package com.example.android.sunshine;

import android.content.Context;
import android.content.res.TypedArray;
import android.preference.EditTextPreference;
import android.util.AttributeSet;

/**
 * @author George Spiridakis <george@codeburrow.com>
 * @since May/22/2016.
 * ===================================================
 * ---------->    http://codeburrow.com    <----------
 * ===================================================
 */
public class LocationEditTextPreference extends EditTextPreference {

    private static final int DEFAULT_MINIMUM_LOCATION_LENGTH = 2;

    private int mMinLength;

    public LocationEditTextPreference(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray a = context.getTheme().obtainStyledAttributes(
                attrs,
                R.styleable.LocationEditTextPreference,
                0,
                0
        );

        try {
            mMinLength = a.getInteger(R.styleable.LocationEditTextPreference_minLength, DEFAULT_MINIMUM_LOCATION_LENGTH);
        } finally {
            a.recycle();
        }
    }

}
package com.example.android.sunshine;

import android.app.AlertDialog;
import android.app.Dialog;
import android.content.Context;
import android.content.res.TypedArray;
import android.os.Bundle;
import android.preference.EditTextPreference;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.AttributeSet;
import android.widget.Button;
import android.widget.EditText;

/**
 * @author George Spiridakis <george@codeburrow.com>
 * @since May/22/2016.
 * ===================================================
 * ---------->    http://codeburrow.com    <----------
 * ===================================================
 */
public class LocationEditTextPreference extends EditTextPreference {

    private static final int DEFAULT_MINIMUM_LOCATION_LENGTH = 2;

    private int mMinLength;

    public LocationEditTextPreference(Context context, AttributeSet attrs) {
        super(context, attrs);
        TypedArray a = context.getTheme().obtainStyledAttributes(
                attrs,
                R.styleable.LocationEditTextPreference,
                0,
                0
        );

        try {
            mMinLength = a.getInteger(R.styleable.LocationEditTextPreference_minLength, DEFAULT_MINIMUM_LOCATION_LENGTH);
        } finally {
            a.recycle();
        }
    }

    @Override
    protected void showDialog(Bundle state) {
        super.showDialog(state);

        EditText editText = getEditText();
        editText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                Dialog dialog = getDialog();
                if (dialog instanceof AlertDialog) {
                    AlertDialog alertDialog = (AlertDialog) dialog;
                    Button positiveButton = alertDialog.getButton(AlertDialog.BUTTON_POSITIVE);
                    // Check if the EditText is empty.
                    if (s.length() < mMinLength){
                        // Disable OK button.
                        positiveButton.setEnabled(false);
                    } else {
                        // Re-enable the button.
                        positiveButton.setEnabled(true);
                    }
                }
            }
        });
    }
}
Use a text changed listener to disable and enable the positive button on the associated AlertDialog as the user enters text. Setting Limits, Part 2.