UserInput.java

package by.andd3dfx.common;

/**
 * <pre>
 * User interface contains two types of user input controls: TextInput, which accepts all characters and NumericInput,
 * which accepts only digits.
 *
 * Implement the class TextInput that contains:
 *  - Public method void add(char c) - adds the given character to the current value
 *  - Public method String getValue() - returns the current value
 *
 * Implement the class NumericInput that:
 *  - Inherits from TextInput
 *  - Overrides the add method so that each non-numeric character is ignored
 *
 * For example, the following code should output "10":
 *   TextInput input = new NumericInput();
 *   input.add('1');
 *   input.add('a');
 *   input.add('0');
 *   System.out.println(input.getValue());
 * </pre>
 */
public class UserInput {

  public static class TextInput {

    private String value = "";

    public void add(char c) {
      this.value = this.value + c;
    }

    public String getValue() {
      return value;
    }
  }

  public static class NumericInput extends TextInput {

    @Override
    public void add(char c) {
      if (Character.isDigit(c)) {
        super.add(c);
      }
    }
  }
}