Android Studio - Building a Calculator

[Ric Cristof]

Hello, guys. How are you?

This is my first post and I'm still working around on the Forum. Sorry if this is the wrong place to post this.

I'm having some issues with an exercise I have to deliver tomorrow - 09/02, where I have to build a calculator. Bellow you will find most of the code that I wrote. Still, I'm having the problems below:

*Can't make it show the operation signs;
*The calculator has to start over after pressing the = but mine gets 0.0 and it adds this to the next number - cannot do that;
- with this error we also have the decimal one: If you use 0.0 and add a number (example: 5), it will be 0.05. If you include a DOT - 0.05.5 - the program crashes.
*I can't make it work as a chain operation, I.e (2+3+5 = 10) in my calculator it goes (2+3+5=8) it forgets the first number;

I spent my whole weekend trying to figure this out. It's probably something simple and if it's possible, an explanation of what I was doing wrong so I can also learn with that.

Thank you very much!

package com.example.ricardo.mycalculator3;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends Activity {

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		textView = (TextView) findViewById(R.id.textView);

		// String with all the Numbers for the calculator

		int Buttons[] = { R.id.zero, R.id.one, R.id.two, R.id.three, R.id.four,
				R.id.five, R.id.six, R.id.seven, R.id.eight, R.id.nine,
				R.id.dot };

		int opButtons[] = { R.id.add, R.id.mult, R.id.sub, R.id.div };

		// New Object for the buttons and operation buttons

		NumberButtonClickListener buttonsClickListener = new NumberButtonClickListener();

		// Assign the listener to the number Buttons

		for (int id : Buttons) {
			View v = findViewById(id);
			v.setOnClickListener(buttonsClickListener);
		}

		for (int id : opButtons) {
			findViewById(id).setOnClickListener(buttonsClickListener);
		}
	}

	TextView textView;
	String currentString = "";
	String previousString = null;
	boolean isTempStringShown = false;
	boolean hasDot = false;

	int previousOpp;

	public void onOperationClick(View v) {
		int id = v.getId();
		if (id == R.id.add || id == R.id.sub || id == R.id.mult
				|| id == R.id.div) {
			hasDot = false;
			previousString = currentString;
			isTempStringShown = true;
		}

		// Method to check the number entered and do the calculation

		// NOTE: Understands that currentString is num1+num2 and not only num1

		double curr = Double.parseDouble(currentString);
		double prev = Double.parseDouble(previousString);
		double result = 0;

		switch (id) {
		case R.id.add:
			previousOpp = R.id.add;
			break;
		case R.id.sub:
			previousOpp = R.id.sub;
			break;
		case R.id.mult:
			previousOpp = R.id.mult;
			break;
		case R.id.div:
			previousOpp = R.id.div;
			break;
		case R.id.equal:
			if (previousOpp == R.id.add) {
				result = prev + curr;
			} else if (previousOpp == R.id.sub) {
				result = prev - curr;
			} else if (previousOpp == R.id.mult) {
				result = prev * curr;
			} else if (previousOpp == R.id.div) {
				result = prev / curr;
			}
			setCurrentString(Double.toString(result));
			previousOpp = 0;
			break;
		}
	}

	void setCurrentString(String s) {
		currentString = s;
		textView.setText(s);
	}

	class NumberButtonClickListener implements OnClickListener {

		@Override
		public void onClick(View v) {

			if (isTempStringShown) {
				previousString = currentString;
				currentString = "";
				isTempStringShown = false;
			}

			String text = (String) ((Button) v).getText();
			if (v.getId() == R.id.dot) {
				if (!hasDot)
					setCurrentString(currentString + text);
				hasDot = true;
			} else
				setCurrentString(currentString + text);
		}
	}
}

[Brian Tkatch]

Sorry, too much for me in too short a time. And without the entire project, it's hard to debug. It would be easier if you just asked about a snippet. A couple comments though. But beware, i'm just a beginner in java and android, so, maybe we'll learn together.

1) The code seems to care if there is a decimal place. Shouldn't the double handle that on its own?
2) The code goes through a lot of trouble setting up an onClickListerner. Why not just add onClick to the XML for each button? It's be cleaner in that you could get rid of those arrays and the for each loops.
3) Why "(String)((Button) v).getText();" and not "((Button) v).getText().toSring();"

In a pinch, you could "cheat" and have a variable store the current number instead of reading it from the display each time.

[Ric Cristof]

Brian Tkatch wrote:Sorry, too much for me in too short a time. And without the entire project, it's hard to debug. It would be easier if you just asked about a snippet. A couple comments though. But beware, i'm just a beginner in java and android, so, maybe we'll learn together.

1) The code seems to care if there is a decimal place. Shouldn't the double handle that on its own?
2) The code goes through a lot of trouble setting up an onClickListerner. Why not just add onClick to the XML for each button? It's be cleaner in that you could get rid of those arrays and the for each loops.
3) Why "(String)((Button) v).getText();" and not "((Button) v).getText().toSring();"

In a pinch, you could "cheat" and have a variable store the current number instead of reading it from the display each time.

Hello, Brian.

Thanks for answering. I already deliver the project, but I'm still trying to figure it out the problem.
I'm also a beginner and would be nice to share and learn more.

So, questions:

1 - Decimal place. Double handles that just fine. I have a friend that used Float and got a large number on the answer.
2 - Yeah.... I noticed that after looking for a while on the Internet. It would be waaaay easier to use onClick on the XML. You're right. I just used the hard way - more coding, less design.
3 - You got me on that one. Had to look out. Thanks for asking. ((Button) v).getText().toSring(); it says that cannot resolve the method .toString();. Still, I understand that .getText() gets the information from the button instead of .toString(); where it gets the info from the XML. (Come to think about, I could really used that instead of my method...)

I made a few changes since yesterday and will post later updating the post.

[Brian Tkatch]

Ric Cristof wrote:
1 - Decimal place. Double handles that just fine. I have a friend that used Float and got a large number on the answer.
2 - Yeah.... I noticed that after looking for a while on the Internet. It would be waaaay easier to use onClick on the XML. You're right. I just used the hard way - more coding, less design.
3 - You got me on that one. Had to look out. Thanks for asking. ((Button) v).getText().toSring(); it says that cannot resolve the method .toString();. Still, I understand that .getText() gets the information from the button instead of .toString(); where it gets the info from the XML. (Come to think about, I could really used that instead of my method...)

I made a few changes since yesterday and will post later updating the post.

1. Double has twice the precision of float. While neither is accurate, for your calculator, i doubt it matters. Just round the (longer) numbers. Do a google search for "what every computer scientist should know about floating point"

2. Heh. So many books don't even teach this method, instead showing callbacks and what you did. BTW, there's another way i found, and i'm using in my current project:

- In the class declaration, add: implements View.OnClickListener
- In the class block implement: public void onClick(View view){}
- for the button: button.setOnClickListener(this);

The "this" refers to the class, as you'd expect, making it search for the onClick(). It works well in my scenario because i am creating a bunch of buttons dynamically.

3. Oops! Android Studio didn't allow me to mess with the code because it wasn't part of my project. Based on the (String) i just assumed that it needed to be cast, in which case, there's probably be a toString(). There usually is.

When you post again, please use the code tags. The "Code" button on top of the editor window will make it look nice and pretty, keep formatting, making it much easier to read.