Kotlin grammar learning (a)

Basic grammar

Defined Functions

• java

public int method(int a,int b){
    // todo
    return a+b;
}
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• kotlin

fun method(a:Int,b:Int):Int{
    return a+b
}
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• In the kotlin everything is an object, the basic data types (int, long, boolean, double , short, float, byte) java be used in its corresponding package in kotlin class (Int, Long, Boolean, Double, Short, Float, Byte) . Note : 1, the number of types can not be installed for implicit in Kotlin (example: java in type long int) 2, when Kotlin function does not return a value, as Unit Int replaced, omitted or

Define the variable

• java

private final int a;
public int b=1;
String c;
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• kotlin

private val a:Int?=1
public val b=1
lateinit var c:String
private val d:Int by lazy { 1 }
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• In kotlin declared with var and val Variables and Constants
• Note: 1, under normal circumstances, when you declare a variable must be initialized 2, if you want to add variables need to be null after the type declaration '? '3, if we are not initialized when they are declared, you can add keywords before var lateinit (Val does not support lateinit) 4, val can be used by lazy key (initialization at its first call) at initialization time delay

And using null null detector

• java

String a=null;
if(a!=null){
    int size=a.length();
}
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• kotlin

var a:Strng?=null
var size=a?.length
var size2=a!!.length
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• When using the parameter may be empty in kotlin in need '? 'Determines whether the current parameter is empty, if empty'? After the logic 'will not be executed. You may also be used '!' To determine whether the air, but which parameter is directly empty null pointer exception is thrown.

Usage type detector and automatic conversion

• java

public String get(Object obj){
    if(obj instanceof String){
        String s=(String)obj;
        return s;
    }
    return null;
}
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• kotlin

fun get(obj:Any):String?{
    if(obj is String){
        return obj
    }
    return null
}
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• by kotlin is to determine whether the type of the front back type instance; if a local variable or non-variable properties has judged that a certain type, then the branch after the detection of this type can be used as directly, without displaying the conversion, For example: obj code above.
• Note: kotlin in Any java equivalent of Object is the parent of all classes.

Use a for loop

• java

public void test(List<Stirng> list){
    int size=list.size;
    for(int i=0;i<size;i++){
        System.out.println("list 第"+i+"个数据："+ list.get(i));
    }
}
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• kotlin

fun test(list:ArrayList<String>){
    for(item in list){
        println("list item:$item")
    }
    
    for(item in list.indices){
        println("list 第$item 个数据：${list[item]}")
    }
    
    for((index,item) in list.withIndex()){
            println("list 第$index 个数据为:$item")
        }
}
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• By kotlin in keyword through the data, the code above, the first for loop out directly in the data list; second for loop index list is taken out by the index data can be extracted; the third loop may be removed for simultaneously indexing and data traversal.

When using expressions

• java

public void test(int a){
    switch(a){
        case 1:
        // todo
            break;
        case 2:
        // todo
            break;
        default:
        // todo
            break;
    }
}
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• kotlin

fun test(a:Int){
    when(a){
        0,1->{
            //todo
        }
        2->{
            //todo
        }
        in 3..10->{
            //todo
        }
        else{
            //todo
        }
    }
}
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• when expressions of java kotlin in similar switch.
• When a plurality of conditions are to perform the same logic, it can be separated with commas;
• For the range of conditions can be used in keywords in in with the former '! 'Indicates negation;

(Range) range

• kotlin

fun test(a:Int){
    //检测一个数是否在某个范围
    if(a in 1..10){
        println(x)
    }
    
    //检测某个数是否在区间之外
    var list=listOf(2,3,4,5)
    if(-1 !in 0..list.lastIndex){
        println("-1 不在 list范围内!")
    }
    
    //区间迭代
    for(x in 1..10){
        println(x)
    }
    
    for(x in 1..10 step 2){
        prinlnt(x)
    }
    
    for(x in 9 downTo 0 step 3){
        println(x)
    }
    
}
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• kotlin section may be used in x..y expressed in the sense that the number x between y and x comprising x and y <y, if x> y using downto ;
• step x represents plus or minus per iteration number x;

Set of arrays

• statement

//声明空数组
var list= arrayOfNulls<Any>(2)
//声明有初始值的数组
var list1= arrayOf(1,2,3,4)

//声明List
var list=ArrayList<String>()
//声明List并且初始化 
var list3= listOf(1,2,3)

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Reproduced in: https: //juejin.im/post/5d06eb146fb9a07ed36ea985