@[toc]
在Java當中一些常量通常情況下都是定義在接口當中，默認情況下所有的成員變量都是public static final類型的，所有的方法都是public abstract類型的，而在Kotlin中的接口是不允許定義常量的，但是我們可以通過伴生對象companion object來解決這個問題。
數(shù)據(jù)實體類
- Java中的實現(xiàn)
public class NewsType {
public int id;
public String typename;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTypename() {
return typename;
}
public void setTypename(String typename) {
this.typename = typename;
}
- Kotlin中的實現(xiàn)
class NewsType {
var id: Int = 0
var typename: String? = null
var addtime: String? = null
}
接口變量
- Java中的定義：
public interface NewsDBConfig {
String DB_NAME = "fly_news.db";
int VERSION_CODE = 1;
interface NewsType {
String TABLE_NAME = "news_type";
String CLUMN_ID = "id";
String CLUMN_TYPE_NAME = "typename";
String CLUMN_ADD_TIME = "addtime";
String CREATE_TABLE = "create table " + TABLE_NAME + " ("+CLUMN_ID+" integer PRIMARY KEY autoincrement," + CLUMN_TYPE_NAME + " varchar(64),"+CLUMN_ADD_TIME+" datetime default (datetime('now','localtime')))";
}
}
- Kotlin中的定義
interface NewsDBConfig {
interface NewsType {
companion object {
val TABLE_NAME = "news_type"
val CLUMN_ID = "id"
val CLUMN_TYPE_NAME = "typename"
val CLUMN_ADD_TIME = "addtime"
val CREATE_TABLE = "create table $TABLE_NAME ($CLUMN_ID integer PRIMARY KEY autoincrement,$CLUMN_TYPE_NAME varchar(64),$CLUMN_ADD_TIME datetime default (datetime('now','localtime')))"
}
}
companion object {
val DB_NAME = "fly_news.db"
val VERSION_CODE = 1
}
}
懶漢式單例
- Java中的實現(xiàn)
public class NewsDBHelper extends SQLiteOpenHelper {
private static NewsDBHelper mNewsDBHelper;
public static NewsDBHelper getInstance(Context context) {
if (mNewsDBHelper == null) {
synchronized (NewsDBHelper.class) {
mNewsDBHelper = new NewsDBHelper(context);
}
}
return mNewsDBHelper;
}
private NewsDBHelper(Context context) {
super(context, NewsDBConfig.DB_NAME, null, NewsDBConfig.VERSION_CODE);
}
}
- Kotlin中的實現(xiàn)
class NewsDBHelper private constructor(context: Context) :
SQLiteOpenHelper(context, NewsDBConfig.DB_NAME, null, NewsDBConfig.VERSION_CODE) {
companion object {
private var mNewsDBHelper: NewsDBHelper? = null
fun getInstance(context: Context): NewsDBHelper? {
if (mNewsDBHelper == null) {
synchronized(NewsDBHelper::class.java) {
if (mNewsDBHelper == null) {
mNewsDBHelper = NewsDBHelper(context)
}
}
}
return mNewsDBHelper
}
}
}
構(gòu)造方法
情況1
- Java中的實現(xiàn)
public class MyView extends View {
public MyView(Context context) {
super(context);
}
public MyView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}
public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
}
- Kotlin中的實現(xiàn)
class MyView : View {
constructor(context: Context) : super(context) {}
constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {}
constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context,attrs,defStyleAttr) {}
}
情況2
- java中的實現(xiàn)
public class MyView1 extends View {
public MyView1(Context context) {
this(context,null);
}
public MyView1(Context context, @Nullable AttributeSet attrs) {
this(context, attrs,0);
}
public MyView1(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
}
- Kotlin中的實現(xiàn)
class MyView1 @JvmOverloads constructor(
context: Context,
attrs: AttributeSet? = null,
defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr)
或者
class MyView2 : View {
@JvmOverloads
constructor(
context: Context,
attrs: AttributeSet? = null,
defStyleAttr: Int = 0
) : super(context, attrs, defStyleAttr) { }
lambda的應用
無參數(shù)
- 常規(guī)寫法
fun print() {
println("HelloWorld")
}
- lambda標準寫法
var print1: () -> Unit = {println("HelloWorld1")}
- lambda簡寫
var print2 = {println("HelloWorld2")}
有參數(shù)加法
- 常規(guī)寫法
fun add(x: Int, y: Int): Int {
return x + y
}
- lambda標準寫法
var add1: (Int, Int) -> Int = { x, y -> x + y }
- lambda簡寫
var add2 = { x: Int, y: Int -> x + y }
作為函數(shù)參數(shù)
- 一般寫法
fun add3(x: Int, y: Int): Int {return x * y}
fun add4(a: Int, b: Int): Int {return a + b}
//調(diào)用
var result = add4(1, add3(2, 3))
- lambda寫法
fun add5(x:Int,add6:(Int,Int)->Int):Int{
return x + add6.invoke(2,3)
}
//調(diào)用
var result5 = add5(1,{x:Int,y:Int -> x*y})
- lambda寫法
fun add7(x:Int,add8:(Int)->Boolean):Int{
if(add8(x)){
return x
}else{
return 0
}
}
//調(diào)用1
var add81 = add7(10,{x:Int -> x > 0})
//調(diào)用2
var add82 = add7(10,{it > 0})
匿名函數(shù)
- 標準寫法
fun add(x: Int, y: Int): Int {
return x + y
}
或者
fun add1(x: Int, y: Int): Int = x + y
- 匿名寫法
var add2 = fun(x: Int, y: Int): Int {
return x + y
}
- 匿名寫法
var add3 = fun(x: Int, y: Int): Int = x + y
閉包
1.函數(shù)嵌套
fun test1() {
println("test1")
fun test2() {
println("test2")
}
}
2.函數(shù)作為返回值
fun test2(x: Int): () -> Int {
var a = 0
return fun(): Int {
a++
return a + x
}
}
//調(diào)用
var test = test2(10)
println(test())
println(test())
println(test())
3.函數(shù)作為返回值
fun test3(v: Int): (Int, Int) -> Int {
var a = 1
var b = 2
return { x, y -> x + y + a + b + v}
}
//調(diào)用
var test2 =test3(10)
var result = test2(12,1)
println(result)
