不建议使用默认构造函数时如何调用父类的构造函数

Question

I have a class called BaseKeyListener that extends android.text.method.DigitsKeyListener. I didn't define a constructor in the BaseKeyListener class so the parents default constructor was called.

As of api level 26 the default constructor of DigitsKeyListener is deprecated . In order to still support lower Android versions I would have to add a constructor to BaseKeyListener that conditionally calls the constructor of the parent. However this results in another error.

public static abstract class BaseKeyListener extends DigitsKeyListener
{
    public BaseKeyListener()
    {
        if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
        {
            // api level 26 constructor
            super(null);
        }
        else 
        {
            // api level 1 constructor (deprecated)
            super(); 
        } 
    }
}

The error I'm getting now:

Call to 'super()' must be first statement in constructor body

I tried a shorthand if statement but that also didn't do the trick. There was another api level 1 constructor but unfortunately it's also deprecated. What can I do to fix these errors?

Answer 1

I would think something like this:

/**
 * only use this if you want to use api level 1
 */
public BaseKeyListener() {
  super(); // implicitly added already
}

/**
 * only use this if you want to use api level 26
 * and add if condition before calling this constructor
 */
public BaseKeyListener(Obect param) {
  super(param);
}

Answer 2

The deprecated constructor still exists in API 26+ and passing in a null locale is the same as calling the default constructor anyway . You can either just override the default constructor or override both and add a static method to call the right constructor depending on which version of Android it's running on.

Option 1 - Default constructor

public static abstract class BaseKeyListener extends DigitsKeyListener {
  public BaseKeyListener() {
    super(); 
  }
}

Option 2 - Two private constructors

public static abstract class BaseKeyListener extends DigitsKeyListener {
  private BaseKeyListener() {
    super(); 
  }

  private BaseKeyListener(Locale locale) {
    super(locale);
  }

  public static BaseKeyListener newInstance(Locale locale) {
    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
      return new BaseKeyListener(locale);
    } else {
      return new BaseKeyListener();
    }
  }
}