How to input data in java

How to Get User Input in Java

Reading user input is the first step towards writing useful Java software. User input can come in many forms — mouse and keyboard interactions, a network request, command-line arguments, files that are updated with data relevant for a program’s execution, etc.

We’re going to focus on keyboard input via something called the standard input stream. You may recognize it as Java’s System.in .

We’re going to use Scanner class to make our interaction with the underlying stream easier. Since Scanner has some downfalls, we’ll also be using the BufferedReader and InputStreamReader classes to process the System.in stream.

In the end, we’ll Decorate the InputStream class and implement our own custom UncloseableInputStream to handle issues with the Scanner class.

Java Scanner Class

The java.util.Scanner class is a simple scanner that can parse and handle primitive inputs, Strings and streams. Since System.in is just an InputStream , we can construct a Scanner as such:

Scanner sc = new Scanner(System.in); 

This Scanner instance can now scan and parse booleans, integers, floats, bytes and Strings.

Let’s see how we can extract information from a Scanner into variables we can work with:

Scanner sc = new Scanner(System.in); // Read an integer into a variable int myInteger = sc.nextInt(); // Read a byte into a variable byte myByte = sc.nextByte(); // Read a line until newline or EOF into a string String myLine = sc.nextLine(); // Closing the scanner sc.close(); 

Again, the constructor doesn’t have to take System.in . It can take any File , InputStream , Readable , ReadableByteChannel , Path (of a file to read), or even String . Additionally, as the second argument, it can specify a character encoding to interpret said characters by:

Scanner sc = new Scanner(new FileInputStream("myFile.txt"), "UTF-8"); 

Note that Scanner has to be closed when you’re done working with it. The easiest way to do this is via the try-with-resources statement.

Scanner Methods for Reading

Available methods for reading the next token using the scanner method are:

Method	Return type	Description
next()	String	Finds and returns the next complete token from the scanner.
nextByte()	byte	Scans the next token of the input as a byte.
nextDouble()	double	Scans the next token of the input as a double.
nextFloat()	float	Scans the next token of the input as a float.
nextInt()	int	Scans the next token of the input as an int.
nextLong()	long	Scans the next token of the input as a long.
nextShort()	short	Scans the next token of the input as a short.
nextBoolean()	boolean	Scans the next token of the input into a boolean value and returns that value.
nextLine()	String	Advances this scanner past the current line and returns the input that was skipped.

A method worth noting is the hasNext() method — a generic method that will return true if there’s any type of token to be read. There are type-specific methods such as hasNextInt() , hasNextFloat() , hasNextLine() etc. which you can use in the same fashion.

Problems When Using System.in With Scanner

A big problem with System.in is that it’s an InputStream . When working with it, the Scanner will always expect more input until the InputStream is closed. Once the stream is closed, we can no longer access the input from the Scanner .

Besides closing itself, the Scanner class will also close the InputStream if it implements Closeable .

Since InputStream does, that means that the Scanner will close the System.in stream to the entirety of your program.

That being said, if you close a Scanner and thus the System.in as well, you can’t use System.in again:

Scanner sc = new Scanner(System.in); System.out.println(sc.nextInt()); sc.close(); System.out.println("Closing the scanner. "); sc = new Scanner(System.in); System.out.println(sc.nextInt()); sc.close(); System.out.println("Closing the scanner. "); 

1 1 Closing the scanner. Exception in thread "main" java.util.NoSuchElementException at java.util.Scanner.throwFor(Scanner.java:862) at java.util.Scanner.next(Scanner.java:1485) at java.util.Scanner.nextInt(Scanner.java:2117) at java.util.Scanner.nextInt(Scanner.java:2076) at com.company.Main.main(Main.java:18) 

This makes work with Scanner and System.in much more complicated. We’ll be fixing this in the final section.

BufferedReader and InputStreamReader

Instead of a Scanner , you can also use a BufferedReader alongisde an InputStreamReader to get the user input:

BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); String line; while((line = br.readLine()) != null)< System.out.println(String.format("The input is: %s", line)); > 

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Here, we just repeat back the input String with a prefix:

Hello! The input is: Hello! I'd like to order some extra large fries. The input is: I'd like to order some extra large fries. ^D Process finished with exit code 0 

The BufferedReader is well-suited for reading Strings, but doesn’t have built-in methods for handling numbers. To read an integer, you’d have to parse it from a String:

BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); int a = Integer.parseInt(br.readLine()); System.out.println(a); 

