Nested class ni class iliyofafanuliwa ndani ya class nyingine. Java ina aina nne — static nested, inner (non-static), local, na anonymous — kila moja ina ufikiaji tofauti kwa class inayozunguka na matumizi tofauti.
{
;
{
{
System.out.println(data);
}
}
}
Outer. .Nested();
Static nested class hutenda kama top-level class lakini imewekwa kwenye namespace ndani ya outer. Haishikilii reference ya outer instance — hutumika kwa kupanga helpers (mfano Builder, Node katika muundo wa data).
public class Outer {
private int value = 5;
class Inner { // non-static → has an implicit reference to Outer
void show() {
System.out.println(value); // can access Outer's INSTANCE members (even private)
}
}
}
Outer outer = new Outer();
Outer.Inner inner = outer.new Inner(); // needs an Outer instance to create
Inner class isiyo-static hushikilia reference ya wazi ya instance inayoizunguka, kwa hivyo inaweza kufikia instance fields za outer object. (Tahadhari: reference hii inaweza kusababisha memory leaks kama inner instance inadumu zaidi ya outer.)
void process() {
class Helper { // local to this method
void run() { ... }
}
new Helper().run();
}
Hutumika nadra; imefungwa kwa method.
// implement an interface/abstract class inline, for single use
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) { handleClick(); }
});
Runnable task = new Runnable() {
public void run() { System.out.println("running"); }
};
Anonymous class ni utekelezaji wa mara moja wa interface au class, uliofafanuliwa na kuundwa kwa pamoja — kihistoria ulijulikana kwa event handlers na callbacks. Java ya kisasa mara nyingi huzibadilisha hizi na lambdas wakati lengo ni functional interface:
button.addActionListener(e -> handleClick()); // ✅ lambda — much cleaner
Runnable task = () -> System.out.println("running");
Nested classes ni chombo cha manufaa cha kupanga kimantiki classes zinazotumika katika muktadha mmoja tu, zikiongeza encapsulation na usomaji.
Kuelewa aina — hasa tofauti ya static-vs-inner (static = huru, inner = hushikilia reference ya enclosing-instance) — ni muhimu kwa muundo na kuepuka bugs: reference ya wazi ya outer ya inner classes inaweza kusababisha memory leaks, na kuchagua static wakati huhitaji outer instance ni chaguo-msingi safi zaidi.
Anonymous classes ndizo zilikuwa njia ya kawaida ya kutoa utekelezaji wa inline (event handlers, comparators), na kujua kuwa kwa kiasi kikubwa zimepitishwa na lambdas kwa functional interfaces huonyesha umahiri wa Java wa kisasa.
Hizi huonekana katika library ya kawaida (mfano Map.Entry ni nested interface, builders ni static nested classes), zikifanya kuwa maarifa ya kivitendo, yanayokutana mara kwa mara.