for situations where we want to maintain a thread-safe cache of expensive one-off computed resources.

Here’s another example of holding a key-value pair where value is a thread-safe counter represented by an AtomicInteger:

private final Map counters = new ConcurrentHashMap();

private void accumulate(String name) {
    counters.computeIfAbsent(name, k -> new AtomicInteger()).incrementAndGet();
}

Guava Splitter

Guava Splitter is a utility class that is offering roughly speaking the opposite of what the Joiner utility class does.

You can have a read at my previous quick article about Guava Joiner.

Splitter is offering similar look-and-feel as Joiner: the on method serves as a static factory method, subsequent method calls are following the builder pattern returning a wrapped this (therefore need to be packed prior the final split method call). Finally it doesn’t try to be complicated returning an Iterable of Strings. It is aspiring in replacing the usage of String.split method.

Its usage when we want to get back an Iterable of Strings looks like:

 
Splitter.on(",").trimResults().split("my,string,to,split")

When we want to perform the operation retrieving a Map out of its String representation:

Splitter.on(",").trimResults().omitEmptyStrings().withKeyValueSeparator("->").split("one->1,two->2,three->3")

In this instance the call to withKeyValueSeparator is yielding a Splitter.MapSplitter inner class that is adding the map context to the splitter.

Guava Joiner

Guava Joiner is a nice utility class that is String-ing data structure contents, useful for debugging, logging, reporting or toString-ing activities in a flexible, builder pattern-esque and fluent way.

What we used to achieve with this conventional code:

public static final String joinIterable(final Iterable iterable, String delimiter){
   if(iterable == null)
      throw new IllegalArgumentException();
   StringBuilder result = new StringBuilder("Conventional iterable join: ");
   for(T elem : iterable){
      result.append(elem).append(delimiter);
   }
   result.setLength(result.length() - 1);
   return result.toString();
}

public static final String join(final List list, String delimiter){
   return joinIterable(list, delimiter);
}

can now be achieved with:

Joiner.on(",").useForNull("null").join(list)

The Joiner class is a utility class and the on method represents a static factory method that returns a new Joiner. The useForNull method follows the builder pattern returning a wrapped version of this (therefore need to be packed prior the final join method call). Be carefull that this call is needed otherwise null elements will throw a NPE. The join method is passing the Iterable in this case data structure.

Similarly for maps, what we used to achieve with this:

    public static final <K,V> String join(final Map<K,V> map, String delimiter){
        if(map == null)
            throw new IllegalArgumentException();
        StringBuilder result = new StringBuilder("Conventional map join: ");
        for(K key : map.keySet()){
            result.append(key).append("->").append(map.get(key)).append(delimiter);
        }
        result.setLength(result.length() - 1);
        return result.toString();
    }

can now be achieved by:

Joiner.on(",").withKeyValueSeparator("->").useForNull("null").join(map)

In this instance, the withKeyValueSeparator method call is returning a Joiner.MapJoiner that is a static class that is adding the map context to the Joiner class.

Spring, JdbcTemplate, Oracle example

This is a demonstration of how Spring and Oracle are playing happily together via JdbcTemplate.

Prerequisites:

Let’s start from the end: this is how the JUnit 4 test case looks like for the DAO of our domain object for all its CRUD operations armored with rollback functionality so our database data are not being altered after our transactional operations. Notice the use of matchers and the spring-context-enabling annotation of the test class:

package com.dimitrisli.springJdbcOracle.dao.impl;

import com.dimitrisli.springJdbcOracle.dao.LocationDao;
import com.dimitrisli.springJdbcOracle.model.Location;
import org.junit.Test;
import org.junit.runner.RunWith;
import static org.junit.Assert.*;
import static org.hamcrest.CoreMatchers.*;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
import org.springframework.test.context.transaction.TransactionConfiguration;
import org.springframework.transaction.annotation.Transactional;

import javax.inject.Inject;
import java.util.List;

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "/spring/context/applicationContext.xml")
@TransactionConfiguration(transactionManager = "jdbcTransactionManager", defaultRollback = true)
@Transactional
public class LocationDaoTest {

    @Inject private LocationDao locationDao;

    @Test
    public void testSelectAllLocations(){
    List<Location> locations = locationDao.getLocations();
    assertThat(locations.size(), is(23));
    }

    @Test
    public void testSelectOneLocation(){
      Location location = locationDao.getLocation(1000L);
      assertNotNull("test entry not found", location);
    }

