Basic MDAS Java Swing Calculator

package calculator; 

Package names should associate the software with the author, like com.github.razemoon.basicmdasjavacaluclator.

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public class GUI extends JFrame { 

For Java nming conventions, you'd normally use UpperCamelCase, and use lowercase even for acronyms, like "Gui", "HtmlWidgetToolkit" or "HtmlCssParser".

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private static final long serialVersionUID = 1L; 

You only need this field if it is highly likely that the class will be serialized...in this case, most likely not.

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final String[] ops = new String[] {"+", "-", "x", "/"}; 

Why is this package-private?

Also, final arrays are not as final as you'd think, the individual values can still be changed. You most likely want an Enum....actually, you want an interface, but in this example, an Enum would most likely do fine enough.

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 private ArrayList<Integer> numHistory = new ArrayList<Integer>(); private ArrayList<String> opHistory = new ArrayList<String>(); 

Always try to use the lowest common interface for declarations, in this case List.

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 private void setFrame() { this.setTitle(title); this.add(panel, BorderLayout.CENTER); this.setBounds(10,10,300,700); this.setResizable(false); this.setVisible(true); this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } 

Why are you using this here but nowhere else?

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 this.setResizable(false); 

Why? Your frame is perfectly resizable as far as I can see. By setting it not-resizable you only make sure that your application becomes unusable under different LaFs and font-sizes.

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for (int i = 0; i < 10; i++) { 

I'm a very persistent advocate for that you're only allowed to use single-letter variable names if your dealing with dimensions.

for (int number = 0; number <= 9; number++) { // Or for (int digit = 0; digit <= 9; digit++) { 

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final int num = i; 

Don't shorten variable names just because you can, the decreased amount of typing is not worth the decreased readability.

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Regarding the creation of buttons, I like to create helper methods and classes which make the code easier to read, in this case I'd go for lambdas, like this:

private JButton createButton(String text, Runnable action) { return new JButton(new AbstractButton(text) { @Override public void actionPerformed(ActionEvent e) { action.run(); } }) } // In createButtons: panel.add(createButton(Integer.toString(number), () -> { // Code for the number button goes here. })); 

Another alternative would be to create a private class NumberAction which accepts a number in its constructor and performs the associated action. That would also allow you to get rid of the final-redeclaration.

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private int i; 

That;s a very bad variable name.

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 public GUI() { setPanel(); setFrame(); } private void setFrame() { this.setTitle(title); this.add(panel, BorderLayout.CENTER); this.setBounds(10,10,300,700); this.setResizable(false); this.setVisible(true); this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } // ... public static void main(String[] args) { new GUI(); } 

It would be better to split the responsibilities here. The frame itself is only responsible with getting its own layout going, while the main method should be responsible for getting the frame displayed.

 public GUI() { setPanel(); setFrame(); } private void setFrame() { this.setTitle(title); this.add(panel, BorderLayout.CENTER); this.setBounds(10,10,300,700); this.setResizable(false); } // ... public static void main(String[] args) { GUI gui = new GUI(); gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); gui.setVisible(true); } 

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Your logic does not seem to contain any sort of error handling, I believe pressing an operator button twice in succession should yield an error.

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Maybe a better approach would be to print the whole expression to the screen as entered, and then apply the Shunting Yard Algorithm to process that expression.

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Your logic doesn't do decimals, neither does it gracefully handle overflows. By changing your logic yo use BigDecimals you could handle both easily. Note that you must create BigDecimals with an appropriate MathContext to have proper accuracy and behavior.

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If you want read an already existing implementation, I can recommend exp4j for a math-expression library using floats, EvalEx for one using BigDecimal, and my own project jMathPaper for a calculator which sport different GUIs (regarding abstraction).