{"id":"class-diagrams-inheritance","slug":"inheritance","title":"Inheritance and abstract classes","text":"### A triangle points to the parent\n\nInheritance (module 34) has a line of its own in a class diagram. The recycling center's glass bins are sorted by color, so a `GlassBin` extends `Bin` with a color:\n\n\n
\n\n
The diagram as text
Bin, a class
private field material: String
private field capacity: int
private field load: int
public constructor Bin(material: String, capacity: int)
public method add(kilos: int): boolean
public method toString(): String
GlassBin, a class that extends Bin
private field color: String
public constructor GlassBin(color: String, capacity: int)
public method toString(): String
\n\n\n- The subclass's box is below its parent's, joined to it by a line with a **hollow triangle** at the parent's end. The triangle points to the class that is extended: GlassBin extends Bin.\n- The subclass's box shows only what GlassBin itself writes: its own field, its constructor, and the `toString` it overrides. It inherits `add`, but `add` is drawn once, in Bin's box.\n- Bin's private fields aren't repeated in GlassBin's box either. They're declared in Bin, even though every GlassBin object has them.\n\n```java\npublic class Main {\n public static void main(String[] args) {\n GlassBin green = new GlassBin(\"green\", 400);\n green.add(120);\n System.out.println(green);\n }\n}\n// ==== Bin.java ====\npublic class Bin {\n private String material;\n private int capacity;\n private int load;\n\n public Bin(String material, int capacity) {\n this.material = material;\n this.capacity = capacity;\n this.load = 0;\n }\n\n public boolean add(int kilos) {\n if (this.load + kilos > this.capacity) {\n return false;\n }\n this.load += kilos;\n return true;\n }\n\n @Override\n public String toString() {\n return this.material + \": \" + this.load + \"/\" + this.capacity + \" kg\";\n }\n}\n// ==== GlassBin.java ====\npublic class GlassBin extends Bin {\n private String color;\n\n public GlassBin(String color, int capacity) {\n super(\"glass\", capacity);\n this.color = color;\n }\n\n @Override\n public String toString() {\n return this.color + \" \" + super.toString();\n }\n}\n```\n\n```output\ngreen glass: 120/400 kg\n```\n\nWhen several classes extend one parent, the children sit side by side under it, and their lines meet at the parent's one triangle.\n\n### Abstract classes\n\nAn abstract class (module 34) is drawn like any other class, with two marks: «abstract» above its name, which is in italics, and its abstract methods in italics too.\n\nPeople also bring the recycling center things that don't go in a bin, and pay a fee that depends on what they bring:\n\n\n
\n\n
The diagram as text
DropOff, an abstract class
private field kilos: int
public constructor DropOff(kilos: int)
public method getKilos(): int
public abstract method fee(): int
public method toString(): String
Furniture, a class that extends DropOff
public constructor Furniture(kilos: int)
public method fee(): int
Electronics, a class that extends DropOff
private field screen: boolean
public constructor Electronics(kilos: int, screen: boolean)
public method fee(): int
\n\n\n- DropOff's name is in italics under «abstract»: there are no objects of DropOff itself, only Furniture and Electronics objects.\n- `fee()` is in italics in DropOff's box: it's abstract there, a header without a body. Each subclass writes it, so it's in their boxes again, in plain letters.\n- A subclass can have no fields of its own, like Furniture, whose field part is empty.\n\n```java\nimport java.util.ArrayList;\n\npublic class Main {\n public static void main(String[] args) {\n ArrayList today = new ArrayList<>();\n today.add(new Furniture(30));\n today.add(new Electronics(8, true));\n today.add(new Electronics(2, false));\n int total = 0;\n for (DropOff item : today) {\n System.out.println(item);\n total += item.fee();\n }\n System.out.println(\"Total \" + total);\n }\n}\n// ==== DropOff.java ====\npublic abstract class DropOff {\n private int kilos;\n\n public DropOff(int kilos) {\n this.kilos = kilos;\n }\n\n public int getKilos() {\n return this.kilos;\n }\n\n public abstract int fee();\n\n @Override\n public String