About the "unfortunately has stopped" error.

Question

I am recently building an app that calculates the mortgage as my school project in Android studio using Java. However, when I try to run the app on my tablet, it keeps crashing and appearing "unfortunately the app has stopped". I wonder what's wrong with my codes, the below are my codes. This is my controller:

package ca.roumani.mcalc;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import hr.YumModel;


public class EntryForum extends AppCompatActivity
{

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_entry_forum);

}

public void buttonClicked(View v)
{
    View principleView = findViewById(R.id.principlebox);
    EditText principleEdit = (EditText) principleView;
    String principle = principleEdit.getText().toString();
    View amortizationView = findViewById(R.id.amortizationbox);
    EditText amortizationEdit = (EditText) amortizationView;
    String amortization = amortizationEdit.getText().toString();
    View annualinterestView = findViewById(R.id.interestbox);
    EditText annualinterestEdit = (EditText) annualinterestView;
    String annualinterest = annualinterestEdit.getText().toString();

    MortgageModel model = new MortgageModel(principle, amortization, annualinterest);
    String answer = model.computePayment();

    ((TextView) findViewById(R.id.answer)).setText( "$" + answer);

}

public void yumButton(View v)
{
    View principleView = findViewById(R.id.principlebox);
    EditText principleEdit = (EditText) principleView;
    String principle = principleEdit.getText().toString();
    View amortizationView = findViewById(R.id.amortizationbox);
    EditText amortizationEdit = (EditText) amortizationView;
    String amortization = amortizationEdit.getText().toString();
    View annualinterestView = findViewById(R.id.interestbox);
    EditText annualinterestEdit = (EditText) annualinterestView;
    String annualinterest = annualinterestEdit.getText().toString();

    YumModel yumModel = new YumModel();
    yumModel.setAmortization(amortization);
    yumModel.setInterest(annualinterest);
    yumModel.setPrinciple(principle);
    String answer2 = yumModel.computePayment();

    ((TextView) findViewById(R.id.answer2)).setText(answer2);

  }


}

The following is my model:

 public class MortgageModel
 {
    private double principle;
    private int amortization;
    private double annualInterest;

    public MortgageModel(String p, String a, String i) 
{
    this.principle = Double.parseDouble(p);
    this.amortization = Integer.parseInt(a);
    this.annualInterest = Double.parseDouble(i);

}

public String computePayment()
{
    double r = this.annualInterest * 0.01 / 12 ;
    double n = this.amortization * 12;
    double index1 = 1 + (n * r) + (n * (n-1) * r * r) / 2;
    double index2 = 1 - (1 / index1);
    double index3 = (r * this.principle) / index2;
    String result = String.format("%.d", index3);
    return result;

}



}

Does anyone know what's wrong with my code?