使用Java中的对象调用静态方法会出现什么问题？

Question

If I have the following:

class A {
  public A() { }
  public static void foo() { System.out.println("foo() called"); }
}

public class Main {
  public static void main(String [] args) {
    A a = new A();
    a.foo(); // <-- static call using an instance.
    A.foo(); // <-- static call using class
  }
}

Are there any problems that may arise from calling foo() using an instance? Does the JVM treat the first call to foo() exactly as a static method, or is there some technical subtlety?

Answer 1

Its very easy to introduce subtle logic errors by calling static methods from instances. Case in point, this doesn't do what you think it does:

Thread t = new Thread(...);
t.sleep(1000);

sleep is a static method which pauses the currently executing thread, not the thread instance.

Answer 2

The two calls are the same. The problem that comes to mind is when overriding class A, you cannot directly override foo().

Answer 3

Its just considered bad form / practice. Avoid this.

Answer 4

One good reason is that you can confuse other people who might need to read or update your code. It really "seems" like the instantiated object should be involved in the method call, when in fact it isn't (and in fact it can be null). It's unnecessary obfuscation.

Answer 5

不是100％肯定，但如果您的实例为null，则可能会遇到空指针异常。