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About Mary Zheng

Mary Zheng
Mary has graduated from Mechanical Engineering department at ShangHai JiaoTong University. She also holds a Master degree in Computer Science from Webster University. During her studies she has been involved with a large number of projects ranging from programming and software engineering. She works as a senior Software Engineer in the telecommunications sector where she acts as a leader and works with others to design, implement, and monitor the software solution.

Java printf() Method Example

1. Introduction

In this post, we feature a comprehensive article on Java printf() Method.

Java PrintStream class has provided the printf method to write a formatted string to the PrintStream object since version 5. Here are the method’s signatures:

public PrintStream printf(Locale l, String format, Object... args)
public PrintStream printf(String format, Object... args)
  • l – a locale to apply during formatting
  • format – a format string as described in format string syntax
  • args – arguments referenced by the format specifiers in the format string

The printf method formats a string which contains formatting specifiers for Integer, float, Date, String, etc. The formatting specifiers start with % and is followed by four optional arguments and one mandatory conversion argument.

%[argument_index$][flags][width][.precision]conversion
  • argument_index – it is an optional decimal integer indicating the position of the argument in the argument list. The first argument is referenced by “1$”, the second by “2$”, etc.
  • flags – it is an optional set of characters that modify the output format. The set of valid flags depends on the conversion.
  • width – it is an optional positive decimal integer indicating the minimum number of characters to be written to the output.
  • precision – it is an optional non-negative decimal integer used to restrict the number of characters.
  • conversion – it is a mandatory character indicating how the argument should be formatted.

In this example, I will demonstrate how to format a string with these formatting specifiers.

2. Technologies Used

The example code in this example was built and run with:

  • Java 11
  • Maven 3.3
  • Eclipse
  • Junit 4.12

3. Maven Project

3.1 Dependencies

I will include Junit in the pom.xml.

pom.xml

<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>java-printf-demo</groupId>
	<artifactId>java-printf-demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<build>
		<sourceDirectory>src</sourceDirectory>
		<plugins>
			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.8.0</version>
				<configuration>
					<release>11</release>
				</configuration>
			</plugin>
		</plugins>
	</build>
	<dependencies>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>4.12</version>
		</dependency>
	</dependencies>
</project>

3.2 PrintfFormatBaseTest

Java Formatter class defines the formatting specifiers. Here are some common conversion specifiers:

  • b(B) – converts a non-null boolean or Boolean object base on String.valueOf(arg).
  • c(C) – transforms a character based on its Unicode
  • h – translates based on the hashcode method
  • d – converts an integer number
  • f – converts a float number
  • n – converts to a platform-specific line separator
  • s(S) – formats a String
  • t(T) – converts to a date and time value

The following are some common flag specifiers:

  • -: Left-justified
  • +: Include a sign for a numeric value
  • 0: Zero-padding for a numeric value
  • ,: locale-specific grouping separators

In this example, I will create several constants which embed the format specifiers. I will use Junit @Rule to print out the test method’s name. I will also create several test methods to demonstrate the three kinds of argument indexing:

  • explicit indexing – specifies the argument with explicit indexing, e.g. 1$
  • ordinary indexing – the argument-index is based on the position
  • relative indexing – uses the same argument with '<'

PrintfFormatBaseTest.java

package jcg.zheng.demo;

import java.util.MissingFormatArgumentException;

import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TestRule;
import org.junit.rules.TestWatcher;
import org.junit.runner.Description;

public class PrintfFormatBaseTest {
	
	@Rule
	public TestRule echoMethodName = new TestWatcher() {
		protected void starting(Description desc) {
			System.out.println("\nTestMethodName:" + desc.getMethodName());
		}
	};
 
	protected static final String BOOLEAN_B = "%%b - format boolean value: %b, %%B - Boolean value: %B";
	protected static final String CHAR_C = "%%c - format char with lower-case value: %c, %%C - with upper-case value: %C.";
	protected static final String DECIMAL_F = "%%f - doubleValue is: %f. floatValue is: %f";
	protected static final String DECIMAL_FLAGS_WIDTH_PRECISION = "%%-+,10.2f - Format a negative number by left-justified, separating with comma, with a sign, 10 width, and 2 precision: %-+,10.2f";
	protected static final String DECIMAL_PADDING_BLANK = "%% 10.3f - Format a decimal with padding empty space: % 10.3f";
	protected static final String DECIMAL_PADDING_ZERO = "%%010.3f - Format a decimal with padding zero: %010.3f";

