We Keep Coding

The below code wont execute with some inputs

Sorry for such a basic level question guys. But I'm starter in programming. Not a computers guy. So kindly help me.
In this code when I give input 1000000000, 1000000000, 999999999 the answer should be 4. But my answer is 1. I expect the if statement to execute but it is not executing here.
if you take m*n as a room and "a" as the side as a square tile. Then I want to count MINIMUM no. of tiles required to fill the floor of room. tiles may cover a bit more area but should not leave the room empty. this is my objective. It's working with inputs like 6,6,4 or 15,20,13 etc.
Now its working guys. I had posted the correct code with those minor changes below.
import java.util.Scanner;
public class TheatreSquare {
private static Scanner input;
public static void main(String[] args) {
input = new Scanner(System.in);
float m=input.nextFloat();
float n=input.nextFloat();
float a=input.nextFloat();
long i=(int)(m/a);
long j=(int)(n/a);
if((a*a*i*j)<m*n){
if(a*i<m){
//to check weather it is entering if()
System.out.println("true");
i+=1;
}
if(a*j<n){
System.out.println("false");
//to check weather it is entering if()
j+=1;
}
}
System.out.println((double)(i*j));
}
}

Your floats are overflowing when you multiply them. Defining m, n and a as doubles will solve the issue:
double m = input.nextDouble();
double n = input.nextDouble();
double a = input.nextDouble();

The int conversion loses precision.
Here in this case, a*a*i*j is equal to m*n Hence the if loop will not execute. Also a*i is equal to m and a*j is equal to n.
Hence i isi and j is 1, so i*j is 1.
You need to allow it to go if it is equal too.
Replace
if((a*a*i*j)<m*n){
if(a*i<m){
//to check weather it is entering if()
System.out.println("true");
i+=1;
}
if(a*j<n){
System.out.println("false");
//to check weather it is entering if()
j+=1;
}
}
with
if((a*a*i*j) <= m*n){
System.out.println("Entered if block");
if(a*i <= m){
//to check weather it is entering if()
System.out.println("true");
i+=1;
}
if(a*j <= n ){
System.out.println("false");
//to check weather it is entering if()
j+=1;
}
System.out.println("i is:"+ i +"j is:"+j);
}

thankyou #Mureinik, #Uma Lakshmi Kanth, #Diego Martinoia for helping to solve this. All your answers contributed to solve my question. this is working now. as #Mureinik said my floats are overflowing( though I dont know the meaning). I used Double instead of float and that's it. its working. :-)
import java.util.Scanner;
public class TheatreSquare {
private static Scanner input;
public static void main(String[] args) {
input = new Scanner(System.in);
double m=input.nextDouble();
double n=input.nextDouble();
double a=input.nextDouble();
long i=(long)(m/a);
long j=(long)(n/a);
if((a*a*i*j) <m*n){
if(a*i < m){
//to check weather it is entering if()
i+=1;
}
if(a*j < n ){
//to check weather it is entering if()
j+=1;
}
}
System.out.println((long)(i*j));
}
}

The reason for your behavior is that you are reading numbers as floats. Floats have limited precision, so your m n and a are the same value (at runtime). Reading them as long (and getting rid of all the decimal stuff) should help. But, as mentioned in the comment, we don't know what you wanted to achieve!
--- EDIT DUE TO NEW INFO ---
You have to cover an area of m times n square meters. You have an unit of computation of 1 tile, i.e. a times a square meters (both assumed to be decimal).
Assuming you can cut your tile with good-enough precision, your result will be:
Math.ceiling((m*n) / (a*a));
i.e., either your area is an exact multiple of your tiles (and you can always cut them in rectangles to match the shape of the room), or you'll have some "spare" space to fill in, thus you will need 1 more tile, a part of which you'll use to cover the remaining space, and a part of which you'll throw away.

Input/Output - Arithmetic Equation

I am brand new to Java, started two weeks ago and am having issues wrapping my mind around this issue. I have a problem in a class I am taking that has been brought up before. Converting kilograms to pounds and rounding to the second decimal place.
I can create the input side of things and bring up a dialog box to prompt the user to enter in a weight. I can also create an output that uses an equation I made to output the answer in a dialog box.
My question is how do I take the information that is input and use it to convert from kilograms to pounds?
I have been reading my book and scouring the internet trying to find an answer and I think I may be over thinking it. Thanks for the help.
Input.java:
//This program asks a user to input a weight in kilograms.
package module2;
import javax.swing.*;
public class Input {
public static void main(String[] args) {
String weight;
weight = JOptionPane.showInputDialog("Enter weight in kilograms");
}
}
Output.java:
//This program outputs a converted weight from kilograms to pounds.
package module2;
import javax.swing.JOptionPane;
public class Output {
public static void main(String[] args) {
double kg = 75.5;
double lb = 2.2;
double sum;
sum = (kg * lb);
JOptionPane.showMessageDialog(null,sum, "Weight Conversion", JOptionPane.INFORMATION_MESSAGE);
}
}

Right now you have 2 main methods. Both of these are entry points for the program. Since they have to share information, it doesn't make sense you have both.
What I'd recommend is to change the Output's main method to a instance method, taking one parameter: the weight from the Input.
Like so:
public void printOutput(final double weight){
//...
}
You can then call that from the Input's main method like so:
public static void main(String[] args) {
String weight;
weight = JOptionPane.showInputDialog("Enter weight in kilograms");
double kg = Double.parseDouble(weight); // Be sure to parse the weight to a number
Output output = new Output(); // Create a new instance of the Output class
output.printOutput(kg); // Call our method to display the output of the conversion
}
One other thing, is that since Output is currently only used for that one method, you could consider just moving that method into Input.

