String Format

Learning Goals

• Learn about the various ways to format a String

Introduction

Sometimes we want to format a String a certain way. For example, we might want to insert so many spaces between words when printing a String value. Let's look at ways we can format a String to ensure it will print out to the console a certain way!

The format() method

The String class has a method to help format a String value called format()! To help explain how to use the format() method, let's look at an example:

public class StringExample {
    public static void main(String[] args) {
        String bootcamp = "Flatiron School";
        
        // Concatenate two strings using the format() method
        String string1 = String.format("%s is the best!", bootcamp);
        
        System.out.println(string1);
    }
}

The code above will have the following output:

Flatiron School is the best!

As we can see in the example above, the format() method is a static method. Remember this means it does not need a String instance to be called. In this case we see the format() method takes in two parameters, a String that we want to format and the objects we want to inject into the String. In the example, the object just happens to be another String but it could be an Integer or a Float. We also see in the String that we want to format a character that looks like this: %s. The % tells Java that some format specifiers will follow and be placed here. The s that follows the % is a conversion character. Conversion characters are only valid for certain data types and signal that a certain object will be placed within the String at the spot where the conversion character appears. Therefore, the %s says that another String will be placed where the %s appears. In the example above, we see that the %s is in the beginning, and will be replaced with the bootcamp value. Hence, the output "Flatiron School is the best!" is printed to the console. It should be noted that when conversion characters are used, the second parameter must be the correlating data type.

Here are some of the conversion characters we may see:

Conversion Character	Data Type
s	String
d	Integer
f	Floating-Point Numbers
n	New line

Formatting Rules

When we want to format a String, we can use the following syntax:

%[flag][width][.precision]conversion-character

To break down the syntax some more:

• The flag format specifier contains a set of characters to specify the format of the output. The character set very much depends on the data type that is to be formatted.
• The width specifier specifies the amount of characters we want to output when formatting the value.
• The precision specifier is usually used when the data type we are formatting is a Float and specifies how many decimal values should be printed.
• Notice that the flat, width, and precision format specifiers are in brackets. These specifiers are optional whereas the % and the conversion character are required when formatting a String.

Let's explore these other optional specifiers some more with some examples:

public class StringExample {
    public static void main(String[] args) {
        String bootcamp = "Flatiron School";
        String show = "Parks and Recreation";
        
        // Concatenate two strings using the format() method
        String string1 = String.format("%s is the best!", bootcamp);
        
        // Use multiple conversion characters
        String string2 = String.format("%s has %d seasons.", show, 7);
        
        // Use a floating-point number
        String string3 = String.format("An ice cream cone costs $%f", 2.99);
        
        System.out.println(string1);
        System.out.println(string2);
        System.out.println(string3);
    }
}

The above will print the following output:

Flatiron School is the best!
Parks and Recreation has 7 seasons.
An ice cream cone costs $2.990000

As we can see with string2, we can actually have multiple conversion characters within a String! When we do, we need to make sure we specify the appropriate number of object parameters afterwards. In this example, since we had a string conversion character and an integer conversion character, we need to provide both a String and an Integer as parameters in the format() method! Order is important, so we should always make sure that the following object parameters are in the same order as their correlating conversion character appears.

With string3, we see that we are going to print out a floating-point number, so we use the %f character. Except, when we print string3 to the console, we get an output with more decimal values than we originally expected. We might have only expected to see "An ice cream cone costs $2.99", but instead we see "An ice cream cone costs $2.990000". This is because when we use the format() method to print a float, we are printing a default of 6 decimal digits. In order to specify how many decimal places we want, we need to adjust its precision. To do so, we can do this:

public class StringExample {
    public static void main(String[] args) {
        // Use a floating-point number with 2 decimal places
        String string3 = String.format("An ice cream cone costs $%.2f", 2.99);

        System.out.println(string3);
    }
}

Here, we are using the precision format specifier to specify we want to display only a specific amount of decimal places. The precision format specifier always starts with a dot . and can then be followed by the number of decimal places. In this case, we specify a 2, therefore we should only print out to 2 decimal places when printing the float:

An ice cream cone costs $2.99

Now what if we want to print some whitespace in front of a given format specifier? We can do so by specifying the width like this:

public class StringExample {
    public static void main(String[] args) {
        // Use a floating-point number with 2 decimal places and a width of 14
        String string3 = String.format("An ice cream cone costs $%14.2f", 2.99);

        System.out.println(string3);
    }
}

By specifying a width, we are saying we want the output to take up a certain amount of space. In this example, we are saying we want the "2.99" to take up 14 characters worth of space. Since there are 4 characters within "2.99", it will prepend 10 spaces prior to printing the "2.99".

An ice cream cone costs $          2.99

Cool! So we have seen the precision and width format specifiers, but what about the flag specifier? Let's say we want to actually prepend "2.99" with leading zeroes now instead of spaces. This is where we can specify a flag with 0 to pad the output with zeroes:

public class StringExample {
    public static void main(String[] args) {
        // Use a floating-point number with 2 decimal places and a width of 14
        // and leading zeroes
        String string3 = String.format("An ice cream cone costs $%014.2f", 2.99);

        System.out.println(string3);
    }
}

This will now output the following:

An ice cream cone costs $00000000002.99

Some other flags include the following:

Flags	Description
-	Print the value left-justified
+	Print the sign of the number (positive/negative)
0	Print the number with leading zeroes

The printf() method

We might not always want to initialize a new String to just format it. Sometimes we just want to print a formatted String if we do not care about holding onto how it is formatted.

This is where the printf() comes into play!

The printf() method can be used instead of a System.out.print() or a System.out.println() to print a formatted String using the same formatting rules as above!

Consider the following:

public class StringExample {
    public static void main(String[] args) {
        String bootcamp = "Flatiron School";
        String show = "Parks and Recreation";
        
        // Use printf() instead of String.format()
        System.out.printf("%s is the best!%n", bootcamp);
        System.out.printf("%s has %d seasons.%n", show, 7);
        System.out.printf("An ice cream cone costs $%014.2f%n", 2.99);
    }
}

Notice that the same formatting rules defined above are being used here as well! We can still make use of the conversion characters, width, precision, and flags as we do with the format() method. This is an easy way to simply print to the console a formatted String without initializing a separate variable. Also note the use of %n to write a new line out after each of the printf() statements.

This will output the following:

Flatiron School is the best!
Parks and Recreation has 7 seasons.
An ice cream cone costs $00000000002.99