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Maps Chris Piech CS106A, Stanford University

This article explores the usage of HashMap in Java with a simple example. Learn how to add elements, retrieve values based on keys, and explore the keys and values of a HashMap.

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Maps Chris Piech CS106A, Stanford University

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  1. Maps Chris Piech CS106A, Stanford University

  2. CS106A Winter 2018 Contest

  3. Why is this so fast?

  4. Where are we?

  5. Where are we? • Karel the Robot • Java • Console Programs • Graphics Programs • Text Processing • Data Structures • GUIs • Defining our own Variable Types

  6. Collections High Level List: ArrayList<type> Array: type[] Matrix: type[][]

  7. Collections High Level List: ArrayList<String> Array: double[] Matrix: int[][]

  8. ArrayList index -> value

  9. Arrays index -> value

  10. Matrix (row, col) -> value

  11. Maps can have any type for key Many examples

  12. HashMap key -> value

  13. Simple Example Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  14. Simple Example Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  15. Simple Example animalSoundMap Values: Keys: Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  16. Simple Example animalSoundMap Values: Keys: Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  17. Simple Example animalSoundMap Values: “bark” Keys: “dog” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  18. Simple Example animalSoundMap Values: “bark” Keys: “dog” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  19. Simple Example animalSoundMap Values: “meow” “bark” Keys: “dog” “cat” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  20. Simple Example animalSoundMap Values: “meow” “bark” Keys: “dog” “cat” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  21. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  22. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  23. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sounds 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  24. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sound 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  25. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sound 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “cat”]

  26. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sound 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “cat”]

  27. Simple Example animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sound 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “cat”]

  28. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>();

  29. My First Map Value Type Key Type HashMap<String, String> animalSoundMap = newHashMap<String, String>();

  30. My First Map HashMap<String, String> animalSoundMap= newHashMap<String, String>();

  31. My First Map HashMap<String, String> animalSoundMap = new HashMap<String, String>();

  32. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”);

  33. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”);

  34. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”);

  35. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”);

  36. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”); animalSoundMap.get(“dog”);

  37. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”); animalSoundMap.get(“dog”);

  38. My First Map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); animalSoundMap.put(“dog”, “bark”); animalSoundMap.get(“dog”);

  39. My First Map animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” Make a new HashMap of animal sound 2. Add elements: Put [key = “dog”, value = “bark”] Put [key=“cat”, value=“meow”] Put [key=“seal”, value=“owowow”] 3. Get elements: Get [key = “dog”]

  40. My First Map animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” // 1. Make a new map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); // 2. Put things into the map animalSoundMap.put("dog", "woof"); animalSoundMap.put("cat", "meow"); animalSoundMap.put("seal", "owowow"); // 3. Get things out of the map animalSoundMap.get("dog"); // "woof”

  41. My First Map animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” // 1. Make a new map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); // 2. Put things into the map animalSoundMap.put("dog", "woof"); animalSoundMap.put("cat", "meow"); animalSoundMap.put("seal", "owowow"); // 3. Get things out of the map animalSoundMap.get("dog"); // "woof" animalSoundMap.get("fox"); // ?

  42. brothers Vegard and BårdYlvisåker Circa 2013

  43. Ylvis– “The Fox”. Permission asked. Pending.

  44. My First Map animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” // 1. Make a new map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); // 2. Put things into the map animalSoundMap.put("dog", "woof"); animalSoundMap.put("cat", "meow"); animalSoundMap.put("seal", "owowow"); // 3. Get things out of the map animalSoundMap.get("dog"); // "woof" animalSoundMap.get("fox"); // ?

  45. My First Map animalSoundMap “ow ow ow” Values: “meow” “bark” Keys: “dog” “cat” “seal” // 1. Make a new map HashMap<String, String> animalSoundMap = newHashMap<String, String>(); // 2. Put things into the map animalSoundMap.put("dog", "woof"); animalSoundMap.put("cat", "meow"); animalSoundMap.put("seal", "owowow"); // 3. Get things out of the map animalSoundMap.get("dog"); // "woof" animalSoundMap.get("fox"); // null

  46. HashMaps on one slide Make a HashMap Put and get values into a map Some useful other methods int size = myMap.size(); myMap.containsKey(key); // returns true or false if key is in map myMap.keySet(); myMap.remove(key); // make like a tree and leave! Iterate using a foreach loop HashMap<keyType, valueType> myMap = new HashMap<keyType, valueType>(); myMap.put(key, value); myMap.get(key) // returns the corresponding value for(keyType key : myMap.keySet()){ // not ordered myMap.get(key); // do something with the key/value pair}

  47. Phone Book 6701678

  48. Make a keyboard

  49. Why is this so fast?

  50. Why is this so fast? inthash(stringkey); * Learn more in CS106B

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