Custom Uncloseable InputStream

Fortunately, there’s a solution to Scanner closing the System.in stream thanks to the Decorator Design Pattern. We can implement our own InputStream and just make the close() method do nothing so that when Scanner calls it, it won’t affect the underlying standard input:

public class UnclosableInputStreamDecorator extends InputStream < private final InputStream inputStream; public UnclosableInputStreamDecorator(InputStream inputStream) < this.inputStream = inputStream; > @Override public int read() throws IOException < return inputStream.read(); > @Override public int read(byte[] b) throws IOException < return inputStream.read(b); > @Override public int read(byte[] b, int off, int len) throws IOException < return inputStream.read(b, off, len); > @Override public long skip(long n) throws IOException < return inputStream.skip(n); > @Override public int available() throws IOException < return inputStream.available(); > @Override public synchronized void mark(int readlimit)  < inputStream.mark(readlimit); >@Override public synchronized void reset() throws IOException < inputStream.reset(); >@Override public boolean markSupported() < return inputStream.markSupported(); > @Override public void close() throws IOException < // Do nothing > > 

When we modify our troublesome code to use the custom InputStream , it will execute without issues:

public class ScannerDemo < public static void main(String[] args) < Scanner sc = new Scanner(new UnclosableInputStreamDecorator(System.in)); System.out.println(sc.nextInt()); sc.close(); System.out.println("Closing the scanner. "); sc = new Scanner(new UnclosableInputStreamDecorator(System.in)); System.out.println(sc.nextInt()); sc.close(); System.out.println("Closing the scanner. "); > > 

Running this will result in:

1 1 Closing the scanner. 1 1 Closing the scanner. 

Conclusion

In this article, we’ve covered how to use the Scanner class to read user input. We’ve then used the BufferedReader class alongisde the InputStreamReader as an alternative approach.

Finally, we’ve implemented our own InputStream to avoid the issue of Scanner closing the System.in stream for the whole program.

Hopefully, you’ve learned how to handle basic console input in Java and some common errors that you might encounter along the way.

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How to input data in java

• Introduction to Java
• The complete History of Java Programming Language
• C++ vs Java vs Python
• How to Download and Install Java for 64 bit machine?
• Setting up the environment in Java
• How to Download and Install Eclipse on Windows?
• JDK in Java
• How JVM Works – JVM Architecture?
• Differences between JDK, JRE and JVM
• Just In Time Compiler
• Difference between JIT and JVM in Java
• Difference between Byte Code and Machine Code
• How is Java platform independent?

• Decision Making in Java (if, if-else, switch, break, continue, jump)
• Java if statement with Examples
• Java if-else
• Java if-else-if ladder with Examples
• Loops in Java
• For Loop in Java
• Java while loop with Examples
• Java do-while loop with Examples
• For-each loop in Java
• Continue Statement in Java
• Break statement in Java
• Usage of Break keyword in Java
• return keyword in Java

• Object Oriented Programming (OOPs) Concept in Java
• Why Java is not a purely Object-Oriented Language?
• Classes and Objects in Java
• Naming Conventions in Java
• Java Methods
• Access Modifiers in Java
• Java Constructors
• Four Main Object Oriented Programming Concepts of Java
• Inheritance in Java
• Abstraction in Java
• Encapsulation in Java
• Polymorphism in Java
• Interfaces in Java
• ‘this’ reference in Java

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Java User Input (Scanner)

The Scanner class is used to get user input, and it is found in the java.util package.

To use the Scanner class, create an object of the class and use any of the available methods found in the Scanner class documentation. In our example, we will use the nextLine() method, which is used to read Strings:

Example

import java.util.Scanner; // Import the Scanner class class Main < public static void main(String[] args) < Scanner myObj = new Scanner(System.in); // Create a Scanner object System.out.println("Enter username"); String userName = myObj.nextLine(); // Read user input System.out.println("Username is: " + userName); // Output user input >> 

If you don’t know what a package is, read our Java Packages Tutorial.

Input Types

In the example above, we used the nextLine() method, which is used to read Strings. To read other types, look at the table below:

Method	Description
nextBoolean()	Reads a boolean value from the user
nextByte()	Reads a byte value from the user
nextDouble()	Reads a double value from the user
nextFloat()	Reads a float value from the user
nextInt()	Reads a int value from the user
nextLine()	Reads a String value from the user
nextLong()	Reads a long value from the user
nextShort()	Reads a short value from the user

In the example below, we use different methods to read data of various types:

Example

import java.util.Scanner; class Main < public static void main(String[] args) < Scanner myObj = new Scanner(System.in); System.out.println("Enter name, age and salary:"); // String input String name = myObj.nextLine(); // Numerical input int age = myObj.nextInt(); double salary = myObj.nextDouble(); // Output input by user System.out.println("Name: " + name); System.out.println("Age: " + age); System.out.println("Salary: " + salary); >> 

Note: If you enter wrong input (e.g. text in a numerical input), you will get an exception/error message (like «InputMismatchException»).

You can read more about exceptions and how to handle errors in the Exceptions chapter.

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