{"id":"class-diagrams-interfaces","slug":"interfaces","title":"Interfaces, and a whole program","text":"### An interface's box\n\nAn interface (module 35) gets a box like a class, with «interface» above its name. It has no instance variables and no constructor, so below its name, its box lists only the methods.\n\nThe recycling center's weighing station weighs anything that can say what it weighs, so it has an interface `Weighable`. A crate of bottles and a bale of paper both implement it:\n\n\n
\n«interface»Weighable+weight(): intCrate-bottles: int+Crate(bottles: int)+weight(): intBale-kilos: int+Bale(kilos: int)+weight(): int\n
The diagram as text
\n\n\n- The interface's methods are public, `+`. The interface only lists them, so a line for each is all there is to show.\n- A class that implements the interface is joined to it by a **dashed** line with a hollow triangle at the interface: the triangle of `extends`, dashed for `implements`.\n- Each class's box lists the interface's methods again, since each class writes them.\n- A class that implements several interfaces has a dashed line to each of them.\n\n```java\nimport java.util.ArrayList;\n\npublic class Main {\n public static void main(String[] args) {\n ArrayList station = new ArrayList<>();\n station.add(new Crate(24));\n station.add(new Bale(250));\n int total = 0;\n for (Weighable item : station) {\n total += item.weight();\n }\n System.out.println(total + \" kg\");\n }\n}\n// ==== Weighable.java ====\npublic interface Weighable {\n int weight();\n}\n// ==== Crate.java ====\npublic class Crate implements Weighable {\n private int bottles;\n\n public Crate(int bottles) {\n this.bottles = bottles;\n }\n\n @Override\n public int weight() {\n return 2 + this.bottles / 2;\n }\n}\n// ==== Bale.java ====\npublic class Bale implements Weighable {\n private int kilos;\n\n public Bale(int kilos) {\n this.kilos = kilos;\n }\n\n @Override\n public int weight() {\n return this.kilos;\n }\n}\n```\n\n```output\n264 kg\n```\n\n### A whole program\n\nA diagram of a whole program uses everything at once. Here's the weighing station with a `Scale` that remembers what it has weighed, and old tires, from step 6, that can be weighed as well:\n\n\n
\n«interface»Weighable+weight(): int«abstract»DropOff-kilos: int+DropOff(kilos: int)+fee(): int...Tires-count: int+Tires(kilos: int, count: int)+fee(): int+weight(): intCrate-bottles: int+Crate(bottles: int)+weight(): intScale+Scale()+weigh(item: Weighable): void+total(): int*weighed\n
The diagram as text
  • Weighable, an interface
    • public method weight(): int
  • DropOff, an abstract class
    • private field kilos: int
    • public constructor DropOff(kilos: int)
    • public abstract method fee(): int
    • more members, not shown
  • Tires, a class that extends DropOff and implements Weighable
    • private field count: int
    • public constructor Tires(kilos: int, count: int)
    • public method fee(): int
    • public method weight(): int
  • Crate, a class that implements Weighable
    • private field bottles: int
    • public constructor Crate(bottles: int)
    • public method weight(): int
  • Scale, a class
    • public constructor Scale()
    • public method weigh(item: Weighable): void
    • public method total(): int

Connections:

  • Each Scale has any number of Weighable objects (weighed).
