Teil-Maps und Bereichsansichten

Bereichsansichten in TreