The document provides code examples comparing Java and Kotlin implementations of common programming concepts and patterns. It shows how Kotlin simplifies concepts like listeners, constructors, properties, and null safety checks compared to Java.

14. // Java
@Override
public int getItemCount() {
return mItems != null ? mItems.size() : 0;
}
// Kotlin
override fun getItemCount() = items.size

17. Swift
~

22. // Java
button.setOnClickListener({
@Override
public void onClick(View v) {
// Make it happen
}
});
// Kotlin
button.setOnClickListener {
// Make it happen
}

23. Java

24. Java
+

31. buildscript {
...
dependencies {
...
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.2.30"
}
}
build.gradle in Project

32. apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'kotlin-kapt'
...
dependencies {
...
implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:1.2.40"
}
...
build.gradle in Module

37. // Read-write
var a: Int = 0
var b: String? = null
var c = "Kotlin"
var d = User()
lateinit var c: Int
// Read-only
val pi = 3.14
val d = User()
const val pi = 3.14

38. var b: String? = null
String? = String + null

40. Button button = findViewById(R.id.button);
button.setOnClickListener({
@Override
public void onClick(View v) {
// Make it happen
}
});

41. @BindView(R.id.button) Button button;
@OnClick(R.id.button) void clickTodoSth() {
// Make it happen
}

42. import kotlinx.android.synthetic.main.fragment_main.*
button.setOnClickListener {
// Make it happen
}

43. public class User {
private String name;
private int age;
public User(String n, int a) {
name = n;
age = a;
}
public int getName() {
return name;
}
public int getAge() {
return age;
}
}
class User(
val name: String,
val age: Int)

45. // Java
private static final String BUNDLE_GET_STRING =
"bundle:str";
public static SampleFragment newInstance(String name) {
SampleFragment fragment = new SampleFragment();
Bundle bundle = new Bundle();
bundle.putString(BUNDLE_GET_STRING name);
fragment.setArguments(bundle);
return fragment;
}

46. // Kotlin
companion object {
private const val BUNDLE_GET_STRING = "bundle:str"
fun newInstance(name: String?): SampleFragment {
val fragment = SampleFragment()
val bundle = Bundle()
bundle.putString(BUNDLE_GET_STRING, name)
fragment.arguments = bundle
return fragment
}
}

47. // Kotlin
if (savedInstanceState == null) {
supportFragmentManager.beginTransaction().
replace(R.id. content, SampleFragment.newInstance( "Cherprang")).
commit()
}

48. // Java
public static SampleFragment newInstance() {
SampleFragment fragment = new SampleFragment();
return fragment;
}
// Kotlin
fun newInstance() = SampleFragment()

49. // Java
public boolean getIsLogin() {
return preferences.getBoolean(PREFERENCE_KEY_IS_LOGIN, false);
}
public void setIsLogin(boolean isLogin) {
editor.putBoolean(PREFERENCE_KEY_IS_LOGIN, isLogin);
editor.commit();
}

50. // Kotlin
var isLogin: Boolean
get() = preferences.getBoolean(PREFERENCE_KEY_IS_LOGIN, false)
set(value) {
editor.putBoolean(PREFERENCE_KEY_IS_LOGIN, value)
editor.commit()
}

52. // Java
class SomethingInTheApp {
public SomethingInTheApp(Context context) {
this.mContext = context;
}
...
}

53. // Kotlin
class SomethingInTheApp {
constructor(context: Context) {
this.mContext = context
}
…
}

55. // Kotlin
class SomethingInTheApp(context: Context) {
private var mContext = context
…
}

56. // Java
public class AnimationView extends LinearLayout {
...
public AnimationView(Context context) {
super(context);
...
}
public AnimationView(Context context , AttributeSet attrs) {
super(context, attrs);
...
}
...
}

57. // Kotlin
class AnimationView: LinearLayout {
constructor(context: Context) : super(context) {
initialize(context)
...
}
constructor(context: Context , attrs: AttributeSet) : super(context, attrs)
{
initialize(context)
...
}
...
}

58. // Kotlin
class AnimationView: LinearLayout {
constructor(context: Context) : super(context) {
initialize(context)
...
}
}

59. // Kotlin
class AnimationView(context: Context): LinearLayout(context) {
init {
initialize(context)
...
}
}

61. open class BaseFragment : Fragment() {
...
}

62. sealed class ProfileRouting: FuelRouting {
val PARAM_AUTH = "Authorization"
override val basePath: String
get() = Constant. BASE_PATH
class getProfile(val token: String): ProfileRouting() {
override val method: Method
get() = Method.GET
override val path: String
get() = "/users"
override val params: List<Pair<String , Any?>>?
get() = null
override val headers: Map<String, String>?
get() = null
}
}

63. data class UserProfile(
val id: String,
val name: String,
val age: Int,
val email: String
val pictureUrl: String)

64. // Java
Button button = findViewById(R.id.button);
button.setOnClickListener({
@Override
public void onClick(View v) {
// Make it happen
}
});

65. // Kotlin
button.setOnClickListener {
// Make it happen
}

66. // Java
new CountDownTimer( 30000, 1000) {
public void onTick(long millisUntilFinished) {
mTextField.setText( "seconds remaining: " + millisUntilFinished / 1000);
}
public void onFinish() {
mTextField.setText( "done!");
}
}.start();