	protected static final String EXPONENTIAL_E = "%%e - exponential number. Value is: %e.";

	protected static final String HASHCODE_H = "The string object's hashcode: %h, the interger's hashcode: %h";
	protected static final String INTEGER_D_BASE10 = "%%d - byteValue is: %d, shortValue is: %d, intValue is: %d, longValue is: %d.";

	protected static final String INTEGER_O_BASE8 = "%%o - Octal format. byteValue is: %o, shortValue is: %o, intValue is: %o, longValue is: %o.";
	protected static final String INTEGER_X_BASE16 = "%%x - Base 16. byteValue is: %x, shortValue is: %x, intValue is: %x, longValue is: %x.";
	protected static final String NEW_LINE = "%n";

	protected byte byteValue = 12;
	protected double doubleValue = 10.123;
	protected float floatValue = 123.45f;
	protected int intValue = 123;
	protected long longValue = 1234567;
	protected short shortValue = 121;

	@Test
	public void explicit_indexing_test() {
		System.out.printf("explicit two arguments: %1$s, %2$s", "Hello", "World!");		 
	}

	@Test
	public void explicit_indexing_test_differentOrder() {
		System.out.printf("explicit two arguments: %2$s, %1$s", "Hao!", "Ni");	 
	}

	@Test(expected = MissingFormatArgumentException.class)
	public void ordinary_indexing_test_exception() {
		System.out.printf("throw MissingFormatArgumentException: %s, %s", "test");	 
	}

	@Test
	public void relative_indexing_test() {
		System.out.printf("relative arguments - %1$s, %<s, %<s, %2$s, and %<s", "Zheng", "Mary");	 
	}
	
}

Execute mvn test -Dtest=PrintfFormatBaseTest and capture the output.

PrintfFormatBaseTest results.

C:\MaryZheng\Workspaces\jdk12\java-printf-demo>mvn test -Dtest=PrintfFormatBaseTest
[INFO] Scanning for projects...
[INFO]
[INFO] -----------------< java-printf-demo:java-printf-demo >------------------
[INFO] Building java-printf-demo 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ java-printf-demo ---
[WARNING] Using platform encoding (Cp1252 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory C:\MaryZheng\Workspaces\jdk12\java-printf-demo\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ java-printf-demo ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding Cp1252, i.e. build is platform dependent!
[INFO] Compiling 7 source files to C:\MaryZheng\Workspaces\jdk12\java-printf-demo\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ java-printf-demo ---
[WARNING] Using platform encoding (Cp1252 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory C:\MaryZheng\Workspaces\jdk12\java-printf-demo\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:testCompile (default-testCompile) @ java-printf-demo ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding Cp1252, i.e. build is platform dependent!
[INFO] Compiling 7 source files to C:\MaryZheng\Workspaces\jdk12\java-printf-demo\target\test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ java-printf-demo ---
[INFO] Surefire report directory: C:\MaryZheng\Workspaces\jdk12\java-printf-demo\target\surefire-reports

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running jcg.zheng.demo.PrintfFormatBaseTest

TestMethodName:explicit_indexing_test_differentOrder
explicit two arguments: Ni, Hao!
TestMethodName:relative_indexing_test
relative arguments - Zheng, Zheng, Zheng, Mary, and Mary
TestMethodName:explicit_indexing_test
explicit two arguments: Hello, World!
TestMethodName:ordinary_indexing_test_exception
throw MissingFormatArgumentException: test, Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.157 sec

Results :

Tests run: 4, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  10.262 s
[INFO] Finished at: 2019-07-07T11:50:17-05:00
[INFO] ------------------------------------------------------------------------

C:\MaryZheng\Workspaces\jdk12\java-printf-demo>

4. Format a Number

4.1 Printf_NumberTest

In this step, I will create Junit tests to format a number in several ways:

  • Formats a number (base 8, 10, and 16) for different data types: byte, short, int, and long
  • Formats a float number in exponential format
  • Formats a decimal number for different data types: float and double
  • Formats a decimal number with different padding characters: space and zero
  • Formats a float number with left-justified (-), sign (+), and comma (,) flag, 2 precision (.2), and width 10 specifiers

Printf_NumberTest.java

package jcg.zheng.demo;

import org.junit.Test;

public class Printf_NumberTest extends PrintfFormatBaseTest {


	@Test
	public void d_for_integer_base10() {
		System.out.printf(INTEGER_D_BASE10, byteValue, shortValue, intValue, longValue);
	}