toString() {\n return this.kilos + \" kg, fee \" + this.fee();\n }\n}\n// ==== Furniture.java ====\npublic class Furniture extends DropOff {\n public Furniture(int kilos) {\n super(kilos);\n }\n\n @Override\n public int fee() {\n return 2 * this.getKilos();\n }\n}\n// ==== Electronics.java ====\npublic class Electronics extends DropOff {\n private boolean screen;\n\n public Electronics(int kilos, boolean screen) {\n super(kilos);\n this.screen = screen;\n }\n\n @Override\n public int fee() {\n if (this.screen) {\n return 20;\n }\n return 5;\n }\n}\n```\n\n```output\n30 kg, fee 60\n8 kg, fee 20\n2 kg, fee 5\nTotal 85\n```\n","task":"Old tires are a drop-off too. `DropOff.java` is ready. Write the class `Tires` in `Tires.java`, as the diagram shows. Tires cost 4 dollars each.\n\n\n
\n\n
The diagram as text
DropOff, an abstract class
private field kilos: int
public constructor DropOff(kilos: int)
public method getKilos(): int
public abstract method fee(): int
public method toString(): String
Tires, a class that extends DropOff
private field count: int
public constructor Tires(kilos: int, count: int)
public method fee(): int
\n\n\nThe check runs:\n\n```java\nDropOff tires = new Tires(36, 4);\nSystem.out.println(tires);\nSystem.out.println(tires.getKilos() + \" \" + tires.fee());\n```\n\nand expects:\n\n```\n36 kg, fee 16\n36 16\n```\n","kind":"code","seed":"// ==== Tires.java ====\npublic class Tires {\n\n}\n\n// ==== DropOff.java ====\npublic abstract class DropOff {\n private int kilos;\n\n public DropOff(int kilos) {\n this.kilos = kilos;\n }\n\n public int getKilos() {\n return this.kilos;\n }\n\n public abstract int fee();\n\n @Override\n public String toString() {\n return this.kilos + \" kg, fee \" + this.fee();\n }\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n DropOff tires = new Tires(36, 4);\n System.out.println(tires);\n System.out.println(tires.getKilos() + \" \" + tires.fee());\n }\n}\n","solution":"// ==== Tires.java ====\npublic class Tires extends DropOff {\n private int count;\n\n public Tires(int kilos, int count) {\n super(kilos);\n this.count = count;\n }\n\n @Override\n public int fee() {\n return 4 * this.count;\n }\n}\n\n// ==== DropOff.java ====\npublic abstract class DropOff {\n private int kilos;\n\n public DropOff(int kilos) {\n this.kilos = kilos;\n }\n\n public int getKilos() {\n return this.kilos;\n }\n\n public abstract int fee();\n\n @Override\n public String toString() {\n return this.kilos + \" kg, fee \" + this.fee();\n }\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n DropOff tires = new Tires(36, 4);\n System.out.println(tires);\n System.out.println(tires.getKilos() + \" \" + tires.fee());\n }\n}\n","hints":["The triangle means `public class Tires extends DropOff`. Tires's own field is `count`. The kilos belong to DropOff, so the constructor passes them on with `super(kilos);`.","`fee()` is abstract in DropOff, so Tires must write it: `public int fee()`, returning 4 times the count. `toString` and `getKilos` are inherited."],"tests":[{"name":"Test 1","stdin":"","call":"DropOff tires = new Tires(36, 4);\nSystem.out.println(tires);\nSystem.out.println(tires.getKilos() + \" \" + tires.fee());\n","expect":"36 kg, fee 16\n36 16","hidden":false},{"name":"Test 2","stdin":"","call":"Tires one = new Tires(9, 1);\nTires stack = new Tires(80, 10);\nSystem.out.println(one + \" / \" + stack);\nSystem.out.println(one.fee() + stack.fee());\n","expect":"9 kg, fee 4 / 80 kg, fee 40\n44","hidden":true}],"require":[{"pattern":"\\bclass\\s+Tires\\s+extends\\s+DropOff\\b","message":"The triangle from Tires to DropOff means `public class Tires extends DropOff`."}],"forbid":[],"more":[{"task":"A music festival's program has this diagram, but the code doesn't match it in two places, and `Main.java` doesn't compile:\n\n\n
\n\n
The diagram as text
Performer, an abstract class
private field name: String
public constructor Performer(name: String)
public method getName(): String
public abstract method slotMinutes(): int
public method toString(): String
Band, a class that extends Performer
private field members: int
public constructor Band(name: String, members: int)
public method slotMinutes(): int
Dj, a class that extends Performer
