Java Input

We make computer programs for users. We solve real-world computer problems for users. So, it is necessary for our computer to interact with the users. This is what you will learn in this chapter.

There can be times when we want the user to enter the value which has to be assigned to a variable. Take an example of a calculator in which the user enters the values to be added or subtracted. So let's proceed to learn how to take input from a user.

In Java, we take input with the help of the Scanner class. Java has a number of predefined classes which we can use. We will learn more about classes later.

Predefined classes are organized in the form of packages. This Scanner class is found in the java.util package. So to use the Scanner class, we first need to include the java.util package in our program.

We include a package in a program with the help of the import keyword. A package has many predefined classes. We can either import the java.util.Scanner class or the entire java.util package.

To import a class or a package, add one of the following lines to the very beginning of your code.

After importing, we need to write the following statement in our program.

Scanner s = new Scanner (System.in);

Here, by writing Scanner s, we are declaring s as an object of the Scanner class. System.in within the round brackets tells Java that this will be System Input i.e. input from a user will be given to the system.

If you have understood all that has been stated till now, that’s great. If you haven't, do not bother. Currently, you just need to know that we have to import the class or package at the beginning of the program and then include the above statement in the program.

How to Input

Taking a value from the user is quite easy. You just have to write a statement and it is done!

Let's start with an integer. Consider the following code.

Scanner s = new Scanner (System.in);
int n;
n = s.nextInt(); // s is object of Scanner class

The statement n = s.nextInt() is used to input an integer value from the user and assign it to the variable n. Here, nextInt() is a method of the object s of the Scanner class.

Let's see an example in which we will input an integer value from the user.

import java.util.*;

class I1 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter an integer");
        int a;
        a = s1.nextInt();
        System.out.println("The entered integer is" + a);
    }
}

In this example, we created an object s1 of the Scanner class. Then we took an integer input from the user using s1.nextInt() and the value entered by the user (14 in this case) is assigned to the variable a.

Wasn’t it easy?

In the last example, we used the nextInt() method to take integer input. Similarly, there are different methods to take input of different data types. For example, the nextDouble() method is used to take input of type double.

The table given below shows the methods used for taking input of different data types.

Now let's see an example in which we will input values of different data types.

import java.util.*;

class I2 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter integer");
        int n = s1.nextInt();

        System.out.println("Enter double");
        double db = s1.nextDouble();

        System.out.println("Integer :" + n);
        System.out.println("Double :" + db);
    }
}

Here, s1.nextInt() takes an integer input (45) and assigns it to the variable n, and s1.nextDouble() takes an input of type double (23.242) and assigns it to the variable db.

Input Word

As we have seen in the above table, the method used to input a word is next().

We know that 10 is an integer, in the same way a word is a string. A string is a collection of characters. Since a word consists of characters, therefore it is a string.

To input a word, we need to write

String n;
n = s.next(); // s is object of Scanner class
or simply
String n = s.next(); // s is object of Scanner class
Let's see an example.

import java.util.*;

class I3 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter your first name");
        String f = s1.next();
        System.out.println("Enter your last name");
        String l = s1.next();
        System.out.println("Your name is " + f + " " + l);
    }
}

Here, the first s1.next() takes a word input (Robert) and assigns it to the variable f, and the second s1.next() takes another word input (Brown) and assigns it to the variable f.

Let's see another example.

import java.util.*;

class I3 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter your first name");
        String f = s1.next();
        System.out.println("Your name is " + f);
    }
}

In the above example, the input that we gave (Robert Brown) has two words, but s1.next() took only the first word as input. Hence, the value of the variable f became equal to “Robert”.

Input a Sentence

In the same way as a word is a string, a sentence is also a string. Therefore, we input a sentence in the same way as a word, the difference is that the method used to input a sentence is nextLine().

String st = s.nextLine(); // s is object of Scanner class

Look at the following example.

import java.util.*;

class I4 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter your name");
        String name = s1.nextLine();
        System.out.println("Your name is " + name);
    }
}

Here, s1.nextLine() takes a sentence as input (Robert Brown) and assigns it to the variable name.

An example will make the difference clear.

import java.util.*;

class I5 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter your name");
        String w = s1.next();
        System.out.println("Your name is " + w);
        System.out.println("Again enter your name");
        String st = s1.nextLine();
        System.out.println("Your name is " + st);
    }
}

Here, next() read only the first word. After this first word has been read, nextLine() read the rest of the words.

So this is how we can take input of values of any data type from the user. Before wrapping up, let’s look at an example in which we will take the length and breadth of a rectangle as input from the user and will then display the area of the rectangle.

import java.util.*;

class I6 {

    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        int l, b;
        System.out.println("Enter length of rectangle");
        l = s1.nextInt();

        System.out.println("Enter breath of rectangle");
        b = s1.nextInt();

        System.out.println("Area of rectangle is " + (l * b));
    }
}