java - 我们可以在空对象上调用任何方法吗？

Question

可能吗？

Object obj=null;

obj.someMethod();

someMethod{/*some code here*/}

Answer 1

您可以在空指针上调用静态方法。在静态方法调用中，指针自然会被完全忽略，但在某些情况下（不查看类定义）似乎应该导致 NullPointerException 运行得很好。

class FooObject {
    public static void saySomething() {
        System.out.println("Hi there!");
    }
}

class Main {
    public static void main(String[] args) {
        FooObject foo = null;
        foo.saySomething();
    }
}

但为了清楚起见 - 不，您不能使用空指针调用实例方法。保护程序员免受这种影响是真正的基本保护措施之一，它将 Java 等语言与 C++ 等“低级语言”区分开来。它允许在调用端报告错误，而不是在方法本身内部导致莫名其妙的段错误/whatnot。

Answer 2

不，我们不能。只要方法不是静态的，它就会抛出 NullPointerException。如果方法是静态的，它将运行。

阅读：null：Java 词汇表

Answer 3

不，在 Java 中，null 不是对象。

Answer 4

“obj”变量没有引用堆上的实例。因此，您将在运行时收到空指针异常。甚至您的 Java IDE（Eclipse、IDEA）也会发现问题并警告您。

Answer 5

是的，如果方法是静态的，您可以调用，因为静态方法是在编译时绑定的，并且只有变量类型用于静态绑定，而不是对象的值。

如果您对非静态方法尝试相同的方法，请准备好捕获空指针异常。

Answer 6

This won't compile as Object doesn't have someMethod(). However if you are talking about something like

MyClass o = null;
o.someMethod();

the answer depends on whether someMethod is static or not. If it is static, the value is ignored and it doesn't matter if its null or not.

e.g.

Thread t = null;
t.yield();

runs fine without an exception.

Answer 7

A null object does not exist. In your example, you have a variable (a pointer) that can either store a reference to an instance or nothing.

If it doesn't point to an instance - well, then we can't use it to call methods or access fields.

---

wait, wait - this compiles and runs:

Math m = null;
System.out.println(m.max(1,2)); 

We can call static methods and access static fields in any variable (we just have to ignore compiler/IDE warnings!) But that is something different, a static method/variable is not called/accessed on the instance but on the class itself.

Answer 8

No, there is no way to call a method on a null reference (unless the method is static!). (null does not represent some "base" object, it represents a reference which does not point to any object at all.)

This works fine however (ideone.com link):

class MethodTest {
    static void someMethod() {
        System.out.println("Hello World");
    }
}

class Test {
    public static void main(String[] args) {
        MethodTest mt = null;
        mt.someMethod();
    }
}

---

Relevant quote from the JLS:

15.12.4.4 Locate Method to Invoke
The strategy for method lookup depends on the invocation mode.

[...]

If the invocation mode is static, no target reference is needed and overriding is not allowed. Method m of class T is the one to be invoked.

Otherwise, an instance method is to be invoked and there is a target reference. If the target reference is null, a NullPointerException is thrown at this point. Otherwise, the target reference is said to refer to a target object and will be used as the value of the keyword this in the invoked method.

[...]

Answer 9

你不能执行

null.someMethod(); ！！！

Answer 10

NullPointerExcpetion除非 someMethod 被声明为静态，否则这将始终抛出一个。但是，在实例上调用静态方法是非常糟糕的做法。