Input in Java

A package is a named collection of Java classes that are grouped on the basis of their functionality. For example, java.util.

import statement is used to import built-in and user-defined packages and classes into our Java program. For example, we can import all the classes present in the java.util package into our program using the below statement:

import java.util.*;

Writing fully commented code is a good programming style. The primary purpose of comments is to document the code so that even a layman can understand the purpose of the written code. Hence, a well-documented code is as important as the correctly working code.

Single line comments in Java can be written in the below two ways:

1. Using Single line comments style — Single line comment begin with a double slash (//) and continues till the end of the line. For example:

// Single line comment example

2. Using Multi line comments style — In this style, comments start with the initiating slash-asterisk (/*) and ends with the terminating asterisk-slash(*/). For example:

/* Single line comment using multi line comment style */

The data entered by the user through the keyboard is provided to our Java program with the help of standard input stream that is specified as System.in in the program. We connect our Scanner class to this stream while creating its object. The input received by Scanner class is broken into tokens using a delimiter pattern. The default delimiter is whitespace. Scanner class provides various next methods to read these tokens from the stream. The different next methods provided by Scanner class are next(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), nextLine(), next().charAt(0).

ii. next() and next().charAt(0)

iii. next() and hasNext()

iv. hasNext() and hasNextLine()

A delimiter is a sequence of one or more characters that separates two tokens. The default delimiter is a whitespace.

Errors are mistakes in a program that prevents it from its normal working. They are often referred to as bugs. Errors are classified into three categories — Syntax Errors, Runtime Errors and Logical Errors.

Syntax Errors are the errors that occur when we violate the rules of writing the statements of the programming language. These errors are reported by the compiler and they prevent the program from executing. For example, the following statement will give an error as we missed terminating it with a semicolon:

int sum

ii. Runtime error

Errors that occur during the execution of the program primarily due to the state of the program which can only be resolved at runtime are called Run Time errors.
Consider the below example:

import java.util.Scanner;

class RunTimeError
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int n = in.nextInt();
        int result = 100 / n;
        System.out.println("Result = " + result);
    }
}

This program will work fine for all non-zero values of n entered by the user. When the user enters zero, a run-time error will occur as the program is trying to perform an illegal mathematical operation of division by 0. When we are compiling the program, we cannot say if division by 0 error will occur or not. It entirely depends on the state of the program at run-time.

iii. Logical error

When a program compiles and runs without errors but produces an incorrect result, this type of error is known as logical error. It is caused by a mistake in the program logic which is often due to human error. Logical errors are not caught by the compiler. Consider the below example for finding the perimeter of a rectangle with length 2 and breadth 4:

class RectanglePerimeter
{
    public static void main(String args[]) {
        int l = 2, b = 4;
        double perimeter = 2 * (l * b);    //This line has a logical error
        System.out.println("Perimeter = " + perimeter);
    }
}

Output

Perimeter = 16

The program gives an incorrect perimeter value of 16 as the output due to a logical error. The correct perimeter is 12. We have written the formula of calculating perimeter incorrectly in the line double perimeter = 2 * (l * b);. The correct line should be double perimeter = 2 * (l + b);.

Successful compilation of the program only ensures that there are no syntax errors. Runtime and logical errors can still be there in the program so we cannot say that the program is error free. Only after comprehensive testing of the program, we can say that the program is error free.

public class KboatInterest
{
    public static void main(String args[]) {
        int p = 20000;
        double r = 8.9;
        int t = 3;
        double si = p * r * t / 100.0;
        double amt = p + si;
        System.out.println("Simple Interest = " + si);
        System.out.println("Total Amount = " + amt);
    }
}

Output

import java.util.Scanner;

public class KboatTimeConvert
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter time in seconds: ");
        long secs = in.nextLong();
        long hrs = secs / 3600;
        secs %= 3600;
        long mins = secs / 60;
        secs %= 60;
        System.out.println(hrs + " Hour(s) " + mins 
            + " Minute(s) " + secs + " Second(s)");
        in.close();
    }
}

Output

import java.util.Scanner;

public class KboatTemperature
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("Enter temperature in Fahrenheit: ");
        double f = in.nextDouble();
        double c = (f - 32) * 5.0 / 9.0;
        System.out.println(f + " degree Fahrenheit = " + c + " degree Celsius");
    }
}

Output