Created a custom preference with a custom attribute.

app/src/main/java/com/example/android/sunshine/app/LocationEditTextPreference.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.example.android.sunshine.app;
+
+import android.content.Context;
+import android.content.res.TypedArray;
+import android.preference.EditTextPreference;
+import android.util.AttributeSet;
+
+public class LocationEditTextPreference extends EditTextPreference {
+    static final private 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();
+        }
+    }
+}
+

app/src/main/res/values/attrs.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+     Copyright (C) 2015 The Android Open Source Project
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+ -->
+<resources>
+    <declare-styleable name="LocationEditTextPreference">
+        <attr name="minLength" format="integer" />
+    </declare-styleable>
+</resources>

app/src/main/res/xml/pref_general.xml

@@ -14,16 +14,19 @@
      See the License for the specific language governing permissions and
      limitations under the License.
  -->
-<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
+<PreferenceScreen
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:custom="http://schemas.android.com/apk/res/com.example.android.sunshine.app"
     android:layout_width="match_parent"
     android:layout_height="match_parent">
 
-    <EditTextPreference
+    <com.example.android.sunshine.app.LocationEditTextPreference
         android:title="@string/pref_location_label"
         android:key="@string/pref_location_key"
         android:defaultValue="@string/pref_location_default"
         android:inputType="text"
-        android:singleLine="true" />
+        android:singleLine="true"
+        custom:minLength="3"/>
 
     <ListPreference
         android:title="@string/pref_units_label"