Guava: Joiner class: Concatenate strings

Joiner class is used to concatenate strings together with some delimiter. You may think that, what extra it is offering to you. Following are some of the things that you can get.

a.   You can concatenate multiple strings using delimeter.

b.   You can concatenate data in any collection like List, Set, Map and any other which implements Iterable interface.

c.    You can write final result to StringBuilder

d.   You can write final concatenated result to a file

e.   You can write final concatenated result to any any Appendable interface implementations like BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer.

f.     You can handle null values while concatenating.

Lets start with simple example,

import com.google.common.base.Joiner;

public class JoinerEx {
  public static void main(String args[]) {
    Joiner joiner = Joiner.on("$");

    String result = joiner.join("HariKrishna", "Karnataka", "Bangalore",
        "Marthalli");
    System.out.println(result);
  }
}

Output

HariKrishna$Karnataka$Bangalore$Marthalli

Joiner.on("$")

Above statement returns Joiner instance, which concatenates strings using the delimiter "$".

joiner.join("HariKrishna", "Karnataka", "Bangalore", "Marthalli")

Joins strings using the separator ($: It specified at the time of Joiner instance initialization).

While joining strings, you must be aware of null values. You must handle null values, otherwise java.lang.NullPointerException thrown. It is because, while joining elements, toString method of the elements called, If element is null, obviously it throws java.lang.NullPointerException.

import com.google.common.base.Joiner;

public class JoinerEx {
  public static void main(String args[]) {
    Joiner joiner = Joiner.on("$");

    String result = joiner.join("HariKrishna", "Karnataka", "Bangalore",
        null);
    System.out.println(result);
  }
}

Output

Exception in thread "main" java.lang.NullPointerException
  at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:210)
  at com.google.common.base.Joiner.toString(Joiner.java:433)
  at com.google.common.base.Joiner.appendTo(Joiner.java:111)
  at com.google.common.base.Joiner.appendTo(Joiner.java:152)
  at com.google.common.base.Joiner.join(Joiner.java:193)
  at com.google.common.base.Joiner.join(Joiner.java:183)
  at com.google.common.base.Joiner.join(Joiner.java:209)
  at guava.JoinerEx.main(JoinerEx.java:9)

You can handle null values in two ways.

a.   By skipping null values

b.   By using separate string for null values

import com.google.common.base.Joiner;

public class JoinerEx {
  public static void main(String args[]) {
    Joiner joiner1 = Joiner.on("$").skipNulls();
    Joiner joiner2 = Joiner.on("$").useForNull("NoData");

    String result1 = joiner1.join(null, "1234", "Hello", null, "How");
    String result2 = joiner2.join(null, "1234", "Hello", null, "How");

    System.out.println(result1);
    System.out.println(result2);
  }
}

Output

1234$Hello$How
NoData$1234$Hello$NoData$How

Joiner.on("$").skipNulls();

Above statement creates Joiner instance, while concatenating strings, it simply ignore null values.

Joiner.on("$").useForNull("NoData");

Above statement creates Joiner instance, while concatenating, it replace null values with string “NoData”.

One important thing to remember is Joiner is immutable and thread safe.

One useful feature is Joiner class iterates through all kinds of collections that implements java.lang.Iterable interface and concatenate the strings.

import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

import com.google.common.base.Joiner;

public class JoinerEx {
  public static void main(String args[]) {
    Joiner joiner = Joiner.on("$").skipNulls();

    List<String> employeeList = new ArrayList<>();
    employeeList.add("Sankalp Dubey");
    employeeList.add("Arpan");
    employeeList.add("Sunil Kumar");
    employeeList.add("Srinath");

    Set<String> employeeSet = new TreeSet<>(employeeList);

    String result1 = joiner.join(employeeList);
    String result2 = joiner.join(employeeSet);
    String result3 = joiner.join(employeeList, employeeSet);

    System.out.println(result1);
    System.out.println(result2);
    System.out.println(result3);
  }
}

Output

Sankalp Dubey$Arpan$Sunil Kumar$Srinath
Arpan$Sankalp Dubey$Srinath$Sunil Kumar
[Sankalp Dubey, Arpan, Sunil Kumar, Srinath]$[Arpan, Sankalp Dubey, Srinath, Sunil Kumar]

Joining Map data

import java.util.HashMap;
import java.util.Map;

import com.google.common.base.Joiner;
import com.google.common.base.Joiner.MapJoiner;

public class JoinerEx {
  public static void main(String args[]) {
    Joiner joiner = Joiner.on("$");

    Map<Integer, String> employee = new HashMap<>();
    employee.put(1, "Hari Krishna Gurram");
    employee.put(2, "Arpan Debroy");
    employee.put(3, "Sujatha");
    employee.put(4, "Preethi Nair");

    MapJoiner mapJoiner = joiner.withKeyValueSeparator("=");
    String result = mapJoiner.join(employee);

    System.out.println(result);
  }
}

Output

1=Hari Krishna Gurram$2=Arpan Debroy$3=Sujatha$4=Preethi Nair

Write concatenated data to StringBuilder

By using appendTo method you can write concatenated data to StringBuilder.

import java.util.ArrayList;
import java.util.List;

import com.google.common.base.Joiner;

public class JoinerEx {
  public static void main(String args[]) {
    Joiner joiner = Joiner.on("$");

    StringBuilder builder = new StringBuilder();

    List<String> employeeList = new ArrayList<>();
    employeeList.add("Sankalp Dubey");
    employeeList.add("Arpan");
    employeeList.add("Sunil Kumar");
    employeeList.add("Srinath");

    joiner.appendTo(builder, employeeList);

    System.out.println(builder);

  }
}

Output

Sankalp Dubey$Arpan$Sunil Kumar$Srinath

Write concatenated data to any Appendable interface 

Implementations like BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer

Writing concatenated data to a file

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import com.google.common.base.Joiner;

public class JoinerEx {
  public static void main(String args[]) throws IOException {
    Joiner joiner = Joiner.on("$");

    StringBuilder builder = new StringBuilder();

    List<String> employeeList = new ArrayList<>();
    employeeList.add("Sankalp Dubey");
    employeeList.add("Arpan");
    employeeList.add("Sunil Kumar");
    employeeList.add("Srinath");

    FileWriter writer = new FileWriter(new File(
        "/Users/harikrishna_gurram/out.txt"));

    joiner.appendTo(writer, employeeList);
    writer.close();

    System.out.println(builder);

  }
}

Above program writes “Sankalp Dubey$Arpan$Sunil Kumar$Srinath” to "/Users/harikrishna_gurram/out.txt".

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