	@Test
	public void e_for_exponential_number() {
		System.out.printf(EXPONENTIAL_E, floatValue);
	}

	@Test
	public void f_for_decimal() {
		System.out.printf(DECIMAL_F, doubleValue, floatValue);
	}

	@Test
	public void o_for_integer_base8() {
		System.out.printf(INTEGER_O_BASE8, byteValue, shortValue, intValue, longValue);
	}

	@Test
	public void padding_space() {
		// % 10.3f - Format a decimal with padding empty space: 125.100
		System.out.printf(DECIMAL_PADDING_BLANK, 125.1f);
	}

	@Test
	public void padding_zero() {
		// %010.3f - Format a decimal with padding zero: 000125.100
		System.out.printf(DECIMAL_PADDING_ZERO, 125.1f);
	}

	@Test
	public void with_all_formatting_flags_width_precision() {
		// %-+,10.2f - Format a negative number by left-justified, separating with comma,
		// with a sign, 10 width, and 2 precision: -12,345.12
		System.out.printf(DECIMAL_FLAGS_WIDTH_PRECISION, -12345.1234f);
	}

	@Test
	public void x_for_integer_base16() {
		System.out.printf(INTEGER_X_BASE16, byteValue, shortValue, intValue, longValue);
	}

}

Execute mvn test -Dtest=Printf_NumberTest and capture the output.

Printf_NumberTest results.

C:\MaryZheng\Workspaces\jdk12\java-printf-demo>mvn test -Dtest=Printf_NumberTest
[INFO] Scanning for projects...
[INFO]
[INFO] -----------------< java-printf-demo:java-printf-demo >------------------
[INFO] Building java-printf-demo 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ java-printf-demo ---
[WARNING] Using platform encoding (Cp1252 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory C:\MaryZheng\Workspaces\jdk12\java-printf-demo\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ java-printf-demo ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding Cp1252, i.e. build is platform dependent!
[INFO] Compiling 7 source files to C:\MaryZheng\Workspaces\jdk12\java-printf-demo\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ java-printf-demo ---
[WARNING] Using platform encoding (Cp1252 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory C:\MaryZheng\Workspaces\jdk12\java-printf-demo\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:testCompile (default-testCompile) @ java-printf-demo ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ java-printf-demo ---
[INFO] Surefire report directory: C:\MaryZheng\Workspaces\jdk12\java-printf-demo\target\surefire-reports

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running jcg.zheng.demo.Printf_NumberTest

TestMethodName:d_for_integer_base10
%d - byteValue is: 12, shortValue is: 121, intValue is: 123, longValue is: 1234567.
TestMethodName:x_for_integer_base16
%x - Base 16. byteValue is: c, shortValue is: 79, intValue is: 7b, longValue is: 12d687.
TestMethodName:o_for_integer_base8
%o - Octal format. byteValue is: 14, shortValue is: 171, intValue is: 173, longValue is: 4553207.
TestMethodName:e_for_exponential_number
%e - exponential number. Value is: 1.234500e+02.
TestMethodName:f_for_decimal
%f - doubleValue is: 10.123000. floatValue is: 123.449997
TestMethodName:with_all_formatting_flags_width_precision
%-+,10.2f - Format a negative number by left-justified, separating with comma, with a sign, 10 width, and 2 precision: -12,345.12
TestMethodName:padding_zero
%010.3f - Format a decimal with padding zero: 000125.100
TestMethodName:padding_space
% 10.3f - Format a decimal with padding empty space:    125.100
TestMethodName:explicit_indexing_test_differentOrder
explicit two arguments: Ni, Hao!
TestMethodName:relative_indexing_test
relative arguments - Zheng, Zheng, Zheng, Mary, and Mary
TestMethodName:explicit_indexing_test
explicit two arguments: Hello, World!
TestMethodName:ordinary_indexing_test_exception
throw MissingFormatArgumentException: test, Tests run: 12, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.193 sec

Results :

Tests run: 12, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  8.983 s
[INFO] Finished at: 2019-07-07T11:52:47-05:00
[INFO] ------------------------------------------------------------------------

C:\MaryZheng\Workspaces\jdk12\java-printf-demo>

4.2 NumberFormatTest

Java provides a NumberFormat class to format a number. In this step, I will create several test methods to format a number. It achieves similar formatting results as the printf method with more readable code.

