Kotlin: Java we are different

Kotlin: We are different
Kotlin: A whole new game you haven’t played yet
Article 3: Kotlin for all: Coroutines Special

This article has been authorized to hongyang wechat public account reprint

As Kotlin has become more and more popular, learning Kotlin has become a rite of passage

Code is the best teacher

Integrated Kotlin

It needs to be in the project APP module firstbuild.gradleAdd configuration to file

apply plugin: 'kotlin-android'

android {

    kotlinOptions {
        jvmTarget = '1.8'}}Copy the code

And then in the project root directorybuild.gradleAdd the Kotlin plug-in configuration to the file

buildscript {

    dependencies {
        / / Kotlin plugin: https://plugins.jetbrains.com/plugin/6954-kotlin
        // noinspection GradleDependency
        classpath 'org. Jetbrains. Kotlin: kotlin - gradle - plugin: 1.5.31'}}Copy the code

It is important to note that hereAndroid Studio 4.2For example, the integration isKotlin - gradle - plugin: 1.5.31There is no issue with the version this Kotlin plugin uses and the current versionAndroid StudioVersion-dependent. If integration fails, you need to modify it to the appropriate version.

The new keyword

Java write

Intent intent = new Intent();
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Kotlin spelled

var intent = Intent()
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This keyword

Java write

MainActivity.this
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Kotlin spelled

this@MainActivity
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The class keyword

Java write

MainActivity.class
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Kotlin spelled

MainActivity::class.java
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The extends keyword

Java write

public class MainActivity extends AppCompatActivity {}Copy the code

Kotlin (in Kotlin, inherited classes must be modified with the open keyword)

class MainActivity : AppCompatActivity() {}Copy the code

The volatile keyword

Java write

volatile ThreadPoolManager instance
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Kotlin spelled

@Volatile
var instance: ThreadPoolManager
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The synchronized keyword

Java write

Method on which synchronization locks are defined
public synchronized ThreadPoolManager getInstance(a) {}Copy the code

// define a synchronization lock in the
public ThreadPoolManager getInstance(a) {

    synchronized (ThreadPoolManager.class) {

    }
}
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Kotlin spelled

@Synchronized
fun getInstance(a): ThreadPoolManager? {}Copy the code

fun getInstance(a): ThreadPoolManager? {

    synchronized(ThreadPoolManager::class.java, {
        
    })
}
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The final keyword

Java write

// indicates that the variable cannot be reassigned
final String text = "";
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// The representation method cannot be overridden
public final void test(a) {}Copy the code

// indicates that the class cannot be inherited
public final class MainActivity extends AppCompatActivity {}Copy the code

Kotlin spelled

// indicates that the variable cannot be reassigned
val text = ""
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// The representation method cannot be overridden
fun test(a){}Copy the code

// indicates that the class cannot be inherited
class MainActivity : AppCompatActivity() {}Copy the code

Note that Kotlin classes and methods cannot be inherited or overridden by default and must be decorated with the open keyword if they are to be inherited or overridden

variable

Java write

String text = "";
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Kotlin spelled

var text: String = ""
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Static constants

Java write

public class MainActivity extends AppCompatActivity {
	
    static final String TEXT = "";
}
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Kotlin’s (note that static variables are defined above the class)

const val TEXT: String = ""
	
class MainActivity : AppCompatActivity() {}Copy the code

Define methods

Java write

public void test(String message) {}Copy the code

Kotlin (Unit has the same effect as void)

fun test(message: String): Unit{}Copy the code

// You can omit the return value Unit in Kotlin
fun test(message: String){}Copy the code

Overloaded methods

Java write

public class MainActivity extends AppCompatActivity {
	
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState); }}Copy the code

Kotlin spelled

class MainActivity : AppCompatActivity() {
	
    override fun onCreate(savedInstanceState: Bundle?). {
        super.onCreate(savedInstanceState)
	}
}
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Basic data types