// addition of two integers using JOptionPane
import javax.swing.JOptionPane;
public class Addition
{
public static void main(String[] args)
{
String firstNumber = JOptionPane.showInputDialog("Input <First Integer>");
String secondNumber = JOptionPane.showInputDialog("Input <Second Integer>");
int num1 = Integer.parseInt(firstNumber);
int num2 = Integer.parseInt(secondNumber);
int sum = num1 + num2;
JOptionPane.showMessageDialog(null, "Sum is" + sum, "Sumof two Integers", JOptionPane.PLAIN_MESSAGE);
}
}

How to get a remainder in java

The Student Union is organising a student trip and they are determining how many cars are required.
Prompt the user to enter the total number of students going on the trip.
Then determine how many cars are required (5 to a car) and how many students are left without transport.
import java.util.Scanner;
class P2Trip
{
public static void main(String []args)
{
Scanner input = new Scanner(System.in);
int students;
int space = 5;
int sum;
System.out.println("How many students are going today?: ");
students = input.nextInt();
sum =(students / space);
System.out.println("Ammount of cars needed: " + sum);
}
}//code works fine, but im not sure how to get a remainder of students without a lift.

From your comment, it looks like you just need help with who will be left without a car. You need to use modular division (some people call it remainder) with the percent sign (%). Here is the line of code you are missing:
System.out.println("Amount of people without a car: "+students%sum);
The full code for the program is:
import java.util.Scanner;
public class Solution {
public static void main(String[] args) [
Scanner s=new Scanner(System.in);
int carCapacity=5;
System.out.println("How many students?");
int students=s.nextInt();
int cars=students/carCapacity;
int leftOver=students%carCapacity;
System.out.println("Cars needed: "+cars);
System.out.println("Students left over: "+leftOver);
}
}

Have a look at the remainder/modulo operator here:
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/opsummary.html

In my view, there are two corrections you can do in your program:
sum = (student/space);
int leftstudent;`
leftstudent=(students%space);
System.out.println("left students without transport are " +leftstudent);

You might want to check the modular division operator in Java.
Division / Divisor = coefficient
Division % Divisor = remainder
To find the remainder of number students for example you will have to use "%".

Incrementing with a fraction based on user input

I'm having trouble i need to take a user input and increment it by 1/10 starting at 0 so if the user enters a 5.2 i need to to go through 0.1 0.2 0.3 etc display each and stop at 5.2 until the method is called again and a new input is entered here is what i have but it just runs through up until 10 i understand why it does this just not enough to be able to fix this any help would be appreciated
import java.util.Scanner;
public class SpeedChange {
public double startSpeed;
public double newSpeed;
public void changeUserSpeed(){
Scanner sc = new Scanner (System.in);
System.out.println("How fast would you like to go between 1-10 mph?");
double newSpeed = sc.nextDouble();
for(newSpeed = sc.nextDouble(); newSpeed <= 10.00; newSpeed+=.1 ){
System.out.println(newSpeed);
}
}
}

You need to change your loop content.Since your newSpeed has been specified by the user,you don't need to alter it.You simply need to create a new double variable,say Speed,which will increment in the limit of 0.1 at each iteration and keep running until your Speed equals newSpeed.
for(double Speed=0;Speed<=newSpeed;Speed+=0.1 ){
System.out.println(Speed); }
I hope this clears and solves your doubt!

Your code should instead be
for(double speed=0; speed <= newSpeed; speed+=.1 ){
System.out.println(speed);
}
P.S: double/float is not suitable for this kind of use. Use BigDecimal class to do this.
For details, see http://javarevisited.blogspot.in/2012/02/java-mistake-1-using-float-and-double.html?m=1

How do I use BigInterger In my code for calculating primes?

I'm using this basic and crude code below for calculating prime numbers then exporting them to a text file:
import java.util.Scanner;
import java.io.*;
public class primeGenerator{
public static void main(String[] args)throws Exception {
Scanner kb = new Scanner(System.in);
String prime;
long num = kb.nextLong();
long i;
long z=0;
while(z==0){
for (i=2; i < num ;i++ ){
long n = num%i;
if (n==0){
break;
}
}
if(i == num){
writer(num);
}
num=num+2;
}
}
public static void writer(long num) throws Exception {
FileWriter writer = new FileWriter("prime.txt",true);
String prime= ""+ num;
writer.write(prime);
writer.write(" ");
writer.flush();
writer.close();
}
}
I would like to find primes beyond the Primative long's range and apparently big integer is the way to go about it. So how do i alter my code to do so?

Do you really need this? Having numbers bigger than can be handled by long means you want to test numbers bigger than 9223372036854775807. If your for-loop can test a hundred million divisions per second, it will still take it 2923 years to determine if that number is prime - and longer for larger numbers, of course.
A common optimization is to only test divisions up to sqrt(num). If you haven't found anything then, the number is prime.

Well, use BigInteger wherever you've currently got long. Instead of using % you'll use mod, instead of incrementing you'll use i = i.add(BigInteger.ONE), instead of == 0 you'll use equals(BigInteger.ZERO) etc.
Use Scanner.nextBigInteger instead of Scanner.nextLong, too.
Given that this looks like homework of some description (possibly self-set, of course) I won't write out the whole code for you - but if you have specific problems, feel free to ask.