10 Java.util.Scanner Best Practices

Java.util.Scanner is a class in the Java API used for obtaining input from the user. It is a convenient way to read data from the keyboard, a file, or any other input source. However, it is important to use it correctly to ensure that the program runs efficiently and without errors.

In this article, we will discuss 10 best practices for using Java.util.Scanner. Following these best practices will help you write better code and make sure your program runs smoothly.

1. Close the Scanner when done

When a Scanner is created, it opens an underlying stream to the source of data. This could be a file, network connection, or other type of input stream. If this stream is not closed when done with the Scanner, then the resource will remain open and can cause memory leaks and other issues. Closing the Scanner also ensures that any buffered data in the scanner is flushed out.

To close the Scanner, use the .close() method. It’s important to note that if you are using try-with-resources syntax, the Scanner will automatically be closed at the end of the block. Otherwise, make sure to call the .close() method after all operations on the Scanner have been completed. Additionally, if there is an exception thrown while using the Scanner, the .close() method should still be called in order to ensure that the underlying stream is properly closed.

2. Use the nextLine() method to read a line of text

The nextLine() method is a great way to read an entire line of text from the user. It reads all characters up until it reaches a newline character, which indicates the end of the line. This means that if the user enters multiple words on one line, they will all be read in as part of the same String.

Using the nextLine() method also allows for more flexibility when reading input from the user. For example, if you are expecting the user to enter a number but they accidentally enter a word instead, the Scanner won’t throw an exception and crash your program. Instead, it will simply read the word as a String and allow you to handle the error gracefully.

To use the nextLine() method, you must first create a Scanner object and pass it the source of the data you want to read. This could be System.in (for standard input) or a File object (for reading from a file). Once you have created the Scanner object, you can call the nextLine() method to read a line of text. The method returns a String containing the line of text that was read. You can then process this String however you need to.

3. Use the nextInt(), nextLong(), and other methods for reading numbers

When using the Scanner class, it is important to use the appropriate method for reading numbers. The nextInt(), nextLong(), and other methods are designed specifically for this purpose. They will read a number from the input stream and convert it into an integer or long value. This ensures that the data type of the value being read is correct and avoids any potential errors due to incorrect data types.

The nextInt() and nextLong() methods also provide additional benefits. For example, they can be used to limit the range of values that can be read in. This is done by passing in a minimum and maximum value as parameters. If the user enters a value outside of this range, then an exception will be thrown. This helps to ensure that only valid values are accepted.

Furthermore, these methods can also be used to validate the format of the input. For example, if the user is expected to enter a number with two decimal places, then the nextDouble() method can be used to check that the input matches this format. If not, then an exception will be thrown.

4. Use hasNextXXX() before calling any nextXXX() method

When using the Scanner class, it is important to check if there is a next token available before calling any of the nextXXX() methods. This is because when the end of the input stream has been reached, an exception will be thrown if the nextXXX() method is called without first checking with hasNextXXX(). The hasNextXXX() method returns true if and only if the next token matches the specified pattern. If the next token does not match the pattern, then false is returned.

The syntax for the hasNextXXX() method is as follows:
hasNextXXX(pattern)
where XXX is the type of data being read (e.g., int, double, String). The pattern parameter is optional and can be used to specify a regular expression that must be matched by the next token in order for the method to return true.

Using the hasNextXXX() method before calling any nextXXX() method is a good idea because it allows you to avoid potential exceptions due to reaching the end of the input stream. It also allows you to ensure that the next token matches the expected format, which can help prevent errors caused by unexpected input.

5. Set delimiter before using the scanner

The delimiter is a character or sequence of characters that separates tokens in the input. By default, the Scanner class uses whitespace as the delimiter, which means it will consider any combination of spaces, tabs, and line breaks to be a single delimiter. This can cause problems when trying to parse certain types of data, such as comma-separated values (CSV) files.

To avoid this issue, you should set the delimiter before using the scanner. To do this, use the useDelimiter() method, passing in the desired delimiter as an argument. For example, if you wanted to use commas as the delimiter for a CSV file, you would call the useDelimiter() method like this:

scanner.useDelimiter(“,”);

Once the delimiter has been set, the scanner will only consider the specified character or sequence of characters to be a delimiter. This makes it easier to parse the data correctly, since the scanner will now recognize each token separately.

It’s also important to note that the delimiter can be changed at any time by calling the useDelimiter() method again with a different argument. This allows you to easily switch between different delimiters depending on the type of data being parsed.

