42 }
43 }
44
45 class Node {46 // Integer data4748 public Integer data;49 public Node nextNode;5051 // Previously, Object data52 Node(Node nextNode, Integer data) {53 this.nextNode = nextNode;54 this.data = data;55 }56 }57
58}This nested class is declared without a static modifier, meaning all instances of the class will hold a reference to an instance of the enclosing class.
The Java analyzer has detected that there are no explicit references to the enclosing class here, meaning this nested class can be safely treated as static.
Consider adding a static modifier to the nested class.
Non-static nested classes are generally known as "inner" classes.
Bad Practice
class Outer {
class Inner {
// ...
}
// ...
}
There are a number of things one should be aware of when using inner classes:
* An instance of Inner will contain a reference to an instance of Outer.
* If the Inner instance continues to exist after all other references to the Outer instance are deleted, the Outer instance will still exist because of the reference held by the still-alive Inner class.
* The syntax for instantiating a nested class is relatively obscure, and may confuse future code maintainers.
* Referring to private fields of the enclosing class from the nested class requires Java to generate synthetic accessor methods for that sole purpose, bloating the class's bytecode.
Recommended
Declare the nested class as static if possible.
class Outer {
static class Inner {
// ...
}
// ...
}
Exceptions
If the inner class refers to the outer class's instance fields, this issue will not be reported.
References
- Oracle Java Tutorials - Nested Classes
- Oracle Java 11 Language Specification - Section 8.1.3 - Inner Classes and Enclosing Instances