public constructor Dj(name: String)
public method slotMinutes(): int
\n\n\nA band plays 30 minutes plus 5 for each member, and a DJ plays 60 minutes. Fix `Performer.java` and `Dj.java` so that they match the diagram, and leave `Main.java` and `Band.java` as they are. The program should print:\n\n```\nThe Low Tides, 50 min\nKora, 60 min\n110 min in all\n```\n","kind":"code","seed":"// ==== Performer.java ====\npublic class Performer {\n private String name;\n\n public Performer(String name) {\n this.name = name;\n }\n\n public String getName() {\n return this.name;\n }\n\n public int slotMinutes() {\n return 0;\n }\n\n @Override\n public String toString() {\n return this.name + \", \" + this.slotMinutes() + \" min\";\n }\n}\n\n// ==== Dj.java ====\npublic class Dj {\n private String name;\n\n public Dj(String name) {\n this.name = name;\n }\n\n public int slotMinutes() {\n return 60;\n }\n}\n\n// ==== Band.java ====\npublic class Band extends Performer {\n private int members;\n\n public Band(String name, int members) {\n super(name);\n this.members = members;\n }\n\n @Override\n public int slotMinutes() {\n return 30 + 5 * this.members;\n }\n}\n\n// ==== Main.java ====\nimport java.util.ArrayList;\n\npublic class Main {\n public static void main(String[] args) {\n ArrayList lineup = new ArrayList<>();\n lineup.add(new Band(\"The Low Tides\", 4));\n lineup.add(new Dj(\"Kora\"));\n int minutes = 0;\n for (Performer performer : lineup) {\n System.out.println(performer);\n minutes += performer.slotMinutes();\n }\n System.out.println(minutes + \" min in all\");\n }\n}\n","solution":"// ==== Performer.java ====\npublic abstract class Performer {\n private String name;\n\n public Performer(String name) {\n this.name = name;\n }\n\n public String getName() {\n return this.name;\n }\n\n public abstract int slotMinutes();\n\n @Override\n public String toString() {\n return this.name + \", \" + this.slotMinutes() + \" min\";\n }\n}\n\n// ==== Dj.java ====\npublic class Dj extends Performer {\n public Dj(String name) {\n super(name);\n }\n\n @Override\n public int slotMinutes() {\n return 60;\n }\n}\n\n// ==== Band.java ====\npublic class Band extends Performer {\n private int members;\n\n public Band(String name, int members) {\n super(name);\n this.members = members;\n }\n\n @Override\n public int slotMinutes() {\n return 30 + 5 * this.members;\n }\n}\n\n// ==== Main.java ====\nimport java.util.ArrayList;\n\npublic class Main {\n public static void main(String[] args) {\n ArrayList lineup = new ArrayList<>();\n lineup.add(new Band(\"The Low Tides\", 4));\n lineup.add(new Dj(\"Kora\"));\n int minutes = 0;\n for (Performer performer : lineup) {\n System.out.println(performer);\n minutes += performer.slotMinutes();\n }\n System.out.println(minutes + \" min in all\");\n }\n}\n","hints":["Performer's name and `slotMinutes()` are in italics, under «abstract»: the class is abstract, and so is that method, a header that ends in a semicolon.","The triangle from Dj points to Performer: `public class Dj extends Performer`. Dj's box has no fields, so the name belongs to Performer, and Dj's constructor passes it on with `super(name);`."],"tests":[{"name":"The program's output","stdin":"","expect":"The Low Tides, 50 min\nKora, 60 min\n110 min in all","hidden":false},{"name":"Test 2","stdin":"","call":"Performer solo = new Band(\"Mira Solen\", 1);\nPerformer dj = new Dj(\"Vex\");\nSystem.out.println(solo.getName() + \" \" + solo.slotMinutes());\nSystem.out.println(dj.getName() + \" / \" + dj);\n","expect":"Mira Solen 35\nVex / Vex, 60 min","hidden":true}],"require":[{"pattern":"\\babstract\\s+class\\s+Performer\\b","message":"The «abstract» above Performer's name means `public abstract class Performer`."},{"pattern":"\\babstract\\s+int\\s+slotMinutes\\s*\\(\\s*\\)\\s*;","message":"`slotMinutes()` is in italics in Performer's box: it's abstract, `public abstract int slotMinutes();`, with no body."},{"pattern":"\\bclass\\s+Dj\\s+extends\\s+Performer\\b","message":"The triangle from Dj to Performer means `public class Dj extends Performer`."}],"forbid":[{"pattern":"\\bclass\\s+Dj\\b[^}]*\\bprivate\\s+String\\s+name\\b","message":"Dj's box has no fields: the name is Performer's. Pass it on with `super(name);`."}]},{"task":"A garden center delivers what its customers buy, by van or by bike. Write the three classes of this diagram, each in its own file:\n\n\n
