How to solve java.lang.NullPointerException?

In Java programming, you may encounter java.lang.NullPointerException (hereinafter referred to as NPE) exception. This exception is usually thrown when you try to use a null object or access a null reference, and it indicates that an unexpected null value was found in the code.

There are many ways to solve NPE. This article will introduce several common solutions.

1. Checking for null references

The most common reason is to access a null object or null reference. When you use an object in code, make sure the object is not null. An NPE exception occurs when the object is null. To prevent this, always check for a null reference before referencing an object.

For example, in the following code, the method checkName() returns null, so you must check whether its return value is null.

String name = checkName();
if(name != null){
  // do something with name
} else {
  // handle null value
}

2. Avoid using null objects

Avoiding the use of null objects can effectively solve the problem of NPE. You can avoid the use of null objects by using the Optional class, which was introduced in Java 8 to provide a better way to represent potentially null values.

For example, in the following code, a variable of type Optional can contain a non-null value of type String.

Optional<String> value = Optional.of("test");
if(value.isPresent()){
  // do something with value
} else {
  // handle null value
}

3. Use default values

Sometimes, you may need to provide a default value for null values ​​to avoid NPEs. Default values ​​can be provided using the ternary operator or if/else statements. The following is an example of using the ternary operator:

String name = checkName();
String result = (name != null) ? name : "default value";

4. Using the @NonNull annotation

Java developers can use the @NonNull annotation to avoid NPE issues. This annotation is used during the code compilation phase to ensure that variables or parameters are not null.

The following is an example of a method annotated with @NonNull:

public void printName(@NonNull String name){
  System.out.println(name);
}

If a null value is passed when calling the printName() method, a warning will be raised at compile time, which can prevent NPE from occurring.

5. Using Objects.requireNonNull()

Java 7 introduced the Objects.requireNonNull() method, which is a static method that can be used to check whether an object is null. If the input parameter value is null, this method will throw a NullPointerException exception, otherwise it will return the parameter itself.

The following is an example of using the Objects.requireNonNull() method:

String value = Objects.requireNonNull(checkValue(), "value cannot be null");

If the checkValue() method returns null, an exception will be thrown.

Summary

The above are our practical methods to solve NPE. When you encounter an NPE exception, don't panic. You can quickly solve the problem through the above methods. At the same time, when writing code, try to avoid null references and empty objects, which can make your code more readable, clear and robust.

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