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空指针异常是什么,要如何修复?

Q:What is a NullPointerException, and how do I fix it?

What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?

What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?

空指针异常(java.lang.NullPointerException)是为什么?

A:

When you declare a reference variable (i.e. an object) you are really creating a pointer to an object. Consider the following code where you declare a variable of primitive type int:

int x;
x = 10;

In this example the variable x is an int and Java will initialize it to 0 for you. When you assign it to 10 in the second line your value 10 is written into the memory location pointed to by x.

But, when you try to declare a reference type something different happens. Take the following code:

Integer num;
num = new Integer(10);

The first line declares a variable named num, but, it does not contain a primitive value. Instead it contains a pointer (because the type is Integer which is a reference type). Since you did not say as yet what to point to Java sets it to null, meaning "I am pointing at nothing".

In the second line, the new keyword is used to instantiate (or create) an object of type Integer and the pointer variable num is assigned this object. You can now reference the object using the dereferencing operator . (a dot).

The Exception that you asked about occurs when you declare a variable but did not create an object. If you attempt to dereference num BEFORE creating the object you get a NullPointerException. In the most trivial cases the compiler will catch the problem and let you know that "num may not have been initialized" but sometime you write code that does not directly create the object.

For instance you may have a method as follows:

public void doSomething(SomeObject obj){
   //do something to obj}

in which case you are not creating the object obj, rather assuming that is was created before the doSomething method was called. Unfortunately it is possible to call the method like this:

doSomething(null);

in which case obj is null. If the method is intended to do something to the passed-in object, it is appropriate to throw the NullPointerException because it's a user error and the user will need that information for debugging purposes.

Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a null parameter and behave differently. You should also explain this in the documentation. For example, doSomething could be written as:

/**@param obj An optional foo for ____. May be null, in which case 
*  the result will be ____. */public void doSomething(SomeObject obj){
    if(obj != null){
       //do something
    } else {
       //do something else
    }}

简单来说就是在使用对象前,判断对象是否等于空(obj!=null)

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