An article to help Java developers quickly learn Kotlin

Today, one of my friends came to ask Xiao Hui, saying that he used JAVA as a thief. He wanted to learn Kotlin again.

How can I put it? In fact, I have always felt that the language aspect is familiar, since JAVA has been stolen, I must learn other languages must be more effective with less effort.

However, if there is a good assistant, it will certainly be easier to learn, today xiao Grey and you write a comparison show Java and Kotlin two languages in the grammar of different places of the article, can quickly help Java partners master Kotlin writing skills! So without further ado, I’ll go ahead and show you the difference in the most practical code.

Print log

• Java

System.out.print("I gray");
System.out.println("I gray");
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• Kotlin

print("I gray")
println("I gray")
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---

Constants and variables

• Java

String name = "I gray";
final String name = "I gray";
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• Kotlin

var name = "I gray"
val name = "I gray"
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A null statement

• Java

String otherName;
otherName = null;
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• Kotlin

var otherName : String?
otherName = null
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Empty judgment

• Java

if(text ! =null) {
    int length = text.length();
}
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• Kotlin

text? .let {val length = text.length
}
/ / or
vallength = text? .lengthCopy the code

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String splicing

• Java

String firstName = "Small";
String lastName = "Grey";
String message = "Name:" + firstName + "" + lastName;
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• Kotlin

val firstName = "Small"
val lastName = "Grey"
val message = "Name:$firstName $lastName"
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A newline

• Java

String text = "A line of \ n" +
              "Two lines \ n" +
              "Three lines";
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• Kotlin

val text = "" "lines | 2 | 3 | line "" ".trimMargin()
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Ternary expression

• Java

String text = x > 5 ? "x > 5" : "x <= 5";
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• Kotlin

val text = if (x > 5)
              "x > 5"
           else "x <= 5"
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The operator

• java

final int andResult  = a & b;
final int orResult   = a | b;
final int xorResult  = a ^ b;
final int rightShift = a >> 2;
final int leftShift  = a << 2;
final int unsignedRightShift = a >>> 2;
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• Kotlin

val andResult  = a and b
val orResult   = a or b
val xorResult  = a xor b
val rightShift = a shr 2
val leftShift  = a shl 2
val unsignedRightShift = a ushr 2
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Type determination and conversion (declarative)

• Java

if (object instanceof Car) {
}
Car car = (Car) object;
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• Kotlin

if (object is Car) {
}
var car = object as Car
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Type judgment and conversion (implicit)

• Java

if (object instanceof Car) {
   Car car = (Car) object;
}
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• Kotlin

if (object is Car) {
   var car = object // Intelligent conversion
}
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Multiple conditions

• Java

if (score >= 0 && score <= 300) {}Copy the code

• Kotlin

if (score in 0.300.) {}Copy the code

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More flexible case statements

• Java

int score = // Some scores;
String grade;
switch (score) {
    case 10:
    case 9:
        grade = "Good";
        break;
    case 8:
    case 7:
    case 6:
        grade = "Good";
        break;
    case 5:
    case 4:
        grade = "Good";
        break;
    case 3:
    case 2:
    case 1:
        grade = "Poor";
        break;
    default:
        grade = "Poor";
}
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• Kotlin

var score = // Some scores;
var grade = when (score) {
    9.10 -> "Good"
    in 6.8. -> "Good"
    4.5 -> ""
    in 1.3. -> "Poor"
    else -> "Poor"
}
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The for loop

• Java

for (int i = 1; i <= 10 ; i++) { }

for (int i = 1; i < 10 ; i++) { }

for (int i = 10; i >= 0 ; i--) { }

for (int i = 1; i <= 10 ; i+=2) {}for (int i = 10; i >= 0 ; i-=2) {}for (String item : collection) { }

for (Map.Entry<String, String> entry: map.entrySet()) { }
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• Kotlin

for (i in 1.10.) {}for (i in 1 until 10) {}for (i in 10 downTo 0) {}for (i in 1.10. step 2) {}for (i in 10 downTo 0 step 2) {}for (item in collection) { }

for ((key, value) in map) { }
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More convenient collection operation