Java write

int i = 1;
long l = 2;
boolean b = true;
float f = 0;
double d = 0;
char c = 'A';
String s = "text";
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Kotlin spelled

var i: Int = 1
var l: Long = 2
var b: Boolean = true
var f: Float = 0F
var d: Double = 0.0
var c: Char = 'A'
var s: String = "text"
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// To be more succinct, the type can be derived automatically
var i = 1
var l = 2
var b = true
var f = 0F
var d = 0.0
var c = 'A'
var s = "text"
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Comparison of type

Java write

if ("1" instanceof String) {

}

if(! ("2" instanceof String)) {

}
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Kotlin spelled

if ("1" is String) {

}

if ("2" !is String) {

}
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Comparison of object

Java write

// Compare two objects to see if they have the same content
if (object1.equals(object2)) {

}
if(! object3.equals(object4)) { }// Compare two objects to see if they are the same
if (object5 == object6) {

}

if(object7 ! = object8) { }Copy the code

Kotlin spelled

// Compare two objects to see if they have the same content
if (object1 == object2) {

}
if(object3 ! = object4) { }// Compare two objects to see if they are the same
if (object5 === object6) {

}

if(object7 ! == object8) { }Copy the code

Note: Kotlin= =And in JavaequalsThe effect is the same, whereas Kotlin= = = 和 Java 中 = =The effect is the same.

Conversion operators

Java write

int number = 100;
System.out.println(String.format("Quantity of goods %d", number));
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Kotlin spelled

var number = 100
println("The quantity of goods is${number}")
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// We can write it succinctly
var number = 100
println("The quantity of goods is$number")
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// You can use \$if you don't want the string to be escaped
var number = 100
println("Item quantity = $number")
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An array of

Java write

int[] array1 = {1.2.3};
float[] array2 = {1f.2f.3f};
String[] array3 = {"1"."2"."3"};
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Kotlin spelled

val array1 = intArrayOf(1.2.3)
val array2 = floatArrayOf(1f.2f.3f)
val array3 = arrayListOf("1"."2"."3")
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cycle

Java write

String[] array = {"1"."2"."3"};

for (int i = 0; i < array.length; i++) {
    System.out.println(array[i]);
}
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Kotlin spelled

val array = arrayListOf("1"."2"."3")

for (i in array.indices) {
    println(array[i])
}
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Angle of the loop

Java write

String[] array = {"1"."2"."3"};

for (int i = 1; i < array.length; i++) {
    System.out.println(array[i]);
}
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Kotlin’s notation (which Kotlin calls intervals)

val array = arrayListOf("1"."2"."3")

for (i in IntRange(1, array.size - 1)) {
    println(array[i])
}
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// We can write it more succinctly
val array = arrayListOf("1"."2"."3")

for (i in 1..array.size - 1) {
    println(array[i])
}
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// The compiler prompts us to write it differently
val array = arrayListOf("1"."2"."3")

for (i in 1 until array.size) {
    println(array[i])
}
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Senior cycle

Java write

String[] array = {"1"."2"."3"};

for (String text: array) {
    System.out.println(text);
}
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Kotlin spelled

val array = arrayListOf("1"."2"."3")

for (text in array) {
    println(text)
}
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Judgment is

Java write

int count = 1;

switch (count) {
    case 0:
        System.out.println(count);
        break;
    case 1:
    case 2:
    case 3:
        System.out.println(count);
        break;
    case 5:
    case 7:
        System.out.println(count);
        break;
    default:
        System.out.println(count);
        break;
}
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Kotlin spelled

val count = 1

when (count) {
    0 -> {
        println(count)
    }
    in 1.3. -> {
        println(count)
    }
    5.7 -> {
        println(count)
    }
    else -> {
        println(count)
    }
}
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Val count = 1 when (count) {0 -> println(count) in 1.. 3 -> println(count) 5, 7 -> println(count) else -> println(count) }Copy the code

The constructor

Java write

public class MyView extends View {

    public MyView(Context context) {
        this(context, null);
    }

    public MyView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr); }}Copy the code

