C＃中var关键字相当于java吗？ [英] C# var keyword equivalent in java?

问题描述

一个使用C＃中的 VAR 的关键字的是隐式类型声明。什么是Java语法等价的 VAR 的？

解决方案

有是没有的。唉，你必须键入了完整的类型名称

编辑被张贴，收集一些评论从下面经过6年：

• 原因C＃有 VAR 关键字，因为它可能有有没有名字类型。净。例如：

    VAR MYDATA的=新{A = 1，B =2}; 
    

  在这种情况下，这将是不可能的，得到适当的类型以 myData的。 6年前，这在Java中是不可能的（各类有名字，即使他们是非常冗长和unweil​​dy）。我不知道这是否在平均时间已经改变了。




• VAR 是不一样的动态。 VAR iables还是静态类型的100％。这不会编译：

    VAR的myString =富; 
  的myString = 3; 
    




• VAR 也时非常有用类型是从上下文显而易见。例如：

    VAR的currentUser = User.GetCurrent（）; 
    

  
  
  

  我可以说，在我负责的任何代码，的currentUser 有一个用户或派生类在里面。很显然，如果你的 User.GetCurrent 实施返回一个int，那么也许这是一个有损于你。




• 这无关与 VAR ，但如果你有，你的影子与其他方法的方法怪异继承heirarchies（如新公共无效DoAThing （）），不要忘记，非虚方法是由它们转换为类型的影响。

  
  
  

  我无法想象一个现实世界的场景，这表明良好的设计，但正如你想到这个可能不工作：

   类Foo {
  公共无效非（）{} 
  公共虚拟无效的Virt（）{} 
  } 
   
  类酒吧：富{
  公共新空间非（）{ } 
  公共覆盖无效的Virt（）{} 
  } 
   
  类巴兹{
  公共静态美孚的getFoo（）{
  返回新的酒吧（）; 
  } 
  } 
   
  无功富= Baz.GetFoo（）; 
   foo.Non（）; //&下;  -  Foo.Non，不Bar.Non 
   foo.Virt（）; //<  -  Bar.Virt 
   
   VAR栏=（酒吧）富; 
   bar.Non（）; //&下;  -  Bar.Non，不Foo.Non 
   bar.Virt（）; //&下;  - 静止Bar.Virt 
    

  
  
  

  正如所指出的，虚拟方法不受此<。 / p>




• 没有，没有初始化 VAR 没有实际的变量非笨拙的方式。

    VAR foo1 =酒吧; //好的
  变种foo2的; //错误的，什么类型的？ 
   VAR foo3 = NULL; //坏，空不具有类型
   VAR foo4 =默认值（VAR）; //什么？ 
   VAR foo5 =（对象）空; //合法的，但回家了，你醉了
    

  在这种情况下，只是做旧办法：

   对象foo6; 
    

One use of the var keyword in C# is implicit type declaration. What is the java equivalent syntax for var?

解决方案

There is none. Alas, you have to type out the full type name.

Edit 6 years after being posted, to collect some of the comments from below:

• The reason C# has the var keyword is because it's possible to have Types that have no name in .NET. Eg:

  var myData = new { a = 1, b = "2" };
  

  In this case, it would be impossible to give a proper type to myData. 6 years ago, this was impossible in Java (all Types had names, even if they were extremely verbose and unweildy). I do not know if this has changed in the mean time.

• var is not the same as dynamic. variables are still 100% statically typed. This will not compile:

  var myString = "foo";
  myString = 3;
  

• var is also useful when the type is obvious from context. For example:

  var currentUser = User.GetCurrent();
  

  I can say that in any code that I am responsible for, currentUser has a User or derived class in it. Obviously, if your implementation of User.GetCurrent return an int, then maybe this is a detriment to you.

• This has nothing to do with var, but if you have weird inheritance heirarchies where you shadow methods with other methods (eg new public void DoAThing()), don't forget that non-virtual methods are affected by the Type they are cast as.

  I can't imagine a real world scenario where this is indicative of good design, but this may not work as you expect:

  class Foo {
      public void Non() {}
      public virtual void Virt() {}
  }
  
  class Bar : Foo {
      public new void Non() {}
      public override void Virt() {}
  }
  
  class Baz {
      public static Foo GetFoo() {
          return new Bar();
      }
  }
  
  var foo = Baz.GetFoo();
  foo.Non();  // <- Foo.Non, not Bar.Non
  foo.Virt(); // <- Bar.Virt
  
  var bar = (Bar)foo;
  bar.Non();  // <- Bar.Non, not Foo.Non
  bar.Virt(); // <- Still Bar.Virt
  

  As indicated, virtual methods are not affected by this.

• No, there is no non-clumsy way to initialize a var without an actual variable.

  var foo1 = "bar";        //good
  var foo2;                //bad, what type?
  var foo3 = null;         //bad, null doesn't have a type
  var foo4 = default(var); //what?
  var foo5 = (object)null; //legal, but go home, you're drunk
  

  In this case, just do it the old fashioned way:

  object foo6;
  

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