    @Test
    public void testDeleteLocation(){
        assertNotNull("entry for test should be there", locationDao.getLocation(1000L));
        locationDao.deleteLocation(1000L);
        assertNull("entry wasn't successfully deleted", locationDao.getLocation(1000L));
    }

    @Test
    public void testInsertLocation(){
        Location location = new Location(1000L,"test","11111","athens","athens","IT");
        int sizeBeforeInsert = locationDao.getLocations().size();
        locationDao.createLocation(location);
        assertThat(locationDao.getLocations().size(),is(sizeBeforeInsert + 1));
    }

    @Test
    public void testUpdateLocation(){
        Location newLocation = new Location(1000L,"test","11111","athens","athens","IT");
        locationDao.updateLocation(newLocation);
        Location changedLocation = locationDao.getLocation(1000L);
        assertThat(changedLocation.getStreetAddress(), is("test"));
    }

}

The POM looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>SpringJdbcOracle</groupId>
    <artifactId>SpringJdbcOracle</artifactId>
    <version>1.0</version>


    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.3.2</version>
                <configuration>
                    <source>1.6</source>
                    <target>1.6</target>
                    <encoding>${project.build.sourceEncoding}</encoding>
                </configuration>
            </plugin>

            <!--Logging related plugin
                this plugin breaks the build if non-wanted logging frameworks are spotted in the classpath
            -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-enforcer-plugin</artifactId>
                <version>1.0.1</version>
                <executions>
                    <execution>
                        <id>enforce-versions</id>
                        <goals>
                            <goal>enforce</goal>
                        </goals>
                        <configuration>
                            <rules>
                                <bannedDependencies>
                                    <excludes>
                                        <exclude>commons-logging:commons-logging</exclude>
                                        <exclude>log4j:log4j</exclude>
                                    </excludes>
                                </bannedDependencies>
                            </rules>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <!--Spring related dependencies -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-jdbc</artifactId>
            <version>${spring.version}</version>
            <exclusions>
                <exclusion>
                    <groupId>commons-logging</groupId>
                    <artifactId>commons-logging</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-test</artifactId>
            <version>${spring.version}</version>
            <exclusions>
                <exclusion>
                    <groupId>commons-logging</groupId>
                    <artifactId>commons-logging</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>javax.inject</groupId>
            <artifactId>javax.inject</artifactId>
            <version>1</version>
        </dependency>


        <!--Oracle jdbc driver-->
        <dependency>
            <groupId>com.oracle</groupId>
            <artifactId>ojdbc6</artifactId>
            <version>11.2.0.3</version>
        </dependency>

        <!-- DB Connection Pool -->
        <dependency>
            <groupId>commons-dbcp</groupId>
            <artifactId>commons-dbcp</artifactId>
            <version>1.4</version>
        </dependency>

        <!--Logging related dependencies
            Further info: http://www.slf4j.org/faq.html#excludingJCL and
                          http://blog.frankel.ch/configuring-maven-to-use-slf4j
        -->
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <!--scope should be runtime but applied at compile time
                to get autocompletion visibility at logback.xml-->
            <!--scope>runtime</scope-->
            <version>0.9.24</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.6.1</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>jcl-over-slf4j</artifactId>
            <version>1.7.2</version>
        </dependency>

        <!-- JUnit 4 -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.10</version>
        </dependency>

        <!-- Misc -->
        <dependency>
            <groupId>cglib</groupId>
            <artifactId>cglib</artifactId>
            <version>2.2.2</version>
            <scope>runtime</scope>
        </dependency>


    </dependencies>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <spring.version>3.1.2.RELEASE</spring.version>
     </properties>


</project>

Notes:
– Note how we explicitly piping any commons logging or log4j logging through our logback slf4j wrapper
– Notice how we are declaring our Oracle driver dependency given that it’s already installed in our local Maven repo (see prerequisites section)

The applicationContext:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

    <import resource="classpath*:spring/database/database.xml"/>
    <context:component-scan base-package="com.dimitrisli.springJdbcOracle" />
</beans>

The imported database context above is:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:tx="http://www.springframework.org/schema/tx"
       xmlns:c="http://www.springframework.org/schema/c"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd">

    <bean  class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"
           p:location="properties/database.properties" />

    <bean  id="dataSource"
           class="org.apache.commons.dbcp.BasicDataSource"
           destroy-method="close"
           p:driverClassName="${jdbc.driverClassName}"
           p:url="${jdbc.url}"
           p:username="${jdbc.username}"
           p:password="${jdbc.password}" />

    <bean  class="org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate"
           c:dataSource-ref="dataSource"  />

    <bean  id="jdbcTransactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"
            p:dataSource-ref="dataSource"/>
    <tx:annotation-driven transaction-manager="jdbcTransactionManager"/>

</beans>

Notes:

– We are using DBCP for our DB connection pool datasource
– We are using the parameter namespace to save on some attribute injection open-close xml characters
– We are explicitly stating NamedParameterJdbcTemplate as our jdbcTemplate and inject it with the needed datasource so we can have it available for injection conveniently in our DAOs.

