What are the common causes of NullPointerException in Java?

Java is an object-oriented programming language. Its exception handling mechanism is extremely powerful and there are many exception types. Among them, the NullPointerException exception has attracted much attention because it often occurs in development. This article will introduce the common causes and solutions of NullPointerException exceptions.

NullPointerException is one of the most common exceptions in Java. This exception is thrown when the reference is null when operating an object. In other words, when we try to call a non-existent object, or when we call a method of an object, the object is empty, and a NullPointerException will occur.

1. The object is empty

An object is empty (null), which is the most common cause of NullPointerException. In the following code, the object str is an empty object. When we call the length() method of str, a NullPointerException exception will be thrown.

String str = null;
str.length();

2. Object is not initialized

If an object is not initialized, it will also Throws NullPointerException.

For example, in the following code, an integer array with a length of 5 is created, but its elements have not been initialized. When we access the attribute value in it, because it has not been initialized, it will throw A NullPointerException occurs.

int[] nums = new int[5];
System.out.println(nums[0]);

3. The object is released

When we try to access an object that has been released, NullPointerException will also be thrown.

For example, in the following code, a reference to the string object str is assigned null. A NullPointerException is thrown when we try to call the str method.

String str = "hello world";
str = null;
str.toUpperCase();

4. Non-null object calls empty method

Calling a null method on a non-null object may also cause a NullPointerException exception.

For example, in the following code, if a non-null object calls a null method, a NullPointerException will be thrown.

String str = "hello world";
str = null;
System.out.println(str.indexOf("world"));

Solution:

1. Determine whether the object is empty

Before using the object, we need to use if statements or ternary operators and other logical structures to determine whether the object is empty. Only when If the object is not empty, then reference the object. For example:

String str = null;
if (str != null) {

System.out.println(str.length());

}

2. Initialize the value of the object

When creating an object, assign values ​​to the properties or array elements of the object to avoid uninitialization. For example:

int[] nums = new int[]{1,2,3,4,5};
System.out.println(nums[0]);

3. Use try-catch block to catch exceptions

In order to ensure the normal operation of the program, you can use try-catch block to catch NullPointerException exception and handle the exception. For example:

String str = null;
try {

System.out.println(str.length());

} catch (NullPointerException e) {

e.printStackTrace();
System.out.println("str为空对象！");

}

4. Check The logic of the code

In the process of writing the code, you need to pay attention to the logic of the code to avoid misoperations.

To sum up, NullPointerException is one of the very common exceptions in Java, and there are various reasons for exceptions. In program development, we need to pay more attention and follow Java's good programming practices to avoid NullPointerException exceptions and ensure the stability and reliability of the program.

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