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<!DOCTYPE html><html><head><title>Kotlin is not like Java</title><meta charset="utf-8"><link rel="stylesheet" href="css/style.css"><link class="codestyle" rel="stylesheet" href="css/androidstudio.css"><script src="js/highlight.pack.js"></script><script src="js/kotlin.js"></script><script src="js/java.js"></script><script>hljs.configure({languages: []});hljs.initHighlightingOnLoad();</script></head><body><a target="_blank" href="https://github.com/kotlin-cloud/kotlin-is-not-like-java"><img id="fork-me" src="http://luckymerlin.me/images/fork-me.png"></a><div id="note">Kotlin is NOT Like Java</div><div class="section"><div class="title">BASIC SYNTAX</div><div class="case"><div class="name">Properties</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">public class MyKotlinClass {
val a: Int = 1
var b: Int = 1
val c = 1
var d = 1
private set
}
fun example() {
val myClass = MyKotlinClass()
myClass.b = 2
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">public class MyJavaClass {
private final int a = 1;
private int b = 1;
private final int c = 1;
private int d = 1;
public int getA() { return a; }
public int getB() { return b; }
public void setB(int b) { this.b = b; }
public int getC() { return c; }
public int getD() { return d; }
private void setD(int d) { this.d = d; }
}
class ExampleClass {
public static void example() {
MyJavaClass myClass = new MyJavaClass()
myClass.setB(2)
}
}</code></pre></div></div></div><div class="case"><div class="name">String Templates</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class MyKotlinClass {
val name = "Omar"
val surname = "Miatello"
val example = "My name is $name $surname"
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">class MyJavaClass {
final String getName() {
return "Omar";
}
final String getSurname() {
return "Miatello";
}
final String getExample() {
return String.format("My name is %s %s",
getName(), getSurname()); }
}</code></pre></div></div></div><div class="case"><div class="name">Get and Set</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun renderItem(item: SomeItem) {
visitCount.text = "${item.visitCount}次拜访"
phone1.visibility =
if(item.cellPhone.isNullOrEmpty())
View.GONE else View.VISIBLE
shopName.text = item.name ?: "暂无商户名称"
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">void renderItem() {
visitCount.setText(String.format(...), ...)
if(item.cellPhone == null ||
item.cellPhone.isEmpty()) {
phone1.setVisibility(View.GONE)
} else {
phone1.setVisibility(View.VISIBLE)
}
shopName.text =
item.name != null ? item.name : "..."
}</code></pre></div></div></div><div class="case"><div class="name">Data Class</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">data class User(val name: String, val email: String)</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">public class User {
private String name;
private String email;
User(String name, String email) {
this.name = name;
this.email = email;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
}</code></pre></div></div></div></div><div class="section"><div class="title">NULL SAFETY</div><div class="case"><div class="name">Null Safety</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class MyKotlinClass {
var a: String = "ciao"
var b: String = null // Error at compile time
var c: String? = null
var d: String? = "ok"
fun example(e: String, f: String?) {
e.length()
f.length() // Error at compile time
f?.length()
if (f != null) {
f.length()
}
}
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">static class MyUtils {
void example(String e, String f) {
e.length(); // throw NullPointerException?
f.length(); // throw NullPointerException?
if (e != null) {
e.length();
}
if (f != null) {
e.length();
}
}
}</code></pre></div></div></div></div><div class="section"><div class="title">FUNCTION AND LAMBDA</div><div class="case"><div class="name">Functions</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val a = 1
val b = 2
val f1 = { a: Int, b: Int -> a + b }
fun f2(a: Int, b: Int): Int {
return a + b
}
fun f2_(a: Int, b: Int): Int = a + b
fun f3(a: Int, b: Int, fn: (Int, Int) -> Int): Int =
if (a > b) fn(a, b) else a - b
class MyKotlinClass {
fun test() {
print(f1(a, b)) // => 3
print(f2(a, b)) // => 3
print(f3(a, b, f1)) // => -1
print(f3(2, 1, ::f2)) // => 3
}
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">public class SomeJavaClass {
static int add(int a, int b) {
return a + b;
}
interface Addable {
int add(int a, int b);
}
static int add2(int a, int b, Addable addable) {
if (a > b) {
return addable.add(a, b);
} else {
return a - b;
}
}
void test() {
System.out.println(add2(2, 1, new Addable() {
@Override
public int add(int a, int b) {
return a + b;
}
}));
}
}</code></pre></div></div></div><div class="case"><div class="name">Lambda</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun f3(a: Int, b: Int, fn: (Int, Int) -> Int): Int =
if (a > b) fn(a, b) else a - b
fun test() {
f3(2, 1) { a1, a2 ->
a1 + a2
} // => 3
f3(2, 1, { a1, a2 -> a1 + a2}) // => 3
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">public class SomeJavaClass {
interface Addable {
int add(int a, int b);
}
static int add2(int a, int b, Addable addable) {
if (a > b) {
return addable.add(a, b);
} else {
return a - b;
}
}
void test() {
System.out.println(add2(2, 1, new Addable() {
@Override
public int add(int a, int b) {
return a + b;
}
}));
}
}</code></pre></div></div></div><div class="case"><div class="name">SAM Conversion</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class MyActivity : Activity() {
fun example() {
val view = findViewById(R.id.button)
view.setOnClickListener {
Log.d("TAG", "Item clicked!")