\n\n
The diagram as text
Delivery, an abstract class
private field address: String
public constructor Delivery(address: String)
public method getAddress(): String
public abstract method cost(): int
public method toString(): String
VanDelivery, a class that extends Delivery
private field pallets: int
public constructor VanDelivery(address: String, pallets: int)
public method cost(): int
BikeDelivery, a class that extends Delivery
public constructor BikeDelivery(address: String)
public method cost(): int
\n\n\n- A van delivery costs 25 dollars, plus 15 for each pallet. A bike delivery costs 8.\n- Every delivery's `toString` returns `8 Hill Street: 55 dollars`, with its own address and cost.\n\nThe check runs:\n\n```java\nDelivery van = new VanDelivery(\"8 Hill Street\", 2);\nDelivery bike = new BikeDelivery(\"3 Pond Lane\");\nSystem.out.println(van);\nSystem.out.println(bike);\n```\n\nand expects:\n\n```\n8 Hill Street: 55 dollars\n3 Pond Lane: 8 dollars\n```\n","kind":"code","seed":"// ==== Delivery.java ====\npublic class Delivery {\n\n}\n\n// ==== VanDelivery.java ====\npublic class VanDelivery {\n\n}\n\n// ==== BikeDelivery.java ====\npublic class BikeDelivery {\n\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n Delivery van = new VanDelivery(\"8 Hill Street\", 2);\n Delivery bike = new BikeDelivery(\"3 Pond Lane\");\n System.out.println(van);\n System.out.println(bike);\n }\n}\n","solution":"// ==== Delivery.java ====\npublic abstract class Delivery {\n private String address;\n\n public Delivery(String address) {\n this.address = address;\n }\n\n public String getAddress() {\n return this.address;\n }\n\n public abstract int cost();\n\n @Override\n public String toString() {\n return this.address + \": \" + this.cost() + \" dollars\";\n }\n}\n\n// ==== VanDelivery.java ====\npublic class VanDelivery extends Delivery {\n private int pallets;\n\n public VanDelivery(String address, int pallets) {\n super(address);\n this.pallets = pallets;\n }\n\n @Override\n public int cost() {\n return 25 + 15 * this.pallets;\n }\n}\n\n// ==== BikeDelivery.java ====\npublic class BikeDelivery extends Delivery {\n public BikeDelivery(String address) {\n super(address);\n }\n\n @Override\n public int cost() {\n return 8;\n }\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n Delivery van = new VanDelivery(\"8 Hill Street\", 2);\n Delivery bike = new BikeDelivery(\"3 Pond Lane\");\n System.out.println(van);\n System.out.println(bike);\n }\n}\n","hints":["Start with Delivery: `public abstract class Delivery`, with the address, a constructor, `getAddress`, `public abstract int cost();`, and a `toString` that calls `cost()`.","Each subclass `extends Delivery`, passes the address on with `super(address);`, and writes `cost()`: 25 plus 15 times the pallets for a van, 8 for a bike."],"tests":[{"name":"Test 1","stdin":"","call":"Delivery van = new VanDelivery(\"8 Hill Street\", 2);\nDelivery bike = new BikeDelivery(\"3 Pond Lane\");\nSystem.out.println(van);\nSystem.out.println(bike);\n","expect":"8 Hill Street: 55 dollars\n3 Pond Lane: 8 dollars","hidden":false},{"name":"Test 2","stdin":"","call":"ArrayList today = new ArrayList<>();\ntoday.add(new VanDelivery(\"1 Oak Row\", 0));\ntoday.add(new BikeDelivery(\"7 Fern Way\"));\ntoday.add(new VanDelivery(\"22 Birch Road\", 5));\nint total = 0;\nfor (Delivery delivery : today) {\n total += delivery.cost();\n}\nSystem.out.println(today.get(2).getAddress() + \" \" + today.get(2).cost() + \" \" + total);\nSystem.out.println(today.get(0));\n","expect":"22 Birch Road 100 133\n1 Oak Row: 25 dollars","hidden":true}],"require":[{"pattern":"\\babstract\\s+class\\s+Delivery\\b","message":"The «abstract» above Delivery's name means `public abstract class Delivery`."},{"pattern":"\\babstract\\s+int\\s+cost\\s*\\(\\s*\\)\\s*;","message":"`cost()` is in italics in Delivery's box: write it as `public abstract int cost();`."},{"pattern":"\\bextends\\s+Delivery\\b","min":2,"message":"Both triangles point to Delivery: VanDelivery and BikeDelivery each `extends Delivery`."}],"forbid":[]}]}