静态的: Obj.someMethod

当你想要提供对方法的类级访问时，使用static，即在没有类实例的情况下，方法应该是可调用的。

静态方法和变量是Java中“全局”函数和变量的受控版本。其中方法可以作为classname.methodName()或classInstanceName.methodName()访问，即静态方法和变量可以使用类名以及类的实例访问。

类不能被声明为静态的(因为它没有意义。如果一个类被声明为public，它可以从任何地方访问)，内部类可以被声明为static。

使用静态方法有一些合理的理由:

Performance: if you want some code to be run, and don't want to instantiate an extra object to do so, shove it into a static method. The JVM also can optimize static methods a lot (I think I've once read James Gosling declaring that you don't need custom instructions in the JVM, since static methods will be just as fast, but couldn't find the source - thus it could be completely false). Yes, it is micro-optimization, and probably unneeded. And we programmers never do unneeded things just because they are cool, right? Practicality: instead of calling new Util().method(arg), call Util.method(arg), or method(arg) with static imports. Easier, shorter. Adding methods: you really wanted the class String to have a removeSpecialChars() instance method, but it's not there (and it shouldn't, since your project's special characters may be different from the other project's), and you can't add it (since Java is somewhat sane), so you create an utility class, and call removeSpecialChars(s) instead of s.removeSpecialChars(). Sweet. Purity: taking some precautions, your static method will be a pure function, that is, the only thing it depends on is its parameters. Data in, data out. This is easier to read and debug, since you don't have inheritance quirks to worry about. You can do it with instance methods too, but the compiler will help you a little more with static methods (by not allowing references to instance attributes, overriding methods, etc.).

如果你想创建一个单例，你还必须创建一个静态方法，但是…不喜欢。我的意思是，三思而后行。

更重要的是，为什么不想创建静态方法?基本上，多态被抛在了脑后。你不能重写这个方法，也不能在接口中声明它(在java 8之前)。这给你的设计带来了很大的灵活性。此外，如果您需要状态，如果您不小心，您将以大量并发错误和/或瓶颈告终。

静态方法是Java中可以在不创建类对象的情况下调用的方法。它属于这个阶级。

当我们不需要使用实例调用方法时，我们使用静态方法。

不，静态方法不与实例关联;他们属于这个阶层。静态方法是第二个例子;实例方法是第一个。

可以使用静态方法，如果

One does not want to perform an action on an instance (utility methods) As mentioned in few of above answers in this post, converting miles to kilometers, or calculating temperature from Fahrenheit to Celsius and vice-versa. With these examples using static method, it does not need to instantiate whole new object in heap memory. Consider below 1. new ABCClass(double farenheit).convertFarenheitToCelcium() 2. ABCClass.convertFarenheitToCelcium(double farenheit) the former creates a new class footprint for every method invoke, Performance, Practical. Examples are Math and Apache-Commons library StringUtils class below: Math.random() Math.sqrt(double) Math.min(int, int) StringUtils.isEmpty(String) StringUtils.isBlank(String) One wants to use as a simple function. Inputs are explictly passed, and getting the result data as return value. Inheritence, object instanciation does not come into picture. Concise, Readable.

注意: 很少有人反对静态方法的可测试性，但是静态方法也可以被测试!使用jMockit，可以模拟静态方法。可测试性。在下面的例子:

new MockUp<ClassName>() {
    @Mock
    public int doSomething(Input input1, Input input2){
        return returnValue;
    }
};