打印日志
Java
System.out.print(
"Amit Shekhar");
System.out.println(
"Amit Shekhar");
Kotlin
print("Amit Shekhar")
println("Amit Shekhar")
常量与变量
Java
String name =
"Amit Shekhar";
final String name =
"Amit Shekhar";
Kotlin
var name =
"Amit Shekhar"
val name =
"Amit Shekhar"
null声明
Java
String otherName;
otherName =
null;
Kotlin
var otherName :
String?
otherName =
null
空判断
Java
if (text !=
null) {
int length = text.length();
}
Kotlin
text?.let {
val
length =
text.
length
}
val
length =
text?.
length
字符串拼接
Java
String firstName =
"Amit";
String lastName =
"Shekhar";
String message =
"My name is: " + firstName +
" " + lastName;
Kotlin
val firstName =
"Amit"
val lastName =
"Shekhar"
val message =
"My name is: $firstName $lastName"
换行
Java
String text =
"First Line\n" +
"Second Line\n" +
"Third Line";
Kotlin
val text =
"""
|First Line
|Second Line
|Third Line
""".trimMargin()
三元表达式
Java
String text = x >
5 ?
"x > 5" :
"x <= 5";
Kotlin
val text =
if (x >
5)
"x > 5"
else "x <= 5"
操作符
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
类型判断和转换 (声明式)
Java
if (object
instanceof Car) {
}
Car car = (Car) object;
Kotlin
if (
object is Car) {
}
var car =
object as Car
类型判断和转换 (隐式)
Java
if (object
instanceof Car) {
Car car = (Car) object;
}
Kotlin
if (
object is Car) {
var car =
object
}
多重条件
Java
if (score >=
0 && score <=
300) { }
Kotlin
if (score
in 0..
300) { }
更灵活的case语句
Java
int score =
String grade;
switch (score) {
case 10:
case 9:
grade =
"Excellent";
break;
case 8:
case 7:
case 6:
grade =
"Good";
break;
case 5:
case 4:
grade =
"Ok";
break;
case 3:
case 2:
case 1:
grade =
"Fail";
break;
default:
grade =
"Fail";
}
Kotlin
var score =
// some score
var grade =
when (score) {
9,
10 -> "Excellent"
in 6..8 -> "Good"
4,
5 -> "Ok"
in 1..3 -> "Fail"
else -> "Fail"
}
for循环
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()) { }
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 1 step 2)
for (item
in collection)
for ((key, value)
in map)
更方便的集合操作
Java
final List<Integer> listOfNumber = Arrays.asList(
1,
2,
3,
4);
final Map<Integer, String> keyValue =
new HashMap<Integer, String>();
map.put(
1,
"Amit");
map.put(
2,
"Ali");
map.put(
3,
"Mindorks");
final List<Integer> listOfNumber = List.of(
1,
2,
3,
4);
final Map<Integer, String> keyValue = Map.of(
1,
"Amit",
2,
"Ali",
3,
"Mindorks");
Kotlin
val listOfNumber = listOf(
1,
2,
3,
4)
val keyValue = mapOf(
1 to "Amit",
2 to "Ali",
3 to "Mindorks")
遍历
Java
for (Car car : cars) {
System.out.println(car.speed);
}
cars.forEach(car -> System.out.println(car.speed));
for (Car car : cars) {
if (car.speed >
100) {
System.out.println(car.speed);
}
}
cars.stream().filter(car -> car.speed >
100).forEach(car -> System.out.println(car.speed));
Kotlin
cars
.forEach {
println(it
.speed)
}
cars
.filter { it
.speed >
100 }
.forEach { println(it
.speed)}
方法定义
Java
void doSomething() {
}
void doSomething(
int... numbers) {
}
Kotlin
fun doSomething() {
}
fun doSomething(vararg numbers: Int) {
}
带返回值的方法
Java
int getScore() {
return score;
}
Kotlin
fun getScore(): Int {
return score
}
fun getScore(): Int = score
无结束符号
Java
int getScore(
int value) {
return 2 * value;
}
Kotlin
fun getScore(
value: Int): Int {
return 2 *
value
}
fun getScore(
value: Int): Int =
2 *
value
constructor 构造器
Java
public class Utils {
private Utils() {
}
public static int getScore(
int value) {
return 2 * value;
}
}
Kotlin
class Utils
private constructor()
}
}
// other way is also there
object Utils
}
Get Set 构造器
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() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
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;
return name !=
null ? name.equals(developer.name) : developer.name ==
null;
}
@Override
public int hashCode() {
int result = name !=
null ? name.hashCode() :
0;
result =
31 * result + age;
return result;
}
@Override
public String
toString() {
return "Developer{" +
"name='" + name +
'\'' +
", age=" + age +
'}';
}
}
Kotlin
data class Developer(val name: String, val age: Int)
Java
public class Utils {
private Utils() {
}
public static int triple(
int value) {
return 3 * value;
}
}
int result = Utils.triple(
3);
Kotlin
fun Int.triple(): Int {
return this *
3
}
var result =
3.triple()
你需要知道一些比较重要的事
Learn Kotlin - lateinit vs lazyLearn Kotlin - apply vs withLearn Kotlin - Data ClassLearn Kotlin - Destructuring DeclarationsLearn Kotlin - Extension FunctionsLearn Kotlin - Sealed Classes