The domain POJO object we are about to play with is the Location object that corresponds to the Locations table on the Oracle HR schema:


package com.dimitrisli.springJdbcOracle.model;

public class Location {

    private Long locationId;
    private String streetAddress;
    private String postalCode;
    private String city;
    private String stateProvince;
    private String countryId;

    public Location(Long locationId, String streetAddress, String postalCode, String city, String stateProvince, String countryId) {
        this.locationId = locationId;
        this.streetAddress = streetAddress;
        this.postalCode = postalCode;
        this.city = city;
        this.stateProvince = stateProvince;
        this.countryId = countryId;
    }

//getters, hashcode(), equals(), toString() ignored for brevity

The RowMapper that will provide Location objects having resultSets coming from the DB. It’s a factory method (although not static but stateless by our design) used internally from Spring per DB line result returned:

package com.dimitrisli.springJdbcOracle.orm;

import com.dimitrisli.springJdbcOracle.model.Location;
import org.springframework.jdbc.core.RowMapper;
import org.springframework.stereotype.Component;

import java.sql.ResultSet;
import java.sql.SQLException;

@Component
public class LocationRowMapper implements RowMapper<Location> {

    @Override
    public Location mapRow(ResultSet rs, int rowNum) throws SQLException {
        return  new Location(rs.getLong("LOCATION_ID"),
                             rs.getString("STREET_ADDRESS"),
                             rs.getString("POSTAL_CODE"),
                             rs.getString("CITY"),
                             rs.getString("STATE_PROVINCE"),
                             rs.getString("COUNTRY_ID"));
    }
}

Here’s the DAO interface responsible for the CRUD operations:

package com.dimitrisli.springJdbcOracle.dao;

import com.dimitrisli.springJdbcOracle.model.Location;

import java.util.List;

public interface LocationDao {

    public void createLocation(Location location);
    public List<Location> getLocations();
    public Location getLocation(Long locationId);
    public void updateLocation(Location location);
    public void deleteLocation(Long locationId);

}

and its implementation looks like this:


package com.dimitrisli.springJdbcOracle.dao.impl;

import com.dimitrisli.springJdbcOracle.dao.LocationDao;
import com.dimitrisli.springJdbcOracle.model.Location;
import com.dimitrisli.springJdbcOracle.orm.LocationRowMapper;
import org.springframework.jdbc.core.namedparam.MapSqlParameterSource;
import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
import org.springframework.jdbc.core.namedparam.SqlParameterSource;
import org.springframework.stereotype.Repository;

import javax.inject.Inject;
import java.util.HashMap;
import java.util.List;

@Repository("locationDao")
public class LocationDaoImpl implements LocationDao {

    private static final String CREATE_SQL = "INSERT INTO LOCATIONS( LOCATION_ID, STREET_ADDRESS, POSTAL_CODE, CITY, " +
                                             "STATE_PROVINCE, COUNTRY_ID) " +
                                             "VALUES (LOCATIONS_SEQ.NEXTVAL, :streetAddress, :postalCode, :city, " +
                                             ":stateProvince, :countryId)";

    private static final String GET_ALL_SQL = "SELECT LOCATION_ID, STREET_ADDRESS, POSTAL_CODE, CITY, STATE_PROVINCE, COUNTRY_ID " +
                                              "FROM LOCATIONS";

    private static final String GET_SQL = "SELECT LOCATION_ID, STREET_ADDRESS, POSTAL_CODE, CITY, STATE_PROVINCE, COUNTRY_ID " +
                                          "FROM LOCATIONS WHERE LOCATION_ID = :locationId";

    private static final String DELETE_SQL = "DELETE LOCATIONS WHERE LOCATION_ID = :locationId";

    private static final String UPDATE_SQL = "UPDATE LOCATIONS SET STREET_ADDRESS = :streetAddress, POSTAL_CODE=:postalCode, " +
                                            "CITY = :city, STATE_PROVINCE = :stateProvince, COUNTRY_ID = :countryId " +
                                            "WHERE LOCATION_ID = :locationId";

    @Inject private NamedParameterJdbcOperations jdbcTemplate;
    @Inject private LocationRowMapper locationRowMapper;

