TreeMap Tutorials

TreeMap Tutorial

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A TreeMap relates keys to values. Often the keys are Strings and the values are more complex objects.

• In these examples, the TreeMap world is of type TreeMap<String,Country>.
• The data is read from a text file http://progzoo.net/bbc.txt
• The key value is the name of a country, the value in each case is an object representing the country:

world

Key	Value
Afghanistan	(name:'Afghanistan',region:'Asia',area:652225sqkm,pop:26000000,gdp:0)
Albania	(name:'Albania',region:'Europe',area:28728sqkm,pop:3200000,gdp:6656000000)
Algeria	(name:'Algeria',region:'Middle East',area:2400000sqkm,pop:32900000,gdp:75012000000)
Andorra	(name:'Andorra',region:'Europe',area:468sqkm,pop:64000,gdp:0)
Angola	(name:'Angola',region:'Africa',area:1250000sqkm,pop:14500000,gdp:14935000000)
...
Zambia	(name:'Zambia',region:'Africa',area:752614sqkm,pop:11000000,gdp:4950000000)
Zimbabwe	(name:'Zimbabwe',region:'Africa',area:390759sqkm,pop:12900000,gdp:6192000000)

Access a value: get

You can access a specific value using get

If you know the name of a country (the key) you can access the associated value.

.

static void doStuff(TreeMap<String,Country> world) { Country c = world.get("France"); System.out.println(c); }

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Missing keys: get

If you try to get a key that is missing it will return null.

The null object is returned of the value is missing.

.

static void doStuff(TreeMap<String,Country> world) { Country c = world.get("No where"); System.out.println(c); }

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Testing keys: containsKey

You can check to see if a key exists using containsKey

.

static void doStuff(TreeMap<String,Country> world) { if (world.containsKey("No where")) { System.out.println("Got that one!"); } else { System.out.println("No such country."); } }

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Adding an entry: put

You can create a new entry using put. You must give the key and the value.

.

static void doStuff(TreeMap<String,Country> world) { //Add a new country world.put("Erewon",new Country("Erewon","Europe",0,0,0)); //Read the new country back System.out.println(world.get("Erewon")); }

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Iterating over the keys: keySet()

You can run over all of the keys using the keySet() method.

.

static void doStuff(TreeMap<String,Country> world) { for (String s:world.keySet()) { System.out.println(s); } }

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Iterating over the values: values()

You can run over all of the values using the values() method.

.

static void doStuff(TreeMap<String,Country> world) { for (Country c:world.values()) { System.out.println(c); } }

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TreeMap Tutorial

Introducing Country

Iteration

Accumulating

Make your own TreeMap

Updating

Harder Examples