Jumla za nini ni generiki (polymorphism ya parametric)?

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**Generiki** inakukamatia kuandika kiasi na njia zinazofikiwa na *aina*, ili msimbo sawa ufanye kazi na aina nyingi wakati wa kuweka **usalama wa aina wakati wa kutengeneza**. Hii ni **polymorphism ya parametric** — "umbo moja, aina yoyote." ## Tatizo wanalotatua ```java // Without generics: untyped, needs casts, can break at runtime List list = new ArrayList(); list.add("hi"); String s = (String) list.get(0); // cast required; wrong type → ClassCastException ``` ```java // With generics: type is known and checked at compile time List list = new ArrayList<>(); list.add("hi"); String s = list.get(0); // no cast; list.add(42) won't compile ``` ## Kuandika aina ya generiki ```java class Box { // T is a type parameter private T value; Box(T value) { this.value = value; } T get() { return value; } } Box b = new Box<>(42); // T = Integer, checked at compile time int n = b.get(); // no cast needed ``` ## Mipaka na tofauti (kwa kufupi) ```java > // bounded: T must be comparable List // covariance: accept List, List ``` ## Njia mbaya Java inafuta aina za generiki wakati wa utekelezaji (*type erasure*) — huwezi kufanya `new T()` au `instanceof List`. Violezo vya C++ na generiki vya C# hutenda tofauti (monomorphization / reified types). ## Kwa nini ni muhimu Generiki hutoa matumizi tena *na* usalama wakati huo huo: `List` moja inatumika kwa kila aina ya kipengele, na compiler inakamata makosa ya aina ambayo vinginevyo yangekuwa maaanguko ya wakati wa utekelezaji. Huondoa sensa zisizoaminiwa na ndilo msingi wa mkutano, na kufanya msimbo kuwa na matumizi zaidi tena na kuwa gumu kuzitumia vibaya.