\n\n\nTo read a diagram like this, go through it in layers:\n\n1. **The boxes.** What each one is: a class, an abstract class («abstract») or an interface («interface»).\n2. **The triangles.** Each solid one is an `extends`, each dashed one an `implements`. Tires does both: it extends DropOff and implements Weighable.\n3. **The arrows.** Each is a field, and a star makes it a list. The Scale keeps a list of Weighables: it can weigh crates and tires alike, and any class that implements Weighable later.\n4. **The members**, box by box. A line `...` stands for members that the diagram leaves out, as in DropOff's box: step 6 showed the rest.\n\nTo write the program, start with what needs no other class of the diagram: here the interface and DropOff. Then write the classes that extend or implement them, Tires and Crate, and last the classes whose arrows point at the others, the Scale. Each file then compiles as soon as it's written, since everything it names is already there.\n","task":"The ward's cleaning staff clean anything that's `Cleanable`. `Cleanable.java` is ready. Write the class `Wheelchair` in `Wheelchair.java`, as the diagram shows:\n\n\n
\n«interface»Cleanable+clean(): void+isClean(): booleanWheelchair-number: int-uses: int+Wheelchair(number: int)+use(): void+clean(): void+isClean(): boolean+toString(): String\n
The diagram as text
  • Cleanable, an interface
    • public method clean(): void
    • public method isClean(): boolean
  • Wheelchair, a class that implements Cleanable
    • private field number: int
    • private field uses: int
    • public constructor Wheelchair(number: int)
    • public method use(): void
    • public method clean(): void
    • public method isClean(): boolean
    • public method toString(): String
\n\n\n- A new wheelchair is clean: it has had no uses since it was last cleaned.\n- `use` counts one use, and `clean` sets the count back to 0.\n- `isClean` says whether the wheelchair has had no uses since it was last cleaned.\n- `toString` returns `Wheelchair 7: 2 uses since cleaning`.\n\nThe check runs:\n\n```java\nWheelchair chair = new Wheelchair(7);\nchair.use();\nchair.use();\nSystem.out.println(chair + \" \" + chair.isClean());\nCleanable job = chair;\njob.clean();\nSystem.out.println(chair + \" \" + chair.isClean());\n```\n\nand expects:\n\n```\nWheelchair 7: 2 uses since cleaning false\nWheelchair 7: 0 uses since cleaning true\n```\n","kind":"code","seed":"// ==== Wheelchair.java ====\npublic class Wheelchair {\n\n}\n\n// ==== Cleanable.java ====\npublic interface Cleanable {\n void clean();\n boolean isClean();\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n Wheelchair chair = new Wheelchair(7);\n chair.use();\n chair.use();\n System.out.println(chair + \" \" + chair.isClean());\n Cleanable job = chair;\n job.clean();\n System.out.println(chair + \" \" + chair.isClean());\n }\n}\n","solution":"// ==== Wheelchair.java ====\npublic class Wheelchair implements Cleanable {\n private int number;\n private int uses;\n\n public Wheelchair(int number) {\n this.number = number;\n this.uses = 0;\n }\n\n public void use() {\n this.uses++;\n }\n\n @Override\n public void clean() {\n this.uses = 0;\n }\n\n @Override\n public boolean isClean() {\n return this.uses == 0;\n }\n\n @Override\n public String toString() {\n return \"Wheelchair \" + this.number + \": \" + this.uses + \" uses since cleaning\";\n }\n}\n\n// ==== Cleanable.java ====\npublic interface Cleanable {\n void clean();\n boolean isClean();\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n Wheelchair chair = new Wheelchair(7);\n chair.use();\n chair.use();\n System.out.println(chair + \" \" + chair.isClean());\n Cleanable job = chair;\n job.clean();\n System.out.println(chair + \" \" + chair.isClean());\n }\n}\n","hints":["The dashed triangle means `public class Wheelchair implements Cleanable`. The two fields are ints, and a new wheelchair starts with 0 uses.","`clean` and `isClean` are Cleanable's methods, so they're `public`, with `@Override` above them. `isClean` returns `this.uses == 0`."],"tests":[{"name":"Test 1","stdin":"","call":"Wheelchair chair = new Wheelchair(7);\nchair.use();\nchair.use();\nSystem.out.println(chair + \" \" + chair.isClean());\nCleanable job = chair;\njob.clean();\nSystem.out.println(chair + \" \" + chair.isClean());\n","expect":"Wheelchair 7: 2 uses since cleaning false\nWheelchair 7: 0 uses since cleaning true","hidden":false},{"name":"Test 2","stdin":"","call":"Wheelchair spare = new Wheelchair(12);\nSystem.out.println(spare + \" \" + spare.isClean());\nspare.use();\nspare.clean();\nspare.use();\nspare.use();\nspare.use();\nSystem.out.println(spare);\n","expect":"Wheelchair 12: 0 uses since cleaning true\nWheelchair 12: 3 uses since cleaning","hidden":true}],"require":[{"pattern":"\\bclass\\s+Wheelchair\\s+implements\\s+Cleanable\\b","message":"The dashed triangle from Wheelchair to Cleanable means `public class Wheelchair implements Cleanable`."