Kotlin Java快速对照

于 2019-05-08 10:18:00 发布
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文章标签： java 移动开发 开发工具
原文链接：https://my.oschina.net/komodo/blog/3046876
此博客为转载内容，原链接为https://my.oschina.net/komodo/blog/3046876 ，涉及Java和移动开发领域，还关联开发工具方面。

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val user = getUser()
final User user = getUser();
 
var user = getUser()
User user = getUser();
 
val user: User = getUser()
final User user = getuser();
 
val a = "$user is ${user.age}"
StringBuilder _builder = new StringBuilder();
_builder.append(user);
_builder.append( " is " );
_builder.append(user.getAge());
final String a = _builder.toString()
 
val a = b ?: ""
final String a = b == null ? "" : b;
 
val a = b ?: throw BusinessException()
if (b == null ) {
     throw new BusinessException();
}
final String a = b;
 
val a = getB() ?: ""
final String _tmp = getB();
final a = _tmp == null ? "" : _tmp;
 
val a = getB()?.getC()?.getD() ?: ""
String _tmp;
final B _b = getB();
if (_b != null ) {
    final C _c = _b.getC();
    if (_c != null ) {
        final String _d = _c.getD();
        if (_d != null ) {
           _tmp = _d;
        } else {
           _tmp = "" ;
    } else {
        _tmp = "" ;
    }
} else {
    _tmp = "" ;
}
 
final String a = _tmp;
 
class User
public final class User {}
 
class User(age: Int, name: String) {
     init {
         println( "$name is $age" )
     }
}
public final class User {
     public User(Integer age, String name) {
         StringBuilder _builder = new StringBuilder();
         _builder.append(name);
         _builder.append( " is " );
         _builder.append(age);
         System.out.println(_builder.toString());
     }
}
 
class User(val age: Int?, open var name: String?)
//没有new
val user = User( 18 , "hello" )
println(user.age)
user.name = "bike"
public final class User {
     private Integer age;
     private String name;
 
     public final User(Integer age, String name) {
         this .age = age;
         this .name = name;
     }
 
     public final Integer getAge() {
         return this .age;
     }
 
     public String getName() {
         return this .name;
     }
 
     public void setName(String name) {
        this .name = name;
     }
}
final User user = new User( 18 , "hello" );
System.out.println(user.getAge());
user.setName( "bike" );

var有setter，val没有。

没有open的setter getter都是 final。

open class User(var age: Int)
public class User {
     private Integer age;
     public class User(Integer age) {
         checkParameterIsNotNull(age, "age" );
         this .age = age;
     }
      
     public Integer getAge() {
         return this .age;
     }
     public void setAge(Integer age) {
         checkParameterIsNotNull(age, "age" );
         this .age = age;
     }
}
自动插入校验空的代码
open class User {
     val age: Int? = 0
     var name: String? = null
}
public class User {
     private Integer age = 0 ;
     private String name = null ;
 
     public final Integer getAge() {
         return this .age;
     }
 
     public final String getName() {
         return this .name;
     }
 
     public final void setName(String name) {
        this .name = name;
     }
}
 
class A(a: String?): B(a), C
public final class A extends B implements C {
     public A (String a) {
         B(a);
     }
}
继承，实现，继承时必须指定构造函数链。
interface B
public interface B {}
 
open class A<T>
public class A<T> {}
 
open class A<T: B>
public class A<T extends B> {}
 
open class A<T> where T: B1, T: B2
public class A<T extends B1 & B2>
 
fun test(age: Int?) : String? {
     return "OK"
}

 

public final String test(Integer age) {
     return "OK" ;
}
 
fun test(age: Int) = "OK"
public final String test(Integer age) {
     checkParameterIsNotNull(age, "age" );
     return "OK" ;
}
 
test( 10 )
test( 10 );
 
test(age = 10 )
test( 10 );
 
fun test(age: Int?, name: String? = "name" ) {}
public final test(Integer age) {
    test(age, "name" );
}
public final test(Integer age, String name) {
}
 
val a = if (getValue()) "OK" else "NO"
final String a = getValue() ? "OK" : "NO" ;
 
for (a in list) {
}
for (T a : list) {
}
a的类型自动推断了
for (i in 1 .. 3 ) {}
for ( int i = 1 ; i < 3 ; i++) {}
 
when (a) {
     is TypeA => println( "TypeA" )
     3 => println( "3" )
     test(a) => println( "test" )
     else => println( "else" )
}
if (a instanceof TypeA) {
     System.out.println( "TypeA" );
} else if (a.equals( 3 )) {
     System.out.println( "3" );
} else if (test(a)) {
     System.out.println( "test" );
} else {
     System.out.println( "else" );
}
 
val a = when (b) {
     is TypeA => 1
     3 => 2
     else => 3
}
Integer _tmp = null ;
if (a instanceof TypeA) {
     _tmp = 1 ;
} else if (a.equals( 3 )) {
     _tmp = 2 ;
} else {
    _tmp = 3 ;
}
 
final Integer a = _tmp;
 
object A {
     var name: String? = null
     fun test(a: String?) = a
}
public final class A {
     private static String name;
 
     public static final A INSTANCE;
 
     private A {}
 
     static {
          INSTANCE = new A();
     }
     
     public final void setName(String name) {
          A.name = name;
     }
 
     public final String getName() {
          return A.name;
     }
     
     public static final String test(String a) {
          return a;
     }
}
A类不能有实例变量。
val maxFun = {a: Int, b: Int -> if (a > b) a else b}
interface _L {
     Integer _f(Integer a, Integer b);
}
 
final _L maxFun = (Integer a, Integer b) -> a > b ? a : b
_L _f等是编译器自动生成的。
fun test(f: (Int, Int) -> Int): Int {
     return f( 1 , 2 )
}
 
test({a, b -> a + b})
//最后一个参数是闭包，该参数可以移动到小括号后
test() {a, b -> a + b}
//仅有一个闭包参数，小括号也可以省略
test {a, b -> a + b}
interface _L {
     Integer _f(Integer a, Integer b);
}
 
Integer test(_L f) {
     return f._f( 1 , 2 );
}
 
test((Integer a, Integer b) -> a + b);
test((Integer a, Integer b) -> a + b);
test((Integer a, Integer b) -> a + b);
 
fun test(f: (Int) -> Int): Int {
     return f( 1 );
}
//闭包只有一个参数时，可用it代表
test { it + 2 }
interface _L {
     Integer _f(Integer a);
}
 
Integer test(_L f) {
     return f._f( 1 );
}
 
test((Integer a) -> a + 2 );
 
fun String.isNullOrEmpty(): Boolean {
     return this == null || this .length() == 0 ;
}
 
"aString" .isNullOrEmpty()
public static Boolean isNullOrEmpty(String _this) {
     return _this == null || _this.length() == 0 ;
}
 
isNullOrEmpty( "aString" )
DSL，或改进易读性用
infix fun Int.add(other: Int) = this + other
 
val a = 3 add 4
public static Integer add(Integer _this, Integer other) {
     return _this + other;
}
 
final Integer a = add( 3 , 4 );
DSL用，一般不用
//该函数会在编译器尾递归优化
tailrec fun recursive(a: Int) : Int {
     return if (a == 0 ) 0 else recursive(a--)
}
  
//该函数会在编译期内联
inline fun isEven(a: Int) = a % 2 == 0
  
fun a() {
     fun b() {}
     b()
}
public void a() {
     class _B {
         static void b() {}
     }
     
     _B.b();
}
 

 

转载于:https://my.oschina.net/komodo/blog/3046876

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