随着Kotlin越来越火，最近开始学习Kotlin，感觉Java和区别还是很大的，这里列举一下Kotlin和Java的一些区别；

1.打印日志

Java
System.out.print("hello world"); 
System.out.println("hello world");

kotlin
print("hello world")
println("hello world")

2.定义变量

可变变量 String name = "xxxx";
不可变变量 final String name = "aaaa";

kotlin
可变变量 var name= "xxxx"
不可变变量val name = "aaaaa"

3.null声明

Java
String name;
name = null;

kotlin
var name: String?
name = null

4.空判断

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

kotlin
text?.let{
val length = text.length
}
或者
val length = text?.length

5.字符串拼接

Java
String firstName = "apple";
Stirng lastName = "enginner";
String message = "My name is:"+firstName + lastName;

kotlin
val firstName= "apple"
val lastName="enginner"
val message = "My name is：$firstName $lastName"

6.三元表达式

Java
String text = x > 5 ? "x > 5" : "x <= 5";
kotlin
val text = if(x > 5)
" x > 5"
else
 "x <= 5"

7.多重表达式

Java
if(score >= 0 && score <= 300){}
Kotlin
if(score in 0..300){}

8.更灵活的case语句

Java
int score = 100;
String grade;
switch(score){
 case 1:
 ...
 case 100:
    grade="100";
    break;
 default:
   break;
}

kotlin
var score = 100;
var grade = when(score){
 9,10 -> "excellent"
 in 6..8 -> "Good"
4,5->"ok"
in 1..3->"fail"
else -> "fail"
}

9.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(item in collection){}
for((key,value) in map){}

10.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,"Android");
map.put(2,"Ali");

kotlin
val listOfNumber = listOf(1,2,3,4)
val keyvalue = mapof(1 to "Android",
                                    2 to "Ali",
                                   3 to "Mindorks")

11.遍历

Java
for (String s: stringList) {
            System.out.println(s);
        }
 stringList.forEach(s -> System.out.println(s));

kotlin
fun A(stringList: List<String>) {
    stringList.forEach { s -> println(s) }
}

12.方法定义

Java
void doSomeThing(){}
void doSomeThing(int a,int b){}

kotlin
fun doSomeThing(){}
fun doSomeThing(a : I}nt,b : Int){

14.constructor构造器

Java
public class A{
private A(){}
public static int getScore(int value){
  return 2*value;
}
}

kotlin
class A private constructor(){
    companion object{
        fun getScore(int: Int):Int{
            return 2*int
        }
    }
}

13.Get Set构造器

Java
public class Teacher {
    private String name;
    private int age;

    public Teacher(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;
    }
}

kotlin

data class Student(val name:String,val  age : Int) 
fun main() {
    var stu = Student("XiaoMin",24)
    println(stu.name)
    println(stu.age)
}

15.操作符

Java
final int and = a&b;
final int or = a | b;
final int xor = a ^b;
final int right = a >>2;
final int left = a << 2;
final int unsingedRightShift = a >> 2;
kotlin
val and = a and b
val or = a or b
val xor = a xor b
val rightShift = a shr 2
val leftShift = a shl 2
val unsignedRightShift  = a ushr 2