}
}
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">class MyActivity extends Activity {
void example() {
View view = findViewById(R.id.button);
view.setOnClickListener(
new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d("TAG", "Item clicked!");
}
}
);
}
}</code></pre></div></div></div></div><div class="section"><div class="title">COLLECTIONS</div><div class="case"><div class="name">Using Collections</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
os.filterNotNull().sortedBy { it.length }
.map { it.toUpperCase() }
.forEach { print(it) }
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">class MyUtils {
static void example() {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
List<String> osNotNull = new ArrayList<>();
for (String name : os) {
if (name != null) osNotNull.add(name);
}
Collections.sort(osNotNull, new
Comparator<String>() { @Override
public int compare(String l, String r) {
return l.length() - r.length();
}
});
for (String name : osNotNull) {
String value = name.toUpperCase();
print(value);
}
}</code></pre></div></div></div></div><div class="section"><div class="title">EXTESIONS</div><div class="case"><div class="name">Extension: String</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun String.isBig(): Boolean {
return length() > 10
}
fun example() {
"why?!".isBig() // false!
"P90, RUSH B, ...".isBig() // true!
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">// file MyUtils.java
class MyUtils {
static boolean isBig(String str) {
return str.length() > 10;
}
}
// file MyJavaClass.java
class MyJavaClass {
void example() {
MyUtils.isBig("why?!");
MyUtils.isBig("P90, RUSH B, ...");
}
}</code></pre></div></div></div><div class="case"><div class="name">Simple reified Extension</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">inline fun <reified T : Any> Activity.service(): T {
return HttpService.getApiService(T::class.java)
}
fun Activity.toast(str: String) {
Toast.makeText(this, str,Toast.LENGTH_SHORT).show()
}
class MyActivity : Activity() {
val baiduService = service<Api>()
fun test() {
toast("hahaha")
}
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">class MyActivity extends Activity {
//leave out
Api baiduService = HttpService.getApiService(Api.class);
void test() {
ToastUtil.showToast(this, "hahaha");
}
}</code></pre></div></div></div><div class="case"><div class="name">Extensions: Permission</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">inline fun Array<String>.asPermissions(activity: Activity, fn: () -> Unit) {
checkNotNull(activity)
val PERMISSION_REQUEST_CODE = 11111
var pred = true
if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
this.filter { ActivityCompat.checkSelfPermission(activity, it) != PackageManager.PERMISSION_GRANTED }
.forEach { pred = false }
}
if (pred) {
fn()
} else {
ActivityCompat.requestPermissions(activity, this, PERMISSION_REQUEST_CODE)
}
}
fun test(activity: Activity) {
listOf(Manifest.permission.ACCESS_FINE_LOCATION,
Manifest.permission.ACCESS_COARSE_LOCATION,
Manifest.permission.READ_PHONE_STATE).asPermissions(activity) {
// do things with required permissions
}
}</code></pre></div></div></div><div class="case"><div class="name">Extensions: Make Phone Call</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun Activity.call(phone: String?) {
phone?.let {
if (ContextCompat.checkSelfPermission(this, Manifest.permission.CALL_PHONE)
== PackageManager.PERMISSION_GRANTED) {
val callIntent = Intent(Intent.ACTION_DIAL)
callIntent.data = Uri.parse("tel:" + it)
this.startActivity(callIntent)
} else {
ActivityCompat.requestPermissions(this,
arrayOf(Manifest.permission.CALL_PHONE),
StaticConfig.RequestCode.REQUEST_PHONE)
toast("请确认是否开启拨打电话权限")
}
}
if (phone == null) {
toast("没有电话！")
}
}</code></pre></div></div></div></div><div class="section"><div class="title">DELEGATES</div><div class="case"><div class="name">Lazy Delegation 1</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class MyItem
class MyKotlinClass {
val item by lazy { MyItem() }
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">// in Kotlin is synchronized
class MyJavaClass {
class MyItem { }
MyItem item;
final MyItem getItem() {
if (item == null) {
item = new MyItem();
}
return item;
}
}</code></pre></div></div></div><div class="case"><div class="name">Lazy Delegation 2</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class MyKotlinActivity : Activity() {