}],"forbid":[],"more":[{"task":"Here are the tires of step 6 in the weighing station's diagram. Fill in the three blanks, words from class headers, so that the code matches the diagram.\n\n\n
\n«interface»Weighable+weight(): int«abstract»DropOff-kilos: int+DropOff(kilos: int)+getKilos(): int+fee(): intTires-count: int+Tires(kilos: int, count: int)+fee(): int+weight(): int\n
The diagram as text
  • Weighable, an interface
    • public method weight(): int
  • DropOff, an abstract class
    • private field kilos: int
    • public constructor DropOff(kilos: int)
    • public method getKilos(): int
    • public abstract method fee(): int
  • Tires, a class that extends DropOff and implements Weighable
    • private field count: int
    • public constructor Tires(kilos: int, count: int)
    • public method fee(): int
    • public method weight(): int
\n\n","kind":"fill","seed":"public class Main {\n public static void main(String[] args) {\n Tires tires = new Tires(36, 4);\n Weighable item = tires;\n DropOff dropOff = tires;\n System.out.println(item.weight() + \" \" + dropOff.fee());\n }\n}\n// ==== Weighable.java ====\npublic interface Weighable {\n int weight();\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// ==== Tires.java ====\npublic class Tires [[extends]] DropOff [[implements]] Weighable {\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 @Override\n public int weight() {\n return this.getKilos();\n }\n}\n","solution":"public class Main {\n public static void main(String[] args) {\n Tires tires = new Tires(36, 4);\n Weighable item = tires;\n DropOff dropOff = tires;\n System.out.println(item.weight() + \" \" + dropOff.fee());\n }\n}\n// ==== Weighable.java ====\npublic interface Weighable {\n int weight();\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// ==== Tires.java ====\npublic class Tires extends DropOff implements Weighable {\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 @Override\n public int weight() {\n return this.getKilos();\n }\n}\n","hints":["Look above DropOff's name, and at the two lines from Tires: which one is solid, which dashed?","«abstract» goes into DropOff's header as `abstract`. A solid line with a triangle is `extends`, a dashed one is `implements`, and `extends` comes first: `public class Tires extends DropOff implements Weighable`."],"tests":[{"name":"Output","stdin":"","expect":"36 16","hidden":false}],"require":[],"forbid":[]},{"task":"To close the module, a whole program from one diagram. A board-game café bills each visit for table time and drinks. Write the four parts of this diagram, each in its own file:\n\n\n
\n«interface»Billable+price(): intTableTime-hours: int-players: int+TableTime(hours: int, players: int)+price(): intDrink-name: String-large: boolean+Drink(name: String, large: boolean)+price(): intVisit-guest: String+Visit(guest: String)+add(item: Billable): void+total(): int+toString(): String*items\n
The diagram as text
  • Billable, an interface
    • public method price(): int
  • TableTime, a class that implements Billable
    • private field hours: int
    • private field players: int
    • public constructor TableTime(hours: int, players: int)
    • public method price(): int
  • Drink, a class that implements Billable
    • private field name: String
    • private field large: boolean
    • public constructor Drink(name: String, large: boolean)
    • public method price(): int
  • Visit, a class
    • private field guest: String
    • public constructor Visit(guest: String)
    • public method add(item: Billable): void
    • public method total(): int
    • public method toString(): String

Connections:

  • Each Visit has any number of Billable objects (items).