NumberFormatTest.java

package jcg.zheng.demo;

import static org.junit.Assert.assertEquals;

import java.text.NumberFormat;
import java.util.Locale;

import org.junit.Test;

public class NumberFormatTest {

	private NumberFormat usNumFormat = NumberFormat.getInstance(new Locale("en", "US"));;

	@Test
	public void formatDecimal() {	 
		usNumFormat.setMaximumFractionDigits(4);
		usNumFormat.setMinimumFractionDigits(2);
	 
		assertEquals("123.4568",  usNumFormat.format(123.45678));
	}

	@Test
	public void formatInteger() {
		assertEquals("100", usNumFormat.format(100l));
	}

	@Test
	public void formatPercentage() {
		NumberFormat nf = NumberFormat.getPercentInstance();
	 
		assertEquals("25%", nf.format(0.25));
	}

}

Execute mvn test -Dtest=NumberFormatTest and capture the output.

NumberFormatTest Output

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running jcg.zheng.demo.NumberFormatTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.174 sec

Results :

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0

5. Format a Date

5.1 Printf_DateTest

Java provides additional specifier suffix after the date/time conversation specifiers: t and T.

Here are some common date suffix specifiers:

  • m – Month, formatted as two digits with leading zeros as necessary, i.e. 01 – 12
  • d – Day of month, formatted as two digits with leading zeros as necessary, i.e. 01 – 31
  • y – Last two digits of the year, formatted with leading zeros as necessary, i.e. 00 – 99
  • Y – Year, formatted to at least four digits with leading zeros as necessary, e.g. 0092 equals 92 CE for the Gregorian calendar

Here are some common time suffix specifiers:

  • H – Hour of the day for the 24-hour clock, formatted as two digits with a leading zero as necessary i.e. 00 – 23.
  • M – Minute within the hour formatted as two digits with a leading zero as necessary, i.e. 00 – 59.
  • S – Seconds within the minute, formatted as two digits with a leading zero as necessary, i.e. 00 – 60 (“60” is a special value required to support leap seconds).
  • r – a 12-hour clock time as "%tI:%tM:%tS %Tp"

In this step, I will create several test methods to format a date with the predefined format specifiers.

  • Format a data time with tc
  • format a date with tD
  • format a ISO8601 date with tF
  • Format a 12 hour time with tr

Printf_DateTest.java

package jcg.zheng.demo;

import java.util.Date;

import org.junit.Test;

public class Printf_DateTest extends PrintfFormatBaseTest  {

	@Test
	public void tc_detail_date_time() {
		// DateTime: Mon Jul 01 23:09:59 CDT 2019
		System.out.printf("DateTime: %1$tc", new Date());
	}
	
	@Test
	public void Tc_detail_date_time() {
		// DateTime: SUN JUL 07 07:19:32 CDT 2019
		System.out.printf("DateTime: %1$Tc", new Date());
	}

	@Test
	public void tD_month_day_year() {
		// Date: 07/01/19
		System.out.printf("Date: %1$tD", new Date());
	}


	@Test
	public void dateTime_format_explicating_indexing() {
		// Time: 01-07-2019 22:54:31
		// td day of month
		System.out.printf("Time: %1$td-%1$tm-%1$tY %1$tH:%1$tM:%1$tS", new Date());
	}

	@Test
	public void dateTime_format_explicating_indexing_2() {
		// Date: July 01 2019
		// tB local-specific full month name
		System.out.printf("Date: %1$tB %1$td %1$tY", new Date());
	}

	@Test
	public void tF_ISO8601() {
		// Date: 2019-07-01
		System.out.printf("Date: %1$tF", new Date());
	}

	@Test
	public void tR_Hour_Minute() {
		// tH:tM
		System.out.printf("Date: %1$tR", new Date());
	}

	@Test
	public void tr_Hour_Minute_Second_12() {
		// Time: 11:07:18 PM
		// tH:tM:tS
		System.out.printf("Time: %1$tr", new Date());
	}

	@Test
	public void tT_Hour_Minute_Second() {
		// tH:tM:tS
		System.out.printf("Date: %1$tT", new Date());
	}
}

Execute mvn test -Dtest=Printf_DateTest and capture the output.