    @Override
    public void createLocation(Location location) {
        SqlParameterSource params = new MapSqlParameterSource()
                .addValue("streetAddress", location.getStreetAddress())
                .addValue("postalCode", location.getPostalCode())
                .addValue("city", location.getCity())
                .addValue("stateProvince", location.getStateProvince())
                .addValue("countryId", location.getCountryId());
        jdbcTemplate.update(CREATE_SQL, params);
    }

    @Override
    public List<Location> getLocations() {
        return jdbcTemplate.query(GET_ALL_SQL, new HashMap<String, Object>(), locationRowMapper);
    }

    @Override
    public Location getLocation(Long locationId) {
        SqlParameterSource params = new MapSqlParameterSource()
                .addValue("locationId", locationId);
        List<Location> locations = jdbcTemplate.query(GET_SQL, params, locationRowMapper);
        return locations.isEmpty()?null:locations.get(0);
    }

    @Override
    public void updateLocation(Location location) {
        SqlParameterSource params = new MapSqlParameterSource()
                .addValue("locationId", location.getLocationId())
                .addValue("streetAddress", location.getStreetAddress())
                .addValue("postalCode", location.getPostalCode())
                .addValue("city", location.getCity())
                .addValue("stateProvince", location.getStateProvince())
                .addValue("countryId", location.getCountryId());
        jdbcTemplate.update(UPDATE_SQL, params);
    }

    @Override
    public void deleteLocation(Long locationId) {
        jdbcTemplate.update(DELETE_SQL, new MapSqlParameterSource("locationId",locationId));
    }
}

Notes:
– Notice how we inject the JdbcTemplate and not fetching it from this class directly
– Notice how we inject the RowMapper and we don’t anonymous-class-it from this class directly
– The CRUD operations are setup parameterized in the top of the file as private static finals
– In all the CRUD operations we are using either jdbcTemplate.update() or jdbcTemplate.query() methods

Here’s the Github repo of the project

Maven install ojdbc6

I really wished the Oracle driver jar was part of any (legal) publicly available Maven repo, but it’s not. So we’ll have to take matters on our hands and install it in our local repo once and for all so we can effortlessly thereafter summon it via our pom file:

       <dependency>
            <groupId>com.oracle</groupId>
            <artifactId>ojdbc6</artifactId>
            <version>11.2.0.3</version>
        </dependency>

Steps:

  • Download the jdbc6.jar from the Oracle website. I tried to automate this step via a Groovy script but this pesky agreement radio-button gets in the way (which is there for a reason to be fair)
  • Supposing mvn is already setup in your path:
  • mvn install:install-file -Dfile=ojdbc6.jar -DgroupId=com.oracle -DartifactId=ojdbc6 -Dversion=11.2.0.3 -Dpackaging=jar -DgeneratePom=true
    

Install Java 6,7,8 on Mac OS X

Here’s a quick guide to have Java versions 6,7,8 installed on Mac OS X above and beyond.

Java 6

Java 6 is the last supported version provided by Apple. Therefore we’ll follow the Apple way to install the JDK although Java 1.6 can be branched out from Oracle or the OpenJDK project.

Successful installation will place Java 6 under /Library/Java/JavaVirtualMachines/1.6.0_37-b06-434.jdk/. Following that you can mark JAVA_HOME or point your IDE towards /Library/Java/JavaVirtualMachines/1.6.0_37-b06-434.jdk/Contents/Home/. Also to browse the JDK under an IDE point your editor to the JDK sources found in /Library/Java/JavaVirtualMachines/1.6.0_37-b06-434.jdk/Contents/Home/src.jar

Java 7

We need to go out in the wild to get JDK 7 installed on Mac OS X since mother-Apple doesn’t support it. We can download it either from Oracle or OpenJDK:

Successful installation will place Java 7 under /Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/. Following that you can mark JAVA_HOME or point your IDE towards /Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/. Also to browse the JDK under an IDE point your editor to the JDK sources found in /Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/src.jar

Java 8

We can get a copy of the latest snapshot of JDK 8 to play around with the lambda expressions (natively supported on IntelliJ IDEA) if you haven’t tried already closures in Groovy, Scala or just Predicates/Functions in Google Guava from this download resource

Successful installation will place Java 8 under /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/. Following that you can mark JAVA_HOME or point your IDE towards /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/. Also to browse the JDK under an IDE point your editor to the JDK sources found in /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/src.jar

Groovy, Scala closures demonstration

This is a quick demo of closures usage for Groovy and Scala side-by-side.