\n\n\n- Table time costs 3 dollars per player per hour.\n- A drink costs 4 dollars, or 6 if it's large.\n- A new visit has nothing on its bill. `add` puts an item on it, and `total` returns the prices of the visit's items added up.\n- A visit's `toString` returns `Visit of Ana: 3 items, 34 dollars`, with the visit's own guest, number of items and total.\n\nThe check runs:\n\n```java\nVisit visit = new Visit(\"Ana\");\nvisit.add(new TableTime(2, 4));\nvisit.add(new Drink(\"cocoa\", true));\nvisit.add(new Drink(\"tea\", false));\nSystem.out.println(visit);\nSystem.out.println(new TableTime(1, 3).price());\n```\n\nand expects:\n\n```\nVisit of Ana: 3 items, 34 dollars\n9\n```\n","kind":"code","seed":"// ==== Billable.java ====\npublic interface Billable {\n\n}\n\n// ==== TableTime.java ====\npublic class TableTime {\n\n}\n\n// ==== Drink.java ====\npublic class Drink {\n\n}\n\n// ==== Visit.java ====\npublic class Visit {\n\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n Visit visit = new Visit(\"Ana\");\n visit.add(new TableTime(2, 4));\n visit.add(new Drink(\"cocoa\", true));\n visit.add(new Drink(\"tea\", false));\n System.out.println(visit);\n System.out.println(new TableTime(1, 3).price());\n }\n}\n","solution":"// ==== Billable.java ====\npublic interface Billable {\n int price();\n}\n\n// ==== TableTime.java ====\npublic class TableTime implements Billable {\n private int hours;\n private int players;\n\n public TableTime(int hours, int players) {\n this.hours = hours;\n this.players = players;\n }\n\n @Override\n public int price() {\n return 3 * this.players * this.hours;\n }\n}\n\n// ==== Drink.java ====\npublic class Drink implements Billable {\n private String name;\n private boolean large;\n\n public Drink(String name, boolean large) {\n this.name = name;\n this.large = large;\n }\n\n @Override\n public int price() {\n if (this.large) {\n return 6;\n }\n return 4;\n }\n}\n\n// ==== Visit.java ====\nimport java.util.ArrayList;\n\npublic class Visit {\n private String guest;\n private ArrayList items;\n\n public Visit(String guest) {\n this.guest = guest;\n this.items = new ArrayList<>();\n }\n\n public void add(Billable item) {\n this.items.add(item);\n }\n\n public int total() {\n int sum = 0;\n for (Billable item : this.items) {\n sum += item.price();\n }\n return sum;\n }\n\n @Override\n public String toString() {\n return \"Visit of \" + this.guest + \": \" + this.items.size() + \" items, \" + this.total() + \" dollars\";\n }\n}\n\n// ==== Main.java ====\npublic class Main {\n public static void main(String[] args) {\n Visit visit = new Visit(\"Ana\");\n visit.add(new TableTime(2, 4));\n visit.add(new Drink(\"cocoa\", true));\n visit.add(new Drink(\"tea\", false));\n System.out.println(visit);\n System.out.println(new TableTime(1, 3).price());\n }\n}\n","hints":["Write the files in the order of the lesson: Billable first (one method header), then TableTime and Drink, which implement it (the dashed triangles), and Visit last. Visit's arrow with a star is `private ArrayList items;`.","Visit's `total` adds up `price()` over its list, and each item runs its own class's `price`. `toString` uses the list's size and `total()`."],"tests":[{"name":"Test 1","stdin":"","call":"Visit visit = new Visit(\"Ana\");\nvisit.add(new TableTime(2, 4));\nvisit.add(new Drink(\"cocoa\", true));\nvisit.add(new Drink(\"tea\", false));\nSystem.out.println(visit);\nSystem.out.println(new TableTime(1, 3).price());\n","expect":"Visit of Ana: 3 items, 34 dollars\n9","hidden":false},{"name":"Test 2","stdin":"","call":"Visit visit = new Visit(\"Leo\");\nSystem.out.println(visit + \" \" + visit.total());\nBillable coffee = new Drink(\"coffee\", false);\nvisit.add(coffee);\nvisit.add(new TableTime(3, 2));\nvisit.add(coffee);\nSystem.out.println(visit.total() + \" \" + coffee.price());\nSystem.out.println(visit);\n","expect":"Visit of Leo: 0 items, 0 dollars 0\n26 4\nVisit of Leo: 3 items, 26 dollars","hidden":true}],"require":[{"pattern":"\\binterface\\s+Billable\\s*\\{[^}]*\\bint\\s+price\\s*\\(\\s*\\)\\s*;","message":"Billable's box lists one method: write `int price();` inside the interface."},{"pattern":"\\bimplements\\s+Billable\\b","min":2,"message":"Both dashed triangles point to Billable: TableTime and Drink each `implements Billable`."},{"pattern":"\\bprivate\\s+ArrayList\\s+\\w+\\s*[;=]","message":"The arrow from Visit to Billable, with a star, is a list: `private ArrayList items;`."}],"forbid":[]}]}