Kotlin spelled

// The first way
class MyView : View {
	
    constructor(context: Context) : this(context, null) {}constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0) {}constructor(context: Context, attrs: AttributeSet? , defStyleAttr:Int) : super(context, attrs, defStyleAttr) {
	
    }
}
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// The second way
class MyView : View {
	
    constructor(context: Context) : this(context, null)
	
    constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0)
	
    constructor(context: Context, attrs: AttributeSet? , defStyleAttr:Int) : super(context, attrs, defStyleAttr)
    
    init{}}Copy the code

// The third way
class RegexEditText @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

    init{}}Copy the code

// If there is only one constructor, you can write it like this
class MyView(context: Context) : View(context) {
	
    init{}}Copy the code

Create a class

Java write

public class Person {

    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName(a) {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge(a) {
        return age;
    }

    public void setAge(int age) {
        this.age = age; }}Copy the code

Person person = new Person(Android Wheels.100);
person.setName("HJQ");
person.setAge(50);
System.out.println("name: " + person.getName() + ", age: " + person.getAge());
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(If you don’t want to expose the set method of a member variable, you can change var to val)

class Person {
	
    var name: String? = null
    get() = field
    set(value) {field = value}
	
    var age: Int = 0
    get() = field
    set(value) {field = value}
}
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// We can write it more succinctly
class Person(var name: String, var age: Int)
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var person = Person(Android Wheels.100)
person.name = "HJQ"
person.age = 50
println("name: {$person.name}, age: {$person.age}")
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Privatized set method

Java write

public class Person {

    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName(a) {
        return name;
    }

    private void setName(String name) {
        this.name = name;
    }

    public int getAge(a) {
        return age;
    }

    private void setAge(int age) {
        this.age = age; }}Copy the code

Kotlin spelled

class Person {
	
    var name: String? = null
    private set
	
    var age: Int = 0
    private set
}
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Privatized GET method

Java write

public class Person {

    String name;
    int age;
	
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
	
    private String getName(a) {
        return name;
    }
	
    private void setName(String name) {
        this.name = name;
    }
	
    private int getAge(a) {
        return age;
    }
	
    private void setAge(int age) {
        this.age = age; }}Copy the code

Kotlin spelled

class Person {
	
    private var name: String? = null
	
    private var age: Int = 0
}
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The enumeration

Java write

enum Sex {

    MAN(1), WOMAN(2);

    Sex(int type) {}
}
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Kotlin spelled

enum class Sex (var type: Int) {
	
    MAN(1), WOMAN(2)}Copy the code

interface

Java write

public interface Callback {

    void onSuccess(a);
    
    // New in JDK 8, interface methods support the default implementation
    default void onFail(a) {}}Copy the code

Kotlin spelled

interface Callback {

    fun onSuccess(a)
    
    fun onFail(a){}}Copy the code

Anonymous inner class

Java write

post(new Runnable() {
    
    @Override
    public void run(a) {}});Copy the code

Kotlin spelled

post(object : Runnable {

    override fun run(a){}})Copy the code

The inner class

Java write

public class MainActivity extends AppCompatActivity {

	public class MyTask {}}Copy the code

Kotlin spelled

class MainActivity : AppCompatActivity() {

	inner class MyTask {}}Copy the code

The inner class accesses a variable of the same name as the outer class

Java write

String name = Android Wheels;

public class MyTask {

    String name = "HJQ";

    public void show(a) {
        System.out.println(name + "-" + MainActivity.this.name); }}Copy the code

Kotlin spelled

var name = Android Wheels

inner class MyTask {

    var name = "HJQ"

    fun show(a) {
        println(name + "-" + this@MainActivity.name)
    }
}
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An abstract class

Java write

public abstract class BaseActivity extends AppCompatActivity implements Runnable {

    abstract void init(a);
}
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Kotlin spelled

abstract class BaseActivity : AppCompatActivity(), Runnable {
	
    abstract fun init(a)
}
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Static variables and methods

