Quick Start Introduction to the powerful String.format() in Java

Preface

Starting in Java 5.0, the String class has added a powerful string format() method. This method is not used so far, and it is really a waste. This article will take you through the functions of this method quickly. When you want to use formatted text in the future, you may no longer need to borrow third-party library or implement it yourself.

Let’s first look at a simple example:

 String formatted = String.format("%s %d this year.", "Xiao Li", 30); // "Xiao Li is 30 years old this year."

Without my explanation, you can also see:

• The first parameter of this method is the format string, and the following parameters are all parameters of the format string, which are used to replace the placeholders in the format string.
• Placeholders are represented in the form of "%x", and different parameter types must be used with different letters. We will introduce it in detail later.
• String.format() returns the value type as a string, which is the formatted result.

1. Placeholder type

The letters following the placeholder "%" determine the type of actual parameters it accepts. There are several placeholder types:

Capital letters indicate that the letters output are all capitalized.

The most common ones we use are %s, %d and %f, and occasionally %t. This article is limited in space, so only these four types are introduced. Please read the API documentation for the rest yourself.

2. Format string and integer

Here is an example to illustrate:

 // Fill out the spaces and align right: String.format("%10s, world", "Hello"); // Output " Hello, world"String.format("%8d", 123); // Output " 123"// Fill out the spaces and align left: String.format("%-10s, world", "Hello"); // Output "Hello, world"String.format("%-8d", 123); // Output "123 "// Complease 0 and align (valid only for numbers)String.format("%08d", 123); // Output "123 "String.format("%-08d", 123); // Error! 0 is not allowed to be filled on the right side// Output up to N characters String.format("%.5s", "Hello, world"); // Output "Hello"String.format("%.5s...", "Hello, world"); // Output "Hello..."String.format("%10.5s...", "Hello, world"); // Output "Hello..."// Output comma separated numbers String.format("%,d", 1234567); // Output "1,234,567"

3. Date formatting

This is a little more complicated, but if you want to mix text numbers and dates in a string, it should be more convenient to use only one method than combining DateFormat and NumberFormat together.

First, let’s add one more knowledge, that is, placeholders can specify parameters at a certain position, in the format of %n$. For example, %2$d represents the second plastic shaping parameter. Note that n here starts with 1 instead of 0.

When formatting a date, multiple placeholders are required to point to the same parameter (to avoid repeating the same parameter several times). At the same time, because "t" represents date and time, the complete format is %n$tX, where X represents which part of the time is taken. Optional values for X are for example:

Y=year; m=month; d=day; H=hour; M=minute; S=second; L=ms; A=day of the week (name); B=month name;

There are other letters, please refer to the API documentation for details. Here is an example:

 // The output format is "Now is 15:04:52, Sunday" // Note that 10 in "%1$10tH" also means that the space is filled with 10 bits and right-aligned String.format("Now is %1$10tH:%1$tM:%1$tS, %1$tA", new Date())

Summarize

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