Null pointer exception, what is a null pointer exception

Is there a file in the directory of this class? If so, is the content in it like this?

classname=

arraylist can also be changed to other types of containers).

A null pointer exception is a pointer that is a null pointer, and you have to manipulate it, since it points to an empty object, it can't use the object's methods; For example, if the s above is null, you still need to use the s method.

Null pointer working meaning:

In many languages, this is not a separate escape sequence, but an octal escape sequence, with a single octal number being 0; Therefore, '0' cannot follow any number 0 through 7; Otherwise it is interpreted as the beginning of a longer octal escape sequence. Other escape sequences are found in various languages in use 000, x00, z, or unicode for u0000.

The ability to represent null characters does not always mean that the resulting string will be interpreted correctly, as many programs will consider null as the end of the string. As a result, the ability to type it (in the case of unchecked user input) creates a vulnerability known as null byte injection and can lead to a security breach.

A null character is indicated by a comma . On Windows, this can be done by holding down the Ctrl key and then pressing (this usually requires holding down Shift and pressing the number 2 key at the same time).

You've revealed the reason for the error: the null pointer exception explanation should be, when the null value calls the method, the null pointer exception is returned, and the value of your fifth line i is null, and it participates in the operation of the fifth line expression, according to the logic of boxing and unboxing, the fifth line ** is equivalent to:

x = i+y).intvalue();

Whereas; It directly leads to a null pointer exception, how can a null object go to intvalue();

So it's going to be wrong.

The idea of automatic unboxing is indeed involved here.

Kindness.. You've said it all...

Integer's anomaly.。。

integer is an encapsulated class. Initialization is required. and int.

The second line integer is the reference type to new

Variables of type int are assigned by default when they are global variables, so you write int i; Even if there is no assignment, it will not give an error, but if you write an int type variable in the method and do not assign a value, the compilation will not pass.

integer is the encapsulation class of int; i nteger is an object that points to this object with a reference; An integer is a class, an extension of an int, that defines a number of conversion methods. Therefore, it must be initialized when calling, otherwise a null pointer exception will run.

An integer is a class name, which is an encapsulated class whose objects must be instantiated before they can be used.

So the second line should be changed to:

integer i = new integer(0);

Or initialize to another value.

Because the first time you go to x, you write int x=0; That's it.

The so-called null pointer exception is that a pointer is a null pointer, and you have to manipulate it, since it points to an empty object, it can't use the object's methods. For example, if the above s is null, you still need to use the s method, such as string x); Then a null pointer exception will occur.

Causes of null pointer exceptions:

1) When an object does not exist, an exception will be generated when its method does not exist, and an exception will be generated when an object does not exist, and 2) an exception will be generated when accessing or modifying a field where an object does not exist, and the method method does not exist.

The resolution for the null pointer exception is as follows:Tools Materials: Lenovo Xiaoxin Air14, Windows 8, MyEclipse.

1. The abnormal information thrown by the null pointer is generally as follows:

exception in thread "main"

at :7)

at :17)

From the exception information, you can see which line of that class it is, for example, I covered it in line 7.

2. Then we can look at the 7th line in **Summoning defeat.

3. From the above figure, we can obviously see that there is a variable "and juxtapotactic y" in line 7, then it is possible that the variable "y" is empty, and we print the value of y.

4. Print out the result. <>

5. So generally speaking, before calling an object, it is necessary to verify the non-null, otherwise it is easy to produce an empty pointer exception. <>