val textView by lazy { findViewById(R.id.text) as TextView }
val res by lazy { resources.getString(R.string.cancel) }
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">class MyJavaActivity extends Activity {
TextView textView;
String res;
protected void onCreate(BundlesavedInstanceState) {
super.onCreate(savedInstanceState);
textView = (TextView) findViewById(R.id.text);
res = getResources().getString(R.id.cancel)
}
}</code></pre></div></div></div><div class="case"><div class="name">Butterknife</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">//usage
class MyActivity : Activity() {
val someText by bindView<TextView>(R.id.text)
}
//implementation
private class Lazy<T, V>(private val initializer: (T, KProperty<*>) -> V) : ReadOnlyProperty<T, V> {
private object EMPTY
private var value: Any? = EMPTY
override fun getValue(thisRef: T, property: KProperty<*>): V {
if (value == EMPTY) {
value = initializer(thisRef, property)
}
@Suppress("UNCHECKED_CAST")
return value as V
}
}
public fun <V : View> Activity.bindView(id: Int)
: ReadOnlyProperty<Activity, V> = required(id, viewFinder)
private val Activity.viewFinder: Activity.(Int) -> View?
get() = { findViewById(it) }
private fun <T, V : View> required(id: Int, finder: T.(Int) -> View?)
= Lazy { t: T, desc -> t.finder(id) as V? ?: viewNotFound(id, desc) }</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">//usage
class MyActivity extends Activity {
@Bind(R.id.text)
TextView someText;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ButterKnife.bind(this);
}
}
//implementation:
//https://github.com/JakeWharton/butterknife
//tons of code</code></pre></div></div></div><div class="case"><div class="name">Observable</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class User {
var name: String by Delegates.observable("<no name>") {
prop, old, new ->
println("$old -> $new")
}
}
fun example() {
val user = User()
user.name = "first" //<no name> -> first
user.name = "second" //first -> second
}</code></pre></div></div></div><div class="case"><div class="name">Implement willSet and didSet</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">//usage
var x: Int by varWithObservableSetter(5)
.willSet { println("before") }
.didSet { println("after") }
var y: String by varWithObservableSetter("abc")
.didSet { println("after") }
fun test() {
x = 10 // before 5 // after 10
y = "haha" //after "haha"
}
//implementation
class VarWithObservableSetter<T,
out TWillSet : (() -> Unit)?,
out TDidSet : (() -> Unit)?>
internal constructor(
val initialValue: T,
internal val wilLSet: TWillSet,
internal val didSet: TDidSet
) : ObservableProperty<T>(initialValue) {
override fun beforeChange(property: KProperty<*>, oldValue: T, newValue: T): Boolean {
wilLSet?.invoke()
return true
}
override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
didSet?.invoke()
}
}
fun <T> varWithObservableSetter(initialValue: T) =
VarWithObservableSetter(initialValue, null, null)
fun <T, R : (() -> Unit)?>
VarWithObservableSetter<T, Nothing?, R>.willSet(action: () -> Unit) =
VarWithObservableSetter(initialValue, action, didSet)
fun <T, R : (() -> Unit)?>
VarWithObservableSetter<T, R, Nothing?>.didSet(action: () -> Unit) =
VarWithObservableSetter(initialValue, wilLSet, action)
</code></pre></div></div></div></div><div class="section"><div class="title">SMART CAST AND PATTERN MATCHING</div><div class="case"><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun example1(myView: View) {
if (myView is ImageView) {
myView.setImageAlpha(10)
} else if (myView is TextView) {
myView.setText("Ciao")
}
}
//or
fun example2(myView: View) {
when (myView) {
is ImageView -> myView.imageAlpha = 10
is TextView -> myView.text = "Ciao"
}
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
ImageView imageView =
(ImageView) myView;
imageView.setImageAlpha(10);
} else if (myView instanceof TextView) {
TextView textView = (TextView) myView;
textView.setText("Ciao");
}
}
}</code></pre></div></div></div></div><div class="section"><div class="title">STANDARD LIBRARY</div><div class="case"><div class="name">let</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">customer?.let {
doSth(it.id)
}</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">if (customer != null) {
doSth(customer.id)
