If a private field is declared but not used locally, its limited visibility makes it dead code.
This is either a sign that some logic is missing or that the code should be cleaned.
Cleaning out dead code decreases the size of the maintained codebase, making it easier to understand and preventing bugs from being introduced.
public class MyClass {
private int foo = 42; // Noncompliant: foo is unused and should be removed
public int compute(int a) {
return a * 42;
}
}
Note that this rule does not take reflection into account, which means that issues will be raised on private fields that are only
accessed using the reflection API.
Exceptions
The rule admits 3 exceptions:
The Java serialization runtime associates with each serializable class a version number called serialVersionUID, which is used during
deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible for
serialization.
A serializable class can declare its own serialVersionUID explicitly by declaring a field named serialVersionUID that
must be static, final, and of type long. By definition, those serialVersionUID fields should not be reported by this rule:
public class MyClass implements java.io.Serializable {
private static final long serialVersionUID = 42L; // Compliant by exception
}
- Annotated fields and classes annotated with Lombok annotations
The unused field in this class will not be reported by the rule as it is annotated, except if annotation class SomeAnnotation is
listed in the ignoreAnnotations parameter (see Parameters).
public class MyClass {
@SomeAnnotation
private int unused; // Compliant by exception
}
- Fields from classes with native methods
The unused field in this class will not be reported by the rule as it might be used by native code.
public class MyClass {
private int unused = 42; // Compliant by exception
private native static void doSomethingNative();
}
