val gankApi: GankApi by lazy {
 val retrofit: Retrofit = Retrofit.Builder()
  .baseUrl(API_URL)
  .addConverterFactory(MoshiConverterFactory.create())
  .build()
 retrofit.create(GankApi::class.java)
}

val name: String by lazy {
 Log.d(TAG, "executed only first time")
 "Double Thunder"
}

lazy(LazyThreadSafeMode.NONE){ ... }

@ParseClassName("Book")
class Book : ParseObject() {

 // getString() and put() are methods that come from ParseObject
 var name: String
 get() = getString("name")
 set(value) = put("name", value)

 var author: String
 get() = getString("author")
 set(value) = put("author", value)
}

button.setOnClickListener { view ->
 startDetailActivity()
}

toolbar.setOnLongClickListener { 
 showContextMenu()
 true
}

data class User(val name: String, val age: Int)

// Example with Gson's @SerializedName annotation
data class User(
 @SerializedName("name") val name: String = "",
 @SerializedName("age") val age: Int = 0
)

val users = api.getUsers()
// we only want to show the active users in one list
val activeUsersNames = items.filter { 
 it.active // the "it" variable is the parameter for single parameter lamdba functions
}
adapter.setUsers(activeUsers)

package com.savvyapps.example.util

import android.os.Handler
import android.os.Looper

// notice that this is object instead of class
object ThreadUtil {

 fun onMainThread(runnable: Runnable) {
 val mainHandler = Handler(Looper.getMainLooper())
 mainHandler.post(runnable)
 }
}

ThreadUtil.onMainThread(runnable)

viewPager.addOnPageChangeListener(object : ViewPager.OnPageChangeListener {
 override fun onPageScrollStateChanged(state: Int) {}

 override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {}

 override fun onPageSelected(position: Int) {
  bindUser(position)
 }
});

class ViewUserActivity : AppCompatActivity() {

 companion object {

  const val KEY_USER = "user"

  fun intent(context: Context, user: User): Intent {
   val intent = Intent(context, ViewUserActivity::class.java)
   intent.putExtra(KEY_USER, user)
   return intent
  }
 }
 
 override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_cooking)
  
  val user = intent.getParcelableExtra<User>(KEY_USER)
  //...
 }
}

val intent = ViewUserActivity.intent(context, user)
startActivity(intent)

const val PRESENTATION_MODE_PRESENTING = "presenting"
const val PRESENTATION_MODE_EDITING = "editing"

fun View.fadeOut(duration: Long = 500): ViewPropertyAnimator {
 return animate()
   .alpha(0.0f)
   .setDuration(duration)
}
icon.fadeOut() // fade out with default time (500)
icon.fadeOut(1000) // fade out with custom time

fun Activity.hideKeyboard(): Boolean {
 val view = currentFocus
 view?.let {
  val inputMethodManager = getSystemService(Context.INPUT_METHOD_SERVICE) 
    as InputMethodManager
  return inputMethodManager.hideSoftInputFromWindow(view.windowToken,
    InputMethodManager.HIDE_NOT_ALWAYS)
 }
 return false
}

private lateinit var mAdapter: RecyclerAdapter<Transaction>

override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 mAdapter = RecyclerAdapter(R.layout.item_transaction)
}

var count: Int by Delegates.notNull<Int>()
var name:String by Delegate()

@BindView(R.id.toolbar) lateinit var toolbar: Toolbar
override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_main)
  ButterKnife.bind(this)
  // you can now reference toolbar with no problems!
  toolbar.setTitle("Hello There")
}

var feedFragment: FeedFragment? = supportFragmentManager
 .findFragmentByTag(TAG_FEED_FRAGMENT) as FeedFragment

var feedFragment: FeedFragment? = supportFragmentManager
 .findFragmentByTag(TAG_FEED_FRAGMENT) as? FeedFragment
if (feedFragment == null) {
 feedFragment = FeedFragment.newInstance()
 supportFragmentManager.beginTransaction()
   .replace(R.id.root_fragment, feedFragment, TAG_FEED_FRAGMENT)
   .commit()
}

//Java
if (currentUser != null) {
 text.setText(currentUser.name)
}

//instead Kotlin
user?.let {
 println(it.name)
}

if (TextUtils.isEmpty(name)) {
 // alert the user!
}
public static boolean isEmpty(@Nullable CharSequence str) {
 return str == null || str.length() == 0;
}

public inline fun CharSequence?.isNullOrEmpty(): Boolean = this == null || this.length == 0

public inline fun CharSequence?.isNullOrBlank(): Boolean = this == null || this.isBlank()

// If we do not care about the possibility of only spaces...
if (number.isNullOrEmpty()) {
 // alert the user to fill in their number!
}

// when we need to block the user from inputting only spaces
if (name.isNullOrBlank()) {
 // alert the user to fill in their name!
}

public interface OnClickListener {
 void onClick(View v);
}

view.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View view) {
  // do something
 }
});

view.setOnClickListener { view ->
 // do something
}

//同时也可以为
view.setOnClickListener {
 // do something
}

view.setOnClickListener() {
 // do something
}

view.setOnClickListener(object : OnClickListener {
 override fun onClick(v: View?) {
  // do things
 }
})

private var onClickListener: ((View) -> Unit)? = null
fun setOnClickListener(listener: (view: View) -> Unit) {
 onClickListener = listener
}

// later, to invoke
onClickListener?.invoke(this)

with(helloWorldTextView) {
 text = "Hello World!"
 visibility = View.VISIBLE
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 >

 <TextView
  android:id="@+id/tvHelloWorld"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"/>
</RelativeLayout>

//导入对应的 xml
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

 override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_main)
  //直接使用
  tvHelloWorld.text = "Hello World!"
 }
}

public class User {
 private String firstName;
 private String lastName;

 public String getFirstName() {
  return firstName;
 }
 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public String getLastName() {
  return lastName;
 }
 public void setLastName(String lastName) {
  this.lastName = lastName;
 }
}

class User {
 var firstName: String? = null
 var lastName: String? = null
}

doAsync {
 var result = expensiveCalculation()
 uiThread {
  toast(result)
 }
}

user = User().apply {
 firstName = Double
 lastName = Thunder
}