Java到Kotlin的一些变化-基础版本(适合有Java基础的查看)

一些不一样的规范

Kotlin不需要在语句末尾加;分号
Kotlin的变量都是要初始化的，而Java则不需要
Kotlin变量可以自动识别变量类型，类似于Python

1.Print打印方式

Java

System.out.print("Hello, World!");
System.out.println("Hello, World!");

Kotlin

print("Hello, World!")
println("Hello, World!")

2.Variables变量

Java

不可变变量
final int x;
final int y = 1;
可变变量
int x;
int y = 2;
x = 3;
y = 1;

Kotlin

不可变变量
val x: Int
val y = 1
可变变量
var x: Int
var y = 2
x = 3
y = 1

3.关于空值NULL

Java

final String name = null;

String lastName;
lastName = null

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

Kotlin

val name: String? = null

var lastName: String?
lastName = null

var firstName: String
firstName = null // 编译错误

val length = text?.length

val length = text!!.length // NullPointerException if text == null

4.String字符串

Java

String name = "John";
String lastName = "Smith";
String text = "My name is: " + name + " " + lastName;
String otherText = "My name is: " + name.substring(2);

String text = "First Line\n" +
              "Second Line\n" +
              "Third Line";

Kotlin

val name = "John"
val lastName = "Smith"
val text = "My name is: $name $lastName"
val otherText = "My name is: ${name.substring(2)}"

val text = """
        |First Line
        |Second Line
        |Third Line
""".trimMargin()

5.三元运算符

Java

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

Kotlin

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

6.位操作

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;

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

7.Is As In

Java

if(x instanceof Integer){ }

final String text = (String) other;

if(x >= 0 && x <= 10 ){}

Kotlin

这里的is指代判断
if (x is Int) { }

这里的as是类型转换
val text = other as String

if (x in 0..10) { }

8.智能转换

Java

if(a instanceof String){
  final String result = ((String) a).substring(1);
}

Kotlin

if (a is String) {
  val result = a.substring(1)
}

9.Switch / When

Java

final int x = // value;
final String xResult;

switch (x){
  case 0:
  case 11:
    xResult = "0 or 11";
    break;
  case 1:
  case 2:
    //...
  case 10:
    xResult = "from 1 to 10";
    break;
  default:
    if(x < 12 && x > 14) {
      xResult = "not from 12 to 14";
      break;
    }

    if(isOdd(x)) {
      xResult = "is odd";
      break;
    }

    xResult = "otherwise";
}



final int y = // value;
final String yResult;

if(isNegative(y)){
  yResult = "is Negative";
} else if(isZero(y)){
  yResult = "is Zero";
}else if(isOdd(y)){
  yResult = "is Odd";
}else {
  yResult = "otherwise";
}

Kotlin

val x = // value
val xResult = when (x) {
  0, 11 -> "0 or 11"
  in 1..10 -> "from 1 to 10"
  !in 12..14 -> "not from 12 to 14"
  else -> if (isOdd(x)) { "is odd" } else { "otherwise" }
}

val y = // value
val yResult = when {
  isNegative(y) -> "is Negative"
  isZero(y) -> "is Zero"
  isOdd(y) -> "is odd"
  else -> "otherwise"
}

10.For

Java

for (int i = 1; i < 11 ; i++) { }

for (int i = 1; i < 11 ; i+=2) { }

for (String item : collection) { }

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

Kotlin

for (i in 1 until 11) { }

for (i in 1..10 step 2) {}

for (item in collection) {}
for ((index, item) in collection.withIndex()) {}

for ((key, value) in map) {}

11.集合(List，Map)

Java

final List<Integer> numbers = Arrays.asList(1, 2, 3);

final Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");


// Java 9
final List<Integer> numbers = List.of(1, 2, 3);

final Map<Integer, String> map = Map.of(1, "One",
                                        2, "Two",
                                        3, "Three");


for (int number : numbers) {
  System.out.println(number);
}

for (int number : numbers) {
  if(number > 5) {
    System.out.println(number);
  }
}



final Map<String, List<Integer>> groups = new HashMap<>();
for (int number : numbers) {
  if((number & 1) == 0){
    if(!groups.containsKey("even")){
      groups.put("even", new ArrayList<>());
    }

    groups.get("even").add(number);
    continue;
  }

  if(!groups.containsKey("odd")){
    groups.put("odd", new ArrayList<>());
  }

  groups.get("odd").add(number);
}

// or

Map<String, List<Integer>> groups = items.stream().collect(
  Collectors.groupingBy(item -> (item & 1) == 0 ? "even" : "odd")
);


final List<Integer> evens = new ArrayList<>();
final List<Integer> odds = new ArrayList<>();
for (int number : numbers){
  if ((number & 1) == 0) {
    evens.add(number);
  }else {
    odds.add(number);
  }
}


final List<User> users = getUsers();

Collections.sort(users, new Comparator<User>(){
  public int compare(User user, User otherUser){
    return user.lastname.compareTo(otherUser.lastname);
  }
});

// or

users.sort(Comparator.comparing(user -> user.lastname));

Kotlin

val numbers = listOf(1, 2, 3)

val map = mapOf(1 to "One",
                2 to "Two",
                3 to "Three")



numbers.forEach {
    println(it)
}

numbers.filter  { it > 5 }
       .forEach { println(it) }


val groups = numbers.groupBy {
                if (it and 1 == 0) "even" else "odd"
             }


val (evens, odds) = numbers.partition { it and 1 == 0 }


val users = getUsers()
users.sortedBy { it.lastname }