Writing an OOP with java

Represent your Equations (Quadratic) using a separate class. Define the operation definition in the class, Which you want to perform on the operation. These operations will be accessed using the instance of the Equation.

The main class assignment will accept the number of equations and create the instance in a array of equations.

import java.util.Scanner;

class Equation {
  double a, b, c;
  Equation(double a, double b, double c) {
    this.a = a;
    this.b = b;
    this.c = c;
  }
  // Perform the operation on equation using specific methods.
}

public class assignment {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int input_number = Integer.parseInt(args[0]);
    Equation objects[];
    double inputA, inputB, inputC;
    if (input_number > 0) {
      objects = new Equation[input_number];
      for (int i = 0; i < input_number; i++) {
        inputA = readCoeff(input);
        inputB = readCoeff(input);
        inputC= readCoeff(input);
        objects[i] = new Equation(inputA, inputB, inputC);
      }
    }
  }
  static double readCoeff(Scanner inn) {
    System.out.print("Please enter coefficient of a quadratic equation: ");
    return inn.nextDouble();
  }
}

import java.util.Scanner;

/**
 * Created by EpicPandaForce on 2015.09.20..
 */
public class Main {
    public static void main(String[] args) {
        int iterationCount;
        if (args.length > 0) {
            iterationCount = Integer.parseInt(args[0]);
        } else {
            iterationCount = 1;
        }
        new Main().execute(iterationCount);
    }

    private Scanner scanner;

    public Main() {
        scanner = new Scanner(System.in);
    }

    public void askForCoefficientInput() {
        System.out.println("Please enter the three coefficients");
    }

    public double readCoefficient() {
        return scanner.nextDouble();
    }

    public void execute(int iterationCount) {
        double inputA, inputB, inputC;
        if (iterationCount > 0) {
            for (int i = 0; i < iterationCount; i++) {
                askForCoefficientInput();
                inputA = readCoefficient();
                inputB = readCoefficient();
                inputC = readCoefficient();
                Equation equation = new Equation(inputA, inputB, inputC);
                equation.printResults();
            }
        }
    }

    public static class Equation {
        private double a;
        private double b;
        private double c;

        private double result1;
        private double result2;
        private boolean hasResult;

        public Equation(double a, double b, double c) {
            this.a = a;
            this.b = b;
            this.c = c;
            calculate();
        }

        protected final void calculate() {
            if (a != 0) {
                if ((b * b - 4 * a * c) >= 0) {
                    this.result1 = ((-1) * b + Math.sqrt(b * b - 4 * a * c)) / (2 * a);
                    this.result2 = ((-1) * b - Math.sqrt(b * b - 4 * a * c)) / (2 * a);
                    this.hasResult = true;
                }
            } else {
                if (b != 0) {
                    this.result1 = ((-1) * c) / b;
                    this.result2 = ((-1) * c) / b;
                    this.hasResult = true;
                } else {
                    if (c == 0) {
                        this.result1 = 0;
                        this.result2 = 0;
                        this.hasResult = true;
                    }
                }
            }
        }

        public void printResults() {
            System.out.println(
                    "The equation [" + a + "x^2 " + (b >= 0 ? "+" + b : b) + "x " + (c >= 0 ? "+" + c : c) + "] had " + (this.hasResult() ? "" : "no ") + "results" + (this.hasResult() ? ", these are: [" + this
                            .getFirstResult() + "] and [" + this.getSecondResult() + "]." : "." + "\n"));
        }

        public double getFirstResult() {
            return this.result1;
        }

        public double getSecondResult() {
            return this.result2;
        }

        public boolean hasResult() {
            return this.hasResult;
        }
    }
}

Result:

>> Executing quadratic.Main

Please enter the three coefficients
1 -8 12
The equation [1.0x^2 -8.0x +12.0] had results, these are: [6.0] and [2.0].

Please enter the three coefficients
3 8 12
The equation [3.0x^2 +8.0x +12.0] had no results.

Process finished with exit code 0

Doing something that can be done much more efficiently and cleanly without objects by forcing OOP is kinda retarded. But, there are teachers like that (I know a few as well). So, here's the way I'd do it:

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int inputNumber = Integer.parseInt(args[0]);

        if (inputNumber > 0) {
            for (int i = 0; i < inputNumber; i++) {
                int[] coefficients;
                //read coefficients into the array
                QuadraticEquation equation = new QuadraticEquation(coefficients);
                System.out.println(equation.solve());
            }
        }
    }
}


class QuadraticEquation {
    private int[] coefficients;

    QuadraticEquation(int[] coefficients) {
        this.coefficients = coefficients;
    }

    double solve() {
        //solve the equation using coefficients and return the result
    }
}

Replace variable/field names with something more sensible if needed. Also, class names should always be uppercase. Compiler won't complain, but it's a widely accepted convention.

Now, what actually happens in the above code is that we define a class named QuadraticEquation which is defined by it's coefficients and we pass them in constructor. Now, when the object has all the necessary information, we can call solve() on it to actually do something, i. e. give us the result, which we print out (you can also save all the results in an array and print them out at the end).

Also note that we have two classes. You can put everything into one, but I don't think you should. Every class should represent an Object that can work on its own (Main class is actually pointless when you look it from that angle; it's just there to prevent main(String[]) to pollute other stuff).

Ideally, you'd make a separate file named QuadraticEquation, change class QuadraticEquation to public class QuadraticEquation and prefix constructor and methods with public to make them usable from anywhere, because they aren't tied to anything specifically. I wanted to keep things simple and dumped everything into one place, and because you can't have two public classes in one file, the other one had to be package-protected (the default visibility level).