Printf_DateTest Output

Running jcg.zheng.demo.Printf_DateTest

TestMethodName:tr_Hour_Minute_Second_12
Time: 12:00:29 PM
TestMethodName:tR_Hour_Minute
Date: 12:00
TestMethodName:dateTime_format_explicating_indexing_2
Date: July 07 2019
TestMethodName:dateTime_format_explicating_indexing
Time: 07-07-2019 12:00:29
TestMethodName:tD_month_day_year
Date: 07/07/19
TestMethodName:tc_detail_date_time
DateTime: Sun Jul 07 12:00:29 CDT 2019
TestMethodName:tT_Hour_Minute_Second
Date: 12:00:29
TestMethodName:Tc_detail_date_time
DateTime: SUN JUL 07 12:00:29 CDT 2019
TestMethodName:tF_ISO8601
Date: 2019-07-07
TestMethodName:explicit_indexing_test_differentOrder
explicit two arguments: Ni, Hao!
TestMethodName:relative_indexing_test
relative arguments - Zheng, Zheng, Zheng, Mary, and Mary
TestMethodName:explicit_indexing_test
explicit two arguments: Hello, World!
TestMethodName:ordinary_indexing_test_exception
throw MissingFormatArgumentException: test, Tests run: 13, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.364 sec

Results :

Tests run: 13, Failures: 0, Errors: 0, Skipped: 0

5.2 DateFormatTest

Java provides a SimpleDateFormat and DateTimeFormatter class to format a date. Both classes have similar format specifiers, but the DateTimeFormatter class provides static constants for the common formatting patterns. In this step, I will create two test methods to show how to format a date.

DateFormatTest.java

package jcg.zheng.demo;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.GregorianCalendar;

import org.junit.Test;

public class DateFormatTest {

	@Test
	public void dateTimeFormat() {
		DateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss");

		System.out.println(df.format((new GregorianCalendar()).getTime()));
	}

	@Test
	public void DateTimeFormatter_java8() {
		DateTimeFormatter formatter = DateTimeFormatter.ISO_DATE_TIME;

		LocalDateTime now = LocalDateTime.now();
		System.out.println("default format:" + now);

		String formatDateTime = now.format(formatter);

		System.out.println("ISO_DATE_TIME format : " + formatDateTime);
	}

}

Execute mvn test -Dtest=DateFormatTest and capture the output.

DateFormatTest Output

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running jcg.zheng.demo.DateFormatTest
default format:2019-07-07T12:03:39.023577700
ISO_DATE_TIME format : 2019-07-07T12:03:39.0235777
07-07-2019 12:03:39
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.242 sec

Results :

Tests run: 2, Failures: 0, Errors: 0, Skipped: 0

6. Format a String

6.1 Printf_StringTest

In this step, I will create several Junit test methods to format a string with several conversion specifiers: %c, %b, %h, %s, and %S.

Printf_StringTest.java

package jcg.zheng.demo;

import java.io.UnsupportedEncodingException;

import org.junit.Test;

public class Printf_StringTest extends PrintfFormatBaseTest {

	private boolean isFalse = false;
	private Boolean isTrue = Boolean.TRUE;

	@Test
	public void b_for_boolean() throws UnsupportedEncodingException {
		System.out.printf(BOOLEAN_B, isFalse, isTrue);
	}
	
	@Test
	public void b_for_String_always_True() throws UnsupportedEncodingException {
		System.out.printf(BOOLEAN_B, "test", 1);
	}

	@Test
	public void c_for_character() {
		System.out.printf(CHAR_C, 'm', 'm');
	}

	@Test
	public void h_for_hashcode() {
		String s = "Hello World";
		Integer number = Integer.valueOf(100);
		System.out.printf(HASHCODE_H, s, number);

	}

	@Test
	public void n_for_newline() {
		System.out.printf("Hello World%n2nd line");
	}

	@Test
	public void s_for_string() {
		String stringVal = "Hi, Mary!";
		System.out.printf("Left-justified is: %-20s\n", stringVal);
		System.out.printf("Uppercase, Right-justified is: %20S\n", stringVal);
	}

	@Test
	public void s_for_string_width_too_small() {
		String stringVal = "Hi, Mary!";
		System.out.printf("width is smaller: %2S", stringVal);
	}

}

Execute mvn test -Dtest=Printf_StringTest and capture the output.

Printf_StringTest Output

Running jcg.zheng.demo.Printf_StringTest

TestMethodName:b_for_String_always_True
%b - format boolean value: true, %B - Boolean value: TRUE
TestMethodName:s_for_string_width_too_small
width is smaller: HI, MARY!
TestMethodName:n_for_newline
Hello World
2nd line
TestMethodName:h_for_hashcode
The string object's hashcode: cc969a84, the interger's hashcode: 64
TestMethodName:b_for_boolean
%b - format boolean value: false, %B - Boolean value: TRUE
TestMethodName:s_for_string
Left-justified is: Hi, Mary!
Uppercase, Right-justified is:            HI, MARY!