6. Be aware of default delimiters used by the scanner

The default delimiter used by the scanner is a whitespace character, which means that any sequence of whitespace characters (spaces, tabs, newlines) will be treated as a single delimiter. This can cause problems when trying to parse input with multiple words or numbers separated by whitespace. For example, if you are expecting an input like “John Doe” and the user enters “John Doe”, the scanner will only recognize one token (“John”) because it treats the two spaces between John and Doe as a single delimiter.

To avoid this problem, you should explicitly set the delimiter for the scanner using the useDelimiter() method. This allows you to specify exactly what characters should be considered delimiters. You can also use regular expressions to define more complex patterns of delimiters. For example, if you want to treat both spaces and commas as delimiters, you could use the following code:

Scanner sc = new Scanner(System.in);
sc.useDelimiter(“[\\s,]+”);

This would tell the scanner to consider any combination of whitespace characters and commas as a single delimiter.

It’s also important to note that the useDelimiter() method does not affect how the scanner reads tokens from the input stream. It only affects how the scanner splits the input into tokens. So even if you have specified a custom delimiter, the scanner will still read each token as a string.

7. Don’t forget to close the scanner after use

When a Scanner object is created, it opens up an underlying stream to the source of data. This could be a file, or in some cases, System.in (standard input). If this stream is not closed after use, then the resource will remain open and can cause memory leaks. In addition, if the same scanner is used multiple times without closing it, then the previous data may still be present in the buffer, which can lead to unexpected results.

To ensure that the resources are properly released, the close() method should always be called when the scanner is no longer needed. The syntax for doing so is as follows:

Scanner myScanner = new Scanner(System.in);
// Use the scanner here
myScanner.close();

It’s also important to note that calling close() on a scanner does not necessarily guarantee that the underlying stream is closed. To make sure that the stream is closed, you must call the close() method on the underlying stream itself. For example, if the scanner was created using System.in, then the following code would need to be executed to ensure that the stream is closed:

Scanner myScanner = new Scanner(System.in);
// Use the scanner here
myScanner.close();
System.in.close();

8. Always check the return value of nextXXX() methods

The nextXXX() methods of the Scanner class are used to read input from a source, such as the keyboard or a file. These methods return true if there is more data available for reading and false otherwise. This means that when using these methods, it is important to check their return value in order to determine whether there is still more data to be read. If the return value is false, then the program should stop trying to read any further data.

It is also important to note that the nextXXX() methods will throw an exception if they encounter an error while attempting to read the data. Therefore, checking the return value can help prevent errors by ensuring that the program does not attempt to read beyond the end of the data stream.

Furthermore, checking the return value of the nextXXX() methods allows the programmer to control the flow of the program based on the availability of data. For example, if the return value is false, then the program could take some other action instead of continuing to try to read data.

9. Avoid using the same instance of scanner multiple times

When using the same instance of scanner multiple times, it can cause unexpected behavior. This is because when a Scanner object reads input from a source, it keeps track of where it left off in the stream. If you use the same instance to read different sources, then the scanner will start reading from the point it left off in the previous source. This could lead to incorrect data being read or skipped over entirely.

To avoid this issue, create a new instance of Scanner for each source that needs to be read. This ensures that the scanner starts at the beginning of the source and does not carry over any state from the previous source. Additionally, if an exception occurs while reading one source, it won’t affect the other sources.

It’s also important to close the Scanner after it has finished reading the source. Closing the Scanner releases any resources associated with it, such as open files or network connections. Failing to do so can result in memory leaks or other issues. To ensure that the Scanner is closed properly, it should be wrapped in a try-with-resources block. This guarantees that the Scanner will be closed even if an exception occurs.

10. Use try-with-resources statement for closing scanners

When using a Scanner, it is important to close the scanner when you are done with it. This is because scanners can take up system resources and if they are not closed properly, these resources will remain in use until the program ends. Closing the scanner also ensures that any data associated with the scanner is released so that other programs can access it.

Using a try-with-resources statement for closing scanners is an effective way of ensuring that the scanner is always closed properly. A try-with-resources statement allows you to declare one or more resources within the statement itself. These resources are then automatically closed at the end of the statement, regardless of whether an exception was thrown or not. This means that even if an exception occurs while using the scanner, the scanner will still be closed properly.

To use a try-with-resources statement for closing scanners, you must first create the scanner object as usual. Then, instead of calling the close() method on the scanner directly, you wrap the code that uses the scanner in a try-with-resources statement. The scanner should be declared as a resource within the statement, like this:

try (Scanner scanner = new Scanner(System.in)) {
// Code that uses the scanner goes here
}

This ensures that the scanner will be closed automatically at the end of the statement, regardless of what happens during execution. It is important to note that the scanner must be declared as a resource within the statement; otherwise, it will not be closed automatically.