}</code></pre></div></div></div><div class="case"><div class="pair"><div class="kcard"><div class="lang">Source Code</div><pre class="code"><code class="java">public inline fun <T, R> T.let(block: (T) -> R): R = block(this)</code></pre></div></div></div><div class="case"><div class="name">apply</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">File(dir).apply { mkdirs() }</code></pre></div><div class="card"><div class="lang">Java</div><pre class="code"><code class="java">File makeDir(String path) {
File result = new File(path);
result.mkdirs();
return result;
}</code></pre></div></div></div><div class="case"><div class="pair"><div class="kcard"><div class="lang">Source Code</div><pre class="code"><code class="java">public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }</code></pre></div></div></div><div class="case"><div class="name">with</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun Activity.toast(str: String) {
Toast.makeText(this, str, Toast.LENGTH_SHORT).show()
}
visitList.setOnClickListener {
with(context as Activity) {
toast("haha")
}
}</code></pre></div></div></div><div class="case"><div class="pair"><div class="kcard"><div class="lang">Source Code</div><pre class="code"><code class="java">public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()</code></pre></div></div></div></div><div class="section"><div class="title">DSL</div><div class="case"><div class="name">Bundle</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun bundle(vararg pair: Pair<String, Any>): Bundle {
val bundle = Bundle()
for ((k, v) in pair) {
when (v) {
is String -> this.putString(k, v)
is Int -> this.putInt(k, v)
is Serializable -> this.putSerializable(k, v)
is Bundle -> this.putBundle(k, v)
is Long -> this.putLong(k, v)
...
}
}
return bundle
}
inline fun <reified T : Activity> Activity.jump(requestCode: Int, extras: Bundle) =
ActivityUtils.jump(this, T::class.java, requestCode, extras)
fun example() {
jump<MainActivity>(SOME_CODE, bundle("int-key" to 1, "string-key" to 2))
}</code></pre></div></div></div><div class="case"><div class="name">Converter</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">//usage
fun testDSL() {
convert("ABC") {
toLower() //ABC => abc
toUpper() //abc => ABC
}
convert(2.2) {
toLower() //2.2 => 2.0
}
convert(2) {
toLower() //编译器：没有这个方法
}
}
//implementation
class MConverter<T>(var value: T) {
val old = value
override fun toString(): String =
"[$old] => [$value]"
}
fun <T> convert(
value: T,
init: MConverter<T>.() -> Unit): MConverter<T> {
val converter = MConverter(value)
converter.init()
return converter
}
fun MConverter<String>.toLower() {
value = value.toLowerCase()
}
fun MConverter<String>.toUpper() {
value = value.toUpperCase()
}
fun MConverter<Double>.toLower() {
value = Math.floor(value)
}</code></pre></div></div></div><div class="case"><div class="name">Retrofit</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">//usage
someService.someApi(param1, param2).enqueue {
onResponse {
toast("succeed")
}
onFailure { toast("failed") }
onFinish { hideLoading() }
}
//implementation
inline fun <T> Call<T>.enqueue(init: CallbackHelper<T>.() -> Unit) {
val listener = CallbackHelper<T>()
listener.init()
enqueue(listener)
}
class CallbackHelper<T> : Callback<T> {
private var _onResponse: ((T) -> Unit)? = null
private var _onFailure: ((Throwable) -> Unit)? = null
private var _onFinish: (() -> Unit)? = null
override fun onResponse(call: Call<T>, response: Response<T>) {
onResponse?.invoke(response.body())
_onFinish?.invoke()
}
fun onResponse(listener: (T) -> Unit) {
_onResponse = listener
}
fun onFinish(listener: () -> Unit) {
_onFinish = listener
}
override fun onFailure(call: Call<T>, t: Throwable) {
_onFailure?.invoke(t)
_onFinish?.invoke()
}
fun onFailure(listener: (Throwable) -> Unit) {
_onFailure = listener
}
}</code></pre></div></div></div></div><div class="section"><div class="title">QUIZ</div><div class="case"><div class="name">step 1: solve this quiz. step 2: ???. step 3: profit.</div><div class="pair"><div class="kcard"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">class SomeHolder(var x: Int, val action: () -> Unit)
val sadHolder = SomeHolder(1) { print("I want to print x, but i don’t know how.") }
//modify the following line to let the magic happen
val happyHolder = SomeHolder(1) { print("I know what x is, it's $x")}</code></pre></div></div></div></div></body></html>