Nested & Inner Classes
Static nested class outer class ke bina bhi independently use ho sakti hai — sirf Outer.Nested se access.
Inner (non-static) class outer object se juda hota hai — usse outer ke instance members bhi directly dikhte hain.
Anonymous class ek naam-less class hai jo turant, one-time use ke liye define ki jaati hai — jaise ek button click listener.
class Outer {
static class Nested { void show() { System.out.println("Nested"); } }
class Inner { void show() { System.out.println("Inner"); } }
}Static nested class outer class ke object ke bina bhi ban sakti hai — Outer.Nested obj = new Outer.Nested(). Ye tab use hoti hai jab helper class ka outer instance se koi connection nahi chahiye, bas naming/grouping ke liye andar rakhi hai (jaise Map.Entry, jo Map interface ke andar static nested interface hai).
class Outer {
static class Nested {
void show() { System.out.println("Static nested!"); }
}
}
Outer.Nested n = new Outer.Nested(); // outer object nahi chahiyeNon-static inner class ko banane ke liye pehle outer class ka object chahiye (outerObj.new Inner()). Iska fayda: inner class outer ke saare instance fields/methods directly access kar sakti hai, bina explicitly pass kiye — kyunki inner class implicitly outer object ki reference rakhti hai.
class Outer {
int x = 10;
class Inner {
void show() { System.out.println(x); } // outer ka x directly
}
}
Outer o = new Outer();
Outer.Inner i = o.new Inner(); // outer object zaroori haiMethod ke andar hi ek class define kar sakte ho — "local class". Ye sirf usi method ke andar use ho sakti hai, aur method ke local (effectively final) variables ko capture kar sakti hai. Rare use-case hai, lekin kabhi-kabhi ek complex helper logic ko method ke andar hi scope karna ho tab useful hai.
Bina naam ke class jo turant define aur instantiate ho jaati hai, one-time use ke liye — purane Java mein event listeners iसी tareeke se likhe jaate the (Java 8 ke lambda se pehle). Aaj bhi useful hai jab ek interface/abstract class ka sirf ek-baar-use implementation chahiye, aur wo interface functional (ek method) nahi hai (jaha lambda kaam nahi karega).
Runnable r = new Runnable() {
public void run() { System.out.println("Running!"); }
};