A non-static inner class has a reference to its outer class, and access to the outer class' fields and methods. That class reference makes the inner class larger and could cause the outer class instance to live in memory longer than necessary.
If the reference to the outer class isn’t used, it is more efficient to make the inner class static
(also called nested). If the
reference is used only in the class constructor, then explicitly pass a class reference to the constructor. If the inner class is anonymous, it will
also be necessary to name it.
However, while a nested/static
class would be more efficient, it’s worth noting that there are semantic differences between an inner
class and a nested one:
static
) class can be instantiated independently of the outer class. public class Fruit { // ... public class Seed { // Noncompliant; there's no use of the outer class reference so make it static int germinationDays = 0; public Seed(int germinationDays) { this.germinationDays = germinationDays; } public int getGerminationDays() { return germinationDays; } } }
public class Fruit { // ... public static class Seed { int germinationDays = 0; public Seed(int germinationDays) { this.germinationDays = germinationDays; } public int getGerminationDays() { return germinationDays; } } }