Jelasake prinsip Liskov Substitution kanthi jero.

Question

Accepted Answer

**Prinsip Liskov Substitution (LSP)** nyatakake manawa obyek saka subtype kudu bisa ditukar karo tipe dasare **tanpa nglanggar kabenaran program**. Yen kode bisa gumana karo `Base`, kudu bisa gumana karo saben `Subclass` — pangarepan perilaku sing padha, ora ana kejutan.

## Pelanggaran kanonik: Square extends Rectangle

```java
class Rectangle {
    protected int w, h;
    void setWidth(int w)  { this.w = w; }
    void setHeight(int h) { this.h = h; }
    int area() { return w * h; }
}

class Square extends Rectangle {       // "a square IS-A rectangle"... seemingly
    void setWidth(int w)  { this.w = w; this.h = w; }   // keep sides equal
    void setHeight(int h) { this.w = h; this.h = h; }
}

void resize(Rectangle r) {
    r.setWidth(5);
    r.setHeight(4);
    assert r.area() == 20;   // holds for Rectangle... FAILS for Square (area=16)
}
```

`Square` *melanggar kontrak* sing diandelake pemaggih: nyetel lebar lan tinggi kanthi mandiri. Lewati tes "is-a" ing basa Inggris nanging **melanggar LSP**.

## Apa sing benere dituntut LSP

```text
A subtype must NOT:
  ✗ strengthen preconditions  (demand more than the base, e.g. reject valid inputs)
  ✗ weaken postconditions     (promise less than the base)
  ✗ break invariants the base guarantees
  ✗ throw new unexpected exceptions for previously valid calls
A subtype MAY:
  ✓ weaken preconditions (accept more)   ✓ strengthen postconditions (promise more)
```

Iki **kontravariansa input, kovariansa output** — inti formal LSP.

## Carane ndandani conto

Droptake warisan: `Square` lan `Rectangle` loro-lorone `Shape` karo `area()`, nanging ora ana sing liya. Modelake perilaku, dudu taksonomi naif.

## Kenapa iki penting

LSP iku sing nggawe polimorfisme *aman*: tanpa iku, saben cek `if (x instanceof Square)` lolos maneh, nglanggar maksud saka subtipe.

Ngerangka ulang desain warisan banter **kontrak perilaku**, dudu kamus — alasan nyata "ndhukung komposisi" ana yen akeh relasi is-a sing diam-diam melanggar LSP.