Java write

public class ToastUtils {

    public static Toast toast;

    public static void show(a) { toast.show(); }}Copy the code

Kotlin’s way of writing it (Kotlin called it an associated object)

companion object ToastUtils {

    var toast: Toast? = null

    fun show(a){ toast? .show() } }Copy the code

Variable parameter

Java write

public int add(int. array) {
    int count = 0;
    for (int i: array) {
        count += i;
    }
    return count;
}
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Kotlin spelled

fun add(vararg array: Int): Int {
    var count = 0
    //for (i in array) {
    // count += i
    / /}
    array.forEach {
        count += it
    }
    return count
}
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The generic

Java write

public class Bean<T extends String> {

    T data;
    
    public Bean(T t) {
        this.data = t; }}Copy the code

Bean<String> bean = new Bean<>("666666");
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Kotlin spelled

class Bean<T : Comparable<String>>(t: T) {
    var data = t
}
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var bean = Bean<String>("666666")
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// We can write it more succinctly
var bean = Bean("666666")
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Construct code block

Java write

public class MainActivity extends AppCompatActivity {
	
    int number;
	
    {
        number = 1; }}Copy the code

Kotlin spelled

class MainActivity : AppCompatActivity() {
	
    var number = 0
	
    init {
        number = 1}}Copy the code

Static code block

Java write

public class MainActivity extends AppCompatActivity {
	
    static int number;
	
    static {
        number = 1; }}Copy the code

Kotlin spelled

class MainActivity : AppCompatActivity() {
	
    companion object {
        
        var number = 0
        
        init {
            number = 1}}}Copy the code

Method code block

Java write

void test(a){{int a = 1; }}Copy the code

Kotlin spelled

fun test(a) {
    run {
        var a =1}}Copy the code

Ternary operator

Java write

int c = a > b ? a : b;
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Kotlin spelled

var c = if (a > b) a else b
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Permission modifier

Java (default isdefault)

The modifier	role
public	All classes visible
protected	Subclasses visible
default	Classes under the same package are visible
private	Only visible to its own class

Kotlin (defaultpublic)

The modifier	role
public	All classes visible
internal	Visible as classes under Module
protected	Subclasses visible
private	Only visible to its own class

Suppression code warning

Java write

@SuppressWarnings("unused")

@SuppressLint("ClickableViewAccessibility")
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Kotlin spelled

@Suppress("unused")

@Suppress("ClickableViewAccessibility")
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Annotation processor

Java write

annotationProcessor 'com. Making. Bumptech. Glide: the compiler: 4.12.0'
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Kotlin spelled

apply plugin : 'kotlin-kapt'
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kapt 'com. Making. Bumptech. Glide: the compiler: 4.12.0'
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If Kotlin still uses annotationProcessor to integrate annotation processors, errors will be reported and compilation will fail

This article has been authorized to hongyang wechat public account reprint

As Kotlin has become more and more popular, learning Kotlin has become a rite of passage

Code is the best teacher

directory

Integrated Kotlin

The new keyword

This keyword

The class keyword

The extends keyword

The volatile keyword

The synchronized keyword

The final keyword

variable

Static constants

Define methods

Overloaded methods

Basic data types

Comparison of type

Comparison of object

Conversion operators

An array of

cycle

Angle of the loop

Senior cycle

Judgment is

The constructor

Create a class

Privatized set method

Privatized GET method

The enumeration

interface

Anonymous inner class

The inner class

The inner class accesses a variable of the same name as the outer class

An abstract class

Static variables and methods

Variable parameter

The generic

Construct code block

Static code block

Method code block

Ternary operator

Permission modifier

Suppression code warning

Annotation processor

Next up:Kotlin: A whole new game you haven’t played yet

In addition, I recommend an open source project written in Kotlin language, you can have a look:AndroidProject-Kotlin

Android technology to share QQ group: 78797078

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