Possible memory leak with rotation and button click

Question

I think my app is taking up extra memory that the GC should be able to reallocate. I don't know if these will be considered memory leaks, but there are 2 places that I have noticed possible issues

• From App start, continuously rotate my device from portrait to landscape to portrait to landscape....

  • +300KB cumulative memory usage per rotate

• With 2 inputs, every button click

  • +30KB cumulative memory usage per click

This issue is that the memory is never released as long as the app is still in view.

Example: Rotate the device 10 times, and click the buttons 50 times -> consumes 4.5MB memory. If I leave the app open and don't do anything for 1 hour, then my app will still consume 4.5MB of memory; even though a lot of the memory should have been released around 59 minutes earlier!!

My concern is why the memory is never released while the app is always in view?

Am I wrong in how this works?

NOTE: The app is called ContrivedCalculator

Code

UI

public class Calculator extends AppCompatActivity implements ICalculatorInteraction {

    private EditText txtNumber1, txtNumber2, txtResult;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calculator);

        Button btnAdd = (Button) findViewById(R.id.btnAddition);
        Button btnSub = (Button) findViewById(R.id.btnSubtract);
        Button btnMul = (Button) findViewById(R.id.btnMultiple);
        Button btnDiv = (Button) findViewById(R.id.btnDivide);

        txtNumber1 = (EditText) findViewById(R.id.txtNumber1);
        txtNumber2 = (EditText) findViewById(R.id.txtNumber2);
        txtResult = (EditText) findViewById(R.id.txtResult);

        btnAdd.setOnClickListener(new OperationClick(Add).listenerOn(this));   
        btnSub.setOnClickListener(new OperationClick(Subtract).listenerOn(this));
        btnMul.setOnClickListener(new OperationClick(Multiply).listenerOn(this));
        btnDiv.setOnClickListener(new OperationClick(Divide).listenerOn(this));

    @Override
    public String getFirstNumber() { return valueOf(this.txtNumber1); }

    @Override
    public String getSecondNumber() { return valueOf(this.txtNumber2); }

    @Override
    public void updateResult(String result) { this.txtResult.setText(result); }

    private String valueOf(EditText textbox) {

        String text = textbox.getText().toString();

        if (text.isEmpty()) {

            textbox.setText("0");
            return "0";

        }

        return text;
    }

// default android activity methods

}

Listener Logic

public class OperationClick {

    private BinaryOperation operation;   // ENUM - advanced
    private View.OnClickListener listener;

    public OperationClick(final BinaryOperation operation) { this.operation = operation; }

    public View.OnClickListener listenerOn(final ICalculatorInteraction UI) {

        if (listener != null) return listener;
        return listener = new View.OnClickListener() {

            @Override
            public void onClick(View v) {

                double  num1, num2, total;
                String result, sign;

                num1 = Double.parseDouble(UI.getFirstNumber());
                num2 = Double.parseDouble(UI.getSecondNumber());
                total = operation.execute(num1, num2);
                sign = operation.getSymbol();

                result = String.format("%s %s %s = %s", num1, sign, num2, total);

                UI.updateResult(result);

            }

        };

    }

Calculation Logic

public enum BinaryOperation {

    Add      ("+") { @Override double execute(final double a, final double b) { return a + b; } },
    Subtract ("-") { @Override double execute(final double a, final double b) { return a - b; } },
    Multiply ("×") { @Override double execute(final double a, final double b) { return a * b; } },
    Divide   ("÷") { @Override double execute(final double a, final double b) { return a / b; } };

    private final String symbol;

    abstract double execute(double a, double b);

    BinaryOperation(String symbol) { this.symbol = symbol; }

    public String getSymbol() { return this.symbol; }

}

Answer 1

Tracking memory leaks is best done with Eclipse MAT (don't worry about the name, it works great with memory dumps from Android Studio as well). The tool has a pretty steep learning curve, but it's the most powerful thing you can imagine for tracking such memory problems.

Indeed you're causing a memory leak in this line:

btnAdd.setOnClickListener(new OperationClick(Add).listenerOn(this));

The problem is that the instance of OperationClick holds a reference to the activity, as it's passed with .listenerOn(this).

When you rotate the device you know that the Activity is recreated. I assume you're not clearing the listeners in onDestroy() method so you end up with a leaked Activity (4 times per rotate to be correct).

Btw last week Leak Canary was released by the awesome dudes from SquareUp. It's a nifty library for finding memory leaks on Android. The first library of this sort, I strongly recommend you try it!

EDIT: To fix your leak don't use anonymous OperationClick objects. Also add a "cleanup" method in them where you get rid of the listener you've created in listenerOn().