67. // Kotlin
object : CountDownTimer( 30000, 1000) {
override fun onTick(millisUntilFinished: Long) {
mTextField.setText( "seconds remaining: " + millisUntilFinished / 1000)
}
override fun onFinish() {
mTextField.setText( "done!")
}
}.start()

69. Run fun <T, R> T.run(block: T.() -> R): R = block()
With fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
Apply fun <T> T.apply(block: T.() -> Unit): T = { block(); return this }Also
Also fun <T> T.also(block: (T) -> Unit): T = { block(this); return this }
Let fun <T, R> T.let(block: (T) -> R): R = block(this)

70. button.text = "Press This"
button.isClickable = true
button.background = resources.getDrawable(
R.drawable.green,
this.theme)

71. button.apply {
text = "Press This"
isClickable = true
background = resources.getDrawable(
R.drawable.green,
this.theme)
}

72. button.also {
it.text = "Press This"
it.isClickable = true
it.background = resources.getDrawable(
R.drawable.green,
this.theme)
}

73. button.apply {
text = "Press This"
isClickable = true
background = resources.getDrawable(
R.drawable.green,
this.theme)
}.also {
Toast.makeText(context, "gogo~”,
Toast.LENGTH_SHORT).show()
}

74. button.let {
it.text = "Press This"
it.isClickable = true
it.background = resources.getDrawable(
R.drawable.green,
this.theme)
}

75. if (player != null) {
playbackPosition = player!!.currentPosition
currentWindow = player!!.currentWindowIndex
playWhenReady = player!!.playWhenReady
player?.release()
player = null
}

76. player?.let {
playbackPosition = player!!.currentPosition
currentWindow = player!!.currentWindowIndex
playWhenReady = player!!.playWhenReady
player?.release()
player = null
}

78. edittext.apply {
text = "username"
...
}.run {
Toast.makeText(context, edittext.text.toString(),
Toast.LENGTH_SHORT).show()
}

79. // Kotlin
if (responseCode == 200OK) {
textResponse.text = "Successful!"
} else if (case == 401Unauthorized) {
textResponse.text = "Please log-in"
} else if (case == 404NotFound) {
textResponse.text = "cannot load data now"
} else if (case == 500ServerError) {
textResponse.text = "Internal Server Error"
}

80. // Kotlin
when (responseCode) {
200OK -> textResponse.text = "Successful!"
401Unauthorized -> textResponse.text = "Please log-in"
404NotFound -> textResponse.text = "cannot load data now"
500ServerError -> textResponse.text = "Internal Server Error"
}

81. // Java
itemSize = isTrue ? itemSize : 0;
// Kotlin
itemSize = if (isTrue) itemSize else 0

83. // Kotlin
print(company.department.employee.name)
// Safe call
print(company?.department?.employee?.name)
// Assertion
print(company!!.department.employee.name)

84. // Elvis operator
val s = str ?: ""

85. // Kotlin Loop
for (i in range) {
print (i)
}

86. val iterator = (1..3).iterator()
// skip an element
if (iterator.hasNext()) {
iterator.next()
}
// do something with the rest of elements
iterator.forEach {
println("The element is $it")
}

87. simpleList.forEachIndexed {
index, element -> println(
"index = $index, element = $element")
}

93. $
// Java
println("a = " + a + "b = " + b + "c = " + c)
// Kotlin
println("a = $a, b = $b, c = $c")

94. // Java
String[] member = {"Cherprang", "Music", "Pun", "Jennis"};
// Kotlin
var member = arrayOf("Cherprang", "Music", "Pun", "Jennis")
mapOf(KEY to "value")
listOf(KEY to "value")

95. // Java
array.get(0).count;
// Kotlin
array[0].count

98. // build.gradle
dependencies {
...
implementation 'com.github.kittinunf.fuel:fuel-android:1.12.0'
implementation 'com.github.kittinunf.fuel:fuel-gson:1.12.0'
}

100. sealed class ProfileRouting: FuelRouting {
val PARAM_AUTH = "Authorization"
override val basePath: String
get() = Constant.BASE_PATH
class getProfile(val token: String): ProfileRouting() {
override val method: Method
get() = Method.GET
override val path: String
get() = "/users"
override val params: List<Pair<String, Any?>>?
get() = null
override val headers: Map<String, String>?
get() = null
}
}

101. class ProfilePresenter(listener: UserProfileListener) {
private var mListener = listener
fun getProfile() {
Fuel.request(ProfileRouting.getProfile())
.responseObject(UserProfile.Deserializer()) {
_, _, result ->
result.fold( success = { userProfile ->
mListener.onProfileSuccess(userProfile)
}, failure = { error ->
mListener.onProfileFailure(error)
})
}
}
}

102. interface UserProfileListener {
fun onProfileSuccess(profile: Profile)
fun onProfileFailure(error: FuelError)
}

103. // Call this
ProfilePresenter(this).getUserProfile()
// handle listener
override fun onProfileSuccess(profile: UserProfile) {
//TODO: handle for success
}
override fun onProfileFailure(error: FuelError) {
//TODO: handle for success
}

116. #31DaysOfKotlin

119. Cheat Sheet?!

125. Songshakes, cover artist community, is coming soon!!