kotlin之对比学习

常量与变量

java

String name = "Pan Zhou";
final String name = "Pan Zhou";

kotlin

var name = "Pan Zhou"
val name = "Pan Zhou"

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
}
// 或者
val length = text?.length

字符串拼接

java

String a ="a";
String b = "b";
String message = a+b;

kotlin

var a = "a";
var b = "b";
var message = $a $b

三元运算符

java

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

kotlin

var num = if(x>5)
            "x>5"
       else "x<5"

操作符

java

final int andResult  = a & b;
final int orResult   = a | b;

kotlin

val andResult  = a and b
val orResult   = a or b

多重条件

java

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

kotlin

if (score in 0..300) { }

Case语句

java

int score = // some 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) { }

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) { }

遍历

java

for (Car car : cars) {
  if (car.speed > 100) {
    System.out.println(car.speed);
  }
}

kotlin

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

方法定义

java

void doSomething() {
   // logic here
}

void doSomething(int... numbers) {
   // logic here
}

kotlin

fun doSomething() {
   // logic here
}

fun doSomething(vararg numbers: Int) {
   // logic here
}

带返回值的方法

java

int getScore() {
   // logic here
   return score;
}

kotlin

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

//动态添加数组
 val emptyArr = arrayOfNulls<String>(categoryList.size)
                for (i in categoryList.indices){
                    emptyArr[i] = categoryList[i].name
                }