javasweeper/CustomTextField.java

import java.awt.event.*;
import javax.swing.*;

public class CustomTextField extends JTextField {
	private static final int COLS = 4;

	private final int MIN_VALUE;
	private final int MAX_VALUE;

	public CustomTextField(int defaultValue, int minValue, int maxValue) {
		super(Integer.toString(defaultValue), COLS);

		MIN_VALUE = minValue;
		MAX_VALUE = maxValue;

		addFocusListener(new FocusAdapter() {
			// Auto select when focused
			@Override
			public void focusGained(FocusEvent e) {
				selectAll();
			}

			// Auto format input when unfocused
			@Override
			public void focusLost(FocusEvent e) {
				formatText();
			}
		});
	}

	protected void formatText() {
		String text = getText();

		// If tried to input a negative number, set it to the minimum
		if (text.isEmpty() || text.charAt(0) == '-') {
			setValue(getMinValue());
			return;
		}

		// Remove all the nondigit characters and all leading zeros
		String filteredText = text.replaceAll("(^[^1-9]+|\\D)", "");
		if (filteredText.isEmpty()) {
			setValue(getMinValue());
			return;
		}

		// To prevent integer overflow when parsing, just set it to max
		// when it's more than 4 digits
		if (filteredText.length() > 4) {
			setValue(getMaxValue());
			return;
		}

		// Now that we finally have the intended int, clamp and set it
		setValue(Difficulty.clampInt(Integer.parseInt(filteredText), getMinValue(), getMaxValue()));
	}

	// Use protected getters and setters so we can override them if necessary
	protected int getMinValue() {
		return MIN_VALUE;
	}

	protected int getMaxValue() {
		return MAX_VALUE;
	}

	private void setValue(int value) {
		setText(Integer.toString(value));
	}
}