Java中C＃'var'关键字的等价物是什么？ [英] What is the equivalent of the C# 'var' keyword in Java?

问题描述

在C＃中使用 var 关键字是隐式类型声明。 var 的Java等效语法是什么？

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

推荐答案

没有。唉，你必须输入完整的类型名称。

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

编辑：发布后7年，输入局部变量的推断（带 var <在Java 10中添加了/ code>。

7 years after being posted, type inference for local variables (with var) was added in Java 10.

编辑：发布后6年，从下面收集一些评论：

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

• C＃具有 var 关键字的原因是因为可能有类型没有.NET中的名称。例如：

• 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" };

在这种情况下，无法为 myData提供正确的类型。 6年前，这在Java中是不可能的（所有类型都有名字，即使它们非常冗长和不适合）。我不知道这是否在平均时间内发生了变化。

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 与以下内容不同动态。 var iables仍然是100％静态类型。这不会编译：

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

var myString = "foo";
myString = 3;

var 在以下情况下也很有用从上下文来看，这种类型很明显例如：

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

var currentUser = User.GetCurrent();

我可以说在我负责的任何代码中， currentUser 中包含用户或派生类。显然，如果你的 User.GetCurrent 的实现返回一个int，那么这可能对你不利。

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.

这与 var 无关，但如果你有奇怪的继承层次结构，你可以用其他方法隐藏方法（例如 new public void DoAThing （）），不要忘记非虚拟方法会受到类型转换的影响。

This has nothing to do with var, but if you have weird inheritance hierarchies 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.

我无法想象一个现实世界的场景，这表明设计很好，但这可能无法按预期工作：

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

如上所示，虚拟方法不受此影响。

As indicated, virtual methods are not affected by this.

不，在没有实际变量的情况下初始化 var 没有非笨拙的方法。

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

在这种情况下，就这样做吧老式的方式：

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

object foo6;

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