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How to implement a recursive method for converting an integer to binary representation

I am writing a method to recursively convert an integer value to its binary representation.
The code that I wrote below accomplishes the task by just using a method, but I'd like to know how to actually write out a full method.
import java.util.Scanner;
public class Exercise18_21 {
public static void main(String args[]){
Scanner input = new Scanner(System.in);
System.out.print("Enter a decimal integer: ");
int decimal = input.nextInt();
System.out.print("Enter a character: ");
System.out.printf("%d decimal is binary %s",decimal,dec2Bin(decimal));
}
//input: integer
//output: binary representation of integer as a string
public static String dec2Bin(int decimal){
return Integer.toBinaryString(decimal);
}
}
My question is how can this be accomplished with recursion?

I want to preserve your code as mush as I can. Therefore I just add a new method successiveDivision(int)
import java.util.Scanner;
public class Exercise18_21 {
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
System.out.print("Enter a decimal integer: ");
int decimal = input.nextInt();
System.out.print("Enter a character: ");
System.out.printf("%d decimal is binary %s", decimal, dec2Bin(decimal));
}
// input: integer
// output: binary representation of integer as a string
public static String dec2Bin(int decimal) {
return successiveDivision(decimal);
}
public static String successiveDivision(int dec) {
if (dec <= 0) {
return "";
} else {
int bit = dec % 2;
return successiveDivision(dec / 2) + bit;
}
}
}

You can implement it the same way you can do it with pen and paper. Use the modulo operation.
divided by 2 is your parameter for the recursive call and mod 2 is your current digit.

Java InputMismatchException Error

I've been given a task to make some conversions from ft and in to cm. I've gotten most of this down and the conversions do work. I also want to include the statement of A negative number... or A non-digit... when I type a string, for example, or a negative number, to display said message.
The problem I am getting is that when I do type up a string or negative number, I get the output of testProgram.NegativeNumberException when I enter -9, for example. And testProgram.NonDigitNumberException, when I enter joe, for example.
I am thinking there is something wrong in the catch but not sure exactly where it won't click.
package testProgram;
import java.util.InputMismatchException;
import java.util.Scanner;
public class conversion{
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
double cm = -1;
while(cm == -1){
cm = convertToCentimeters(scan);
if(cm!=-1){
System.out.println("Your result = " +cm);
}
else{
System.out.println("Please enter the values again.");
}
scan.nextLine();
}
}
public static double convertToCentimeters(Scanner scan){
double centimeters = -1;
try{
double foot = getFootValue(scan);
double inch = getInchValue(scan);
double totalInches = foot * 12 + inch;
centimeters = totalInches * 2.54;
}catch(NegativeNumberException e1){
System.out.println(e1);
}
catch(NonDigitNumberException e2){
System.out.println(e2);
}
return centimeters;
}
public static double getFootValue(Scanner scan) throws NegativeNumberException, NonDigitNumberException{
try{
System.out.println("Enter the foot value: ");
double foot = scan.nextDouble();
if(foot <= 0){
throw new NegativeNumberException ("A negative foot value has been entered.");
}
return foot;
}
catch(InputMismatchException e){
throw new NonDigitNumberException ("A non-digit foot value has been entered.");
}
}
public static double getInchValue(Scanner scan)throws NegativeNumberException, NonDigitNumberException{
try{
System.out.println("Enter the inch value: ");
double inch = scan.nextDouble();
if(inch <= 0){
throw new NegativeNumberException ("A negative inch value has been entered.");
}
return inch;
}
catch(InputMismatchException e){
throw new NonDigitNumberException ("A non-digit inch value has been entered.");
}
}
}

Alternatively to #Scary's suggestion, you can add a constructor to your custom exceptions as -
NegativeNumberException(String str) {
System.out.println(str);
}
This shall help you print the message when you
throw new NegativeNumberException ("A n....");

Simple Java way to input decimal and round to nearest integer

I am using the Java SDK to compile. Need I say, I am a beginner.
Here is the code I tried to use to "Ask user to input decimal and code should output an integer. (round to nearest integer)
import java.util.*;
public class readDecimal {
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
double decimalNumber;
long intNumber;
System.out.println(“Please enter a decimal number:“);
decimalNumber = input.nextDouble();
intNumber = Math.round(decimalNumber);
System.out.println(decimalNumber +
“ rounded to the nearest integer is “ + intNumber);
}
}
What am I doing wrong? I saw the other posts however they seem much to complicated for a beginner. Can you please help?
Thank you,
Diane

Your quotation marks are incorrect; they are unicode for some reason. Replace all the quotations by manual typing them in, in you System.out.println statements.
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
double decimalNumber;
long intNumber;
System.out.println("Please enter a decimal number:");
decimalNumber = input.nextDouble();
intNumber = Math.round(decimalNumber);
System.out.println(decimalNumber +
" rounded to the nearest integer is " + intNumber);
}

You can round double numbers using Math.round method.
import java.util.*;
public class RoundingDecimal {
public static void main(String args[]) {
Scanner sc = new Scanner(System.in);
double num1;
double num2;
System.out.print("Please enter a decimal number: ");
num1 = sc.nextDouble();
num2 = Math.round(num1);
System.out.println(" Rounded to the nearest integer is " + num2);
}
}

How to fix user input error when a non number is entered in .nextDouble?

