Java Scanner Class

In this tutorial, we will learn about Java Scanner and its methods through examples.

The PrintWriter class in the java.io package can be used to write output data in a normally readable form (text).

The Scanner class in the java.util package is used to read input data from different sources (such as input streams, users, files, etc.). Let's take an example.

Example1：Use the scanner to read a line of text

import java.util.Scanner;
class Main {
　　　　public static void main(String[] args) {
　　　　　　　　//Create an object　Scanner
　　　　　　　　Scanner input = new Scanner(System.in);
　　　　　　　　System.out.print("Enter name:　");
　　　　　　　　//　Receive input from the keyboard
　　　　　　　　String　name　=　input.nextLine();
　　　　　　　　//　Print output name
　　　　　　　　System.out.println("My name is　"　+　name);
　　　　　　　　//　Closes　the　scanner
　　　　　　　　input.close();
　　　　}
}

Output Result

Enter name:　Jack
My name is　　Jack

Please note the following line in the above example

Scanner input = new Scanner(System.in);

Here, we create a Scanner object named input.

The System.in parameter is used to obtain input from the standard input. It's like getting input from the keyboard.

Then, we use the nextLine() method of the Scanner class to read a line of text from the user.

Now that you have some understanding of Scanner, let's explore it further.

Import the scanner class

From the above example, you can see that we need to import the java.util.Scanner package first before we can use the Scanner class.

import java.util.Scanner;

Create a scanning program object in Java

As mentioned above, after importing the package, you can create a Scanner object.

//Read input from an input stream
Scanner sc1　= new Scanner(InputStream input);
//Read input from a file
Scanner sc2　= new Scanner(File file);
//Read input from a string
Scanner sc3　= new Scanner(String str);

Here, we create objects of the Scanner class, which will read inputs fromInputStream，file and string input reading.

Java Scanner input methods

The Scanner class provides various methods that allow us to read different types of input.

Example2：Java Scanner nextInt()

import java.util.Scanner;
class Main {
　　　　public static void main(String[] args) {
　　　　　　　　//Create a Scanner object
　　　　　　　　Scanner input = new Scanner(System.in);
　　　　　　　　System.out.println("Input an integer: ")
　　　　　　　　//Read an int value
　　　　　　　　int data1　= input.nextInt();
　　　　　　　　System.out.println("Using nextInt(): ")　+　data1);
　　　　　　　　input.close();
　　　　}
}

Output Result

Input an integer:
22
Using nextInt():　22

In the above example, we used nextInt() method to read integer values.

Example3：Java Scanner nextDouble() method

import java.util.Scanner;
class Main {
　　　public static void main(String[] args) {
　　　　　　//Create a Scanner object
　　　　　　Scanner input = new Scanner(System.in);
　　　　　　System.out.print("Input a double precision value: ")
　　　　　　//Read a double precision value
　　　　　　double value = input.nextDouble();
　　　　　　System.out.println("Using nextDouble(): ")　+　value);
　　　　　　input.close();
　　　}
}

Output Result

Input a double precision value:　33.33
Using nextDouble():　33.33

In the above example, we used nextDouble() method to read floating-point values.

Example4：Java Scanner next() method

import java.util.Scanner;
class Main {
　　　public static void main(String[] args) {
　　　　　　//Create a Scanner object
　　　　　　Scanner input = new Scanner(System.in);
　　　　　　System.out.print("Enter your name: ");
　　　　　　//Read the entire word
　　　　　　String value = input.next();
　　　　　　System.out.println("Using next(): ")　+　value);
　　　　　　input.close();
　　　}
}

Output Result

Enter your name: Jonny Walker
Using next(): Jonny

In the above example, we used the next() method to read a string from the user.

Here, we provide the full name. However, the next() method only reads the name.

This is because the next() method reads the input up tospacescharacters. Once a space is encountered, it returns the string (excluding spaces).

Example5：Java scanner nextLine() method

import java.util.Scanner;
class Main {
　　　public static void main(String[] args) {
　　　　　　//Create a Scanner object
　　　　　　Scanner input = new Scanner(System.in);
　　　　　　System.out.print("Enter your name: ");
　　　　　　//Read a full line
　　　　　　String value = input.nextLine();
　　　　　　System.out.println("Using nextLine(): ");　+　value);
　　　　　　input.close();
　　　}
}

Output Result

Enter your name: Jonny Walker
Using nextLine(): Jonny Walker

In the first example, we used the nextLine() method to read a string from the user.

Unlike next(), the nextLine() method reads the entire input line including spaces. It terminates when it encounters the next line character \n.

Java scanner BigInteger and BigDecimal

Java scanner can also be used to read large integers and large decimals.

• nextBigInteger() - Read large integer values from the user

• nextBigDecimal() - Read large decimal values from the user

Example4：Read BigInteger and BigDecimal

import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.Scanner;
class Main {
　　　public static void main(String[] args) {
　　　　　　//Create a Scanner object
　　　　　　Scanner input = new Scanner(System.in);
　　　　　　System.out.print("Enter a large integer: ");
　　　　　　//Read a large integer
　　　　　　BigInteger value1　= input.nextBigInteger();
　　　　　　System.out.println("Using nextBigInteger(): ");　+　value1);
　　　　　　System.out.print("Enter a large decimal: ");
　　　　　　//Read a large decimal
　　　　　　BigDecimal value2　= input.nextBigDecimal();
　　　　　　System.out.println("Using nextBigDecimal(): ")　+　value2);
　　　　　　input.close();
　　　}
}

Output Result

Enter a large integer:　987654321
Using nextBigInteger():　987654321
Enter a large decimal:　9.55555
Using nextBigDecimal():　9.55555

In the above example, we use the java.math.BigInteger and java.math.BigDecimal packages to read BigInteger and BigDecimal, respectively.

How Java Scanner Works

The scanner class reads the entire line and splits the line into tokens. Tokens are small elements that have a certain meaning to the Java compiler. For example

Suppose there is an input string:

He is　22

In this case, the scanner object will read the entire line and split the string into tokens: “ He ”，“ is 22 ”. Then, the object traverses each token and uses its different methods to read each token.

NoteBy default, spaces are used to delimit tokens.