• Java

final List<Integer> listOfNumber = Arrays.asList(1.2.3.4);

final Map<Integer, String> keyValue = new HashMap<Integer, String>();
map.put(1."i");
map.put(2."Small");
map.put(3."Grey");

// Java 9
final List<Integer> listOfNumber = List.of(1.2.3.4);

final Map<Integer, String> keyValue = Map.of(1."i".2."Small".3."Grey");
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• Kotlin

val listOfNumber = listOf(1.2.3.4)
val keyValue = mapOf(1 to "i".2 to "Small".3 to "Grey")
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traverse

• Java

Java 7 and below
for (Car car : cars) {
  System.out.println(car.speed);
}

// Java 8+
cars.forEach(car -> System.out.println(car.speed));

Java 7 and below
for (Car car : cars) {
  if (car.speed > 100) { System.out.println(car.speed); }}// Java 8+
cars.stream().filter(car -> car.speed > 100).forEach(car -> System.out.println(car.speed));
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• Kotlin

cars.forEach {
    println(it.speed)
}

cars.filter { it.speed > 100 }
      .forEach { println(it.speed)}
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Method definition

• Java

void doSomething(a) {
   / / logic...
}

void doSomething(int. numbers) {
   / / logic...
}
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• Kotlin

fun doSomething(a) {
   / / logic...
}

fun doSomething(vararg numbers: Int) {
   / / logic...
}
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A method with a return value

• Java

int getScore(a) {
   / / logic...
   return score;
}
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• Kotlin

fun getScore(a): Int {
   / / logic...
   return score
}

// as a single expression function

fun getScore(a): Int = score
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Termineless symbol

• Java

int getScore(int value) {
   / / logic...
    return 2 * value;
}
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• Kotlin

fun getScore(value: Int): Int {
   / / logic...
   return 2 * value
}

// as a single expression function

fun getScore(value: Int): Int = 2 * value
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The constructor constructor

• Java

public class Utils {

    private Utils(a) { 
      // This class cannot be instantiated publicly
    }
    
    public static int getScore(int value) {
        return 2* value; }}Copy the code

• Kotlin

class Utils private constructor() {

    companion object {
    
        fun getScore(value: Int): Int {
            return 2 * value
        }
        
    }
}

// Another way

object Utils {

    fun getScore(value: Int): Int {
        return 2 * value
    }

}
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Get Set constructor

• Java

public class Developer {

    private String name;
    private int age;

    public Developer(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;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null|| getClass() ! = o.getClass())return false;

        Developer developer = (Developer) o;

        if(age ! = developer.age)return false;
        returnname ! =null ? name.equals(developer.name) : developer.name == null;

    }

    @Override
    public int hashCode(a) {
        intresult = name ! =null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }

    @Override
    public String toString(a) {
        return "Developer{" +
                "name='" + name + '\' ' +
                ", age=" + age +
                '} '; }}Copy the code

• Kotlin

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

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Prototype extension

• Java

public class Utils {

    private Utils(a) { 
      // This class cannot be instantiated publicly
    }
    
    public static int triple(int value) {
        return 3* value; }}int result = Utils.triple(3);

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• Kotlin

fun Int.triple(a): Int {
  return this * 3
}

var result = 3.triple()
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• Java

public enum Direction {
        NORTH(1),
        SOUTH(2),
        WEST(3),
        EAST(4);

        int direction;

        Direction(int direction) {
            this.direction = direction;
        }

        public int getDirection(a) {
            returndirection; }}Copy the code

• Kotlin

enum class Direction(val direction: Int) {
    NORTH(1),
    SOUTH(2),
    WEST(3),
    EAST(4);
}
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