So I am taking Java as part of math degree requirements and have stumbled on a problem with this code. Essentially the code is supposed to take in numbers from the user until they type a zero. It works fine as long as only numbers are entered. However if the user enters a letter or symbol the program gets an exception. Is there a simple way I can validate user input as a number without getting an exception?
import java.util.Scanner;
public class SamsAdder
{
public static void main(String[] args)
{
double userInput = 1;
double sum = 0;
Scanner in = new Scanner (System.in);
while(userInput != 0)
{
System.out.println("Enter a number. (0 to quit):");
userInput = in.nextDouble();
sum = sum + userInput;
}
System.out.println("The sum of the numbers is " + sum + ".");
}
}
So I've tried the try/catch as you showed it. I'm still getting an exception with non numbers though. Entered the code as follows:
while(userInput != 0)
{
System.out.println("Enter a number. (0 to quit):");
try{
userInput = in.nextDouble();
}
catch(NumberFormatException nfe){
System.out.println("Invalid Number");
}
sum = sum + userInput;
}

import java.util.InputMismatchException;
import java.util.Scanner;
public class SamsAdder {
public static void main(String[] args) {
double userInput = 1;
double sum = 0;
Scanner in = new Scanner(System.in);
while (userInput != 0) {
try {
System.out.println("Enter a number. (0 to quit):");
userInput = in.nextDouble();
sum = sum + userInput;
} catch (InputMismatchException nfe) {
System.out.println("Invalid Number");
in.next();
}
}
in.close();
System.out.println("The sum of the numbers is " + sum + ".");
}
}

Code for only accepting integers

import javax.swing.*;
import java.awt.*;
public class JCD {
public static void main(String[] args) {
String input, inputs;
int input1, input2;
input = JOptionPane.showInputDialog("Enter first number");
inputs = JOptionPane.showInputDialog("Enter second number");
input1 = Integer.parseInt(input);
input2 = Integer.parseInt(inputs);
JOptionPane.showMessageDialog(null, "The GCD of two numbers " + input
+ "and" + inputs + " is: " + findGCD(input1, input2));
}// close void
private static int findGCD(int number1, int number2) {
// base case
if (number2 == 0) {
return number1;
}// end if
return findGCD(number2, number1 % number2);
}// end static
} // close class
What can I add so that it will only accept integers? If not given an integer then it will go back to ask again.....

Put your input request in a while statement, check if it's an int, if not repeat the loop, otherwise exit. Do it for both your inputs.
Something like this
public static void main(String[] args) {
String input=null, inputs=null;
int input1 = 0, input2=0;
boolean err=true;
do{
try{
input = JOptionPane.showInputDialog("Enter first number");
input1 = Integer.parseInt(input);
err=false;
}catch(NumberFormatException e){
e.printStackTrace();
}
}while(err);
err=true;
do{
try{
inputs = JOptionPane.showInputDialog("Enter second number");
input2 = Integer.parseInt(inputs);
err=false;
}catch(NumberFormatException e){
e.printStackTrace();
}
}while(err);
JOptionPane.showMessageDialog(null, "The GCD of two numbers " + input
+ "and" + inputs + " is: " + findGCD(input1, input2));
}
Note that this solution requires you to initialize your variables when you declare them
String input = null, inputs = null;
int input1=0, input2=0;

You should try with "try and catch".
input = JOptionPane.showInputDialog("Enter first number");
inputs=JOptionPane.showInputDialog("Enter second number");
try {
input1=Integer.parseInt(input);
input2=Integer.parseInt(inputs);
// establish and use the variables if the characters inserted are numbers
}
catch(NumberFormatException e){
JOptionPane.showMessageDialog(null, e+ "is not a number");
//display a warning to le the user know
}

You could use something like this :-
boolean inputAccepted = false;
while(!inputAccepted) {
try {
input1=Integer.parseInt(input);
input2=Integer.parseInt(inputs);
inputAccepted = true;
} catch(NumberFormatException e) {
JOptionPane.showMessageDialog("Please input a number only");
}
... do stuff with good input value
}