What are the common causes of NullPointerException in Java?
Java is a widely used object-oriented programming language, and NullPointerException is one of the common exceptions in Java programming. When a null object is accessed in the program, a NullPointerException is thrown. This article will delve into the common causes of NullPointerException exceptions in Java.
- Object is null
The most common cause of NullPointerException is that the accessed object is null. For example:
String str = null; System.out.println(str.length());
Since the str object is empty, a NullPointerException will be thrown when printing its length. You can check if an object is null by adding an if statement:
String str = null;
if (str != null) {
System.out.println(str.length());
}- Using uninitialized variables
A NullPointerException is also thrown when trying to use an uninitialized variable . For example:
String str; System.out.println(str.length());
Since the str variable is not initialized, a NullPointerException will be thrown when its method is called. This problem can be solved by initializing the variable:
String str = ""; System.out.println(str.length());
- Accessing empty elements of the array
The array can also be empty. If you try to access a null element in the array, a NullPointerException will be thrown. For example:
int[] nums = null; System.out.println(nums[0]);
Since the nums array is empty, trying to access its first element will throw a NullPointerException. This problem can be avoided by checking whether the array is empty:
int[] nums = null;
if (nums != null && nums.length > 0) {
System.out.println(nums[0]);
}- Inheritance problem
When inheritance is used, the subclass can access the parent class by calling its method Instance variables of the class. If the instance variables of the parent class are null, the subclass cannot access these variables, thus throwing a NullPointerException. For example:
class Parent {
String name;
}
class Child extends Parent {
void printName() {
System.out.println(name.length());
}
}
Child child = new Child();
child.printName();Because the instance variable name of the parent class is empty, a NullPointerException exception will be thrown when the subclass tries to call its length. This problem can be avoided by checking in the child class whether the variable of the parent class is empty:
class Child extends Parent {
void printName() {
if (name != null) {
System.out.println(name.length());
}
}
}- When closing the JVM
When closing the JVM, it may be thrown NullPointerException exception. This is usually due to incorrect resource release methods used in Java programs. For example:
InputStream stream = new FileInputStream("file.txt");
// 这里应该添加stream.close()方法来关闭流
System.exit(0);Since the stream.close() method is not called to close the file stream, a NullPointerException exception will be thrown when the JVM is closed.
To sum up, NullPointerException is one of the common exceptions in Java programming. When writing a program, you need to pay attention to the above common causes to avoid this problem. Strengthening the control of hollow objects in the program and rationally calling resource release methods can avoid the occurrence of NullPointerException exceptions.
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