Let’s get an easy study case where we are trying to identify whether a string contains unique characters. Starting with Java we would have:

Java

public class StringAllUniqueChars {

    public static boolean hasStringAllUniqueChars(String str){

        //cache data structure
        final HashSet<Character> stackConfinedCache = new HashSet<Character>();
        //imperative iteration
        for(Character c : str.toCharArray()){
            if(stackConfinedCache.remove(c))
                //fail fast
                return false;
            else
                stackConfinedCache.add(c);
        }
        return true;
    }

    public static void main(String[] args) {
        System.out.println("hello\t" + StringAllUniqueChars.hasStringAllUniqueChars("hello"));
        System.out.println("helo\t" + StringAllUniqueChars.hasStringAllUniqueChars("helo"));
    }
}

Keypoints:

  • We create a data structure to store the findings along the way and fail fast in case of the first dup char found
  • We iterate over the chars of the string using a for loop

Groovy

def hasStringAllUniqueChars(str){

    str.collect {         //in a collection
            str.count(it)    //the occurrences of each char
        }
        .findAll {
            it>1    //filter those with more than one occurrences
        }
        .size()==0  //make sure they don't exist
}

def hasStringAllUniqueChars2(str){
    !   //if we don't
    str.any{       //find any
        str.count(it)>1    //character occurrence in string more than once
    }
}

println "hello\t" + hasStringAllUniqueChars2("hello")
println "hello\t" + hasStringAllUniqueChars("helo")

Keypoints:

  • We are running it in a form of Groovy script
  • We are making use of the default it reference while being on a closure
  • In the first implementation we are first transforming the string characters into a collection of corresponding character sizes. Then we are filtering only duplicate characters and finally we are taking the decision based on whether we have any dups across all chars.
  • In the second implementation we are taking a shortcut using Groovy’s any method hand-picking in its closure only dup characters

Scala

object StringAllUniqueChars {

  def hasStringAllUniqueChars(str: String) =

    !str.exists{  //if we don't find any case where
      c =>    //each character's
        str.count(_==c)>1       //count in the string is greater than 1
    }

  def main(args: Array[String]){
    println("hello\t"+hasStringAllUniqueChars("hello"))
    println("helo\t"+hasStringAllUniqueChars("helo"))
  }

}

Keypoints:

  • We are using an Object since we want to host our main method somewhere. It’s Scala’s way to address static and normally the so-called companion object is grouping all the static content of its corresponding class.
  • We are employing closures in a similar way as the second Groovy implementation using Groovy’s exists and count methods

For a matter of completeness this is how Scala, Groovy and Java are co-existing happily together during compile/run time under the Maven umbrella:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>ScalaGroovyClosures</groupId>
    <artifactId>ScalaGroovyClosures</artifactId>
    <version>1.0</version>

    <build>
        <plugins>
            <!--java compiler-->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.3.2</version>
                <configuration>
                    <source>1.6</source>
                    <target>1.6</target>
                </configuration>
            </plugin>

            <!--scala compiler-->
            <plugin>
                <groupId>org.scala-tools</groupId>
                <artifactId>maven-scala-plugin</artifactId>
                <version>2.15.2</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                            <goal>testCompile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>

            <!--groovy compiler-->
            <plugin>
                <groupId>org.codehaus.groovy.maven</groupId>
                <artifactId>gmaven-plugin</artifactId>
                <version>1.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                            <goal>testCompile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
    <dependencies>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-library</artifactId>
            <version>2.9.2</version>
        </dependency>
        <dependency>
            <groupId>org.codehaus.groovy.maven.runtime</groupId>
            <artifactId>gmaven-runtime-1.6</artifactId>
            <version>1.0</version>
        </dependency>
    </dependencies>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

</project>

Java 7 in Maven Project

If you have both jdk6 and jdk7 installed and you would want to interchange between the two compilers in a maven project, this can be easily done by the maven-compiler-plugin and its source and target parameters.

Assuming that we have created a JAVA_7_HOME environment variable we can point the maven-compiler-plugin to the jdk7 compiler as follows:

  		<plugin>
  			<groupId>org.apache.maven.plugins</groupId>
  			<artifactId>maven-compiler-plugin</artifactId>
  			<version>2.3.2</version>
  			<configuration>
  				<source>1.7</source>
  				<target>1.7</target>
  				<encoding>${project.build.sourceEncoding}</encoding>			
  				<executable>${env.JAVA_7_HOME}</executable>
  			</configuration>
  		</plugin>

I have also created a 2-module Maven project one for each jdk that you can find in this Github repository.