TestMethodName:c_for_character
%c - format char with lower-case value: m, %C - with upper-case value: M.
TestMethodName:explicit_indexing_test_differentOrder
explicit two arguments: Ni, Hao!
TestMethodName:relative_indexing_test
relative arguments - Zheng, Zheng, Zheng, Mary, and Mary
TestMethodName:explicit_indexing_test
explicit two arguments: Hello, World!
TestMethodName:ordinary_indexing_test_exception
throw MissingFormatArgumentException: test, Tests run: 11, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.211 sec

Results :

Tests run: 11, Failures: 0, Errors: 0, Skipped: 0

6.2 Printf_FileTest

In this step, I will demonstrate how to write the formatted data to a text file with the printf method.

Printf_FileTest.java

package jcg.zheng.demo;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.PrintStream;
import java.util.Date;

import org.junit.Test;

public class Printf_FileTest extends PrintfFormatBaseTest {

	@Test
	public void writetoFile() {
		PrintStream writeToFile;
		try {
			writeToFile = new PrintStream(new FileOutputStream("Printf_demo.txt", true));

			writeToFile.printf(BOOLEAN_B, false, Boolean.TRUE);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(CHAR_C, 'a', 'a');
			writeToFile.printf(NEW_LINE);
			
			writeToFile.printf("%%-20s - Format string with Left-justified and 20 width is: %-20s", "Mary Zheng");
			writeToFile.printf(NEW_LINE);
			writeToFile.printf("%%20S - Format string with Uppercase, Right-justified : %20S", "Hello");
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(DECIMAL_F, doubleValue, floatValue);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(EXPONENTIAL_E, floatValue);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(INTEGER_D_BASE10, byteValue, shortValue, intValue, longValue);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(INTEGER_O_BASE8, byteValue, shortValue, intValue, longValue);		
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(INTEGER_X_BASE16, byteValue, shortValue, intValue, longValue);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(DECIMAL_PADDING_BLANK, 125.1f);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(DECIMAL_PADDING_ZERO, 125.1f);
			writeToFile.printf(NEW_LINE);
			writeToFile.printf(DECIMAL_FLAGS_WIDTH_PRECISION, -12345.1234f);
			writeToFile.printf(NEW_LINE);

			writeToFile.printf("%%1$tc - Format a DateTime: %1$tc", new Date());
			writeToFile.printf(NEW_LINE);
			writeToFile.printf("%%1$tD - Format a Date: %1$tD", new Date());
			writeToFile.printf(NEW_LINE);		 
			writeToFile.printf("%%1$tT - Format a Time: %1$tT", new Date());
			writeToFile.printf(NEW_LINE);
		 
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}

	}

}

Execute mvn test -Dtest=Printf_FileTest and capture the content of Printf_demo.txt.

Printf_demo.txt

%b - format boolean value: false, %B - Boolean value: TRUE
%c - format char with lower-case value: a, %C - with upper-case value: A.
%-20s - Format string with Left-justified and 20 width is: Mary Zheng          
%20S - Format string with Uppercase, Right-justified :                HELLO
%f - doubleValue is: 10.123000. floatValue is: 123.449997
%e - exponential number. Value is: 1.234500e+02.
%d - byteValue is: 12, shortValue is: 121, intValue is: 123, longValue is: 1234567.
%o - Octal format. byteValue is: 14, shortValue is: 171, intValue is: 173, longValue is: 4553207.
%x - Base 16. byteValue is: c, shortValue is: 79, intValue is: 7b, longValue is: 12d687.
% 10.3f - Format a decimal with padding empty space:    125.100
%010.3f - Format a decimal with padding zero: 000125.100
%-+,10.2f - Format a negative number by left-justified, separating with comma, with a sign, 10 width, and 2 precision: -12,345.12
%1$tc - Format a DateTime: Sun Jul 07 12:06:42 CDT 2019
%1$tD - Format a Date: 07/07/19
%1$tT - Format a Time: 12:06:42

7. Java printf() Method – Summary

In this example, I demonstrated how to use the printf method to:

  • Format a number
  • Format a date
  • Format a string
  • Write to a file

I also compared printf to NumberFormat, SimpleDateFormat, and DateTimeFormatter. Developers must understand the formatting specifiers when using the printf method.

8. Download the Source Code

This was an example on Java printf() Method .This example consists of a Maven project which defines seven test classes to demonstrate the printf method’s usages.

Download
You can download the full source code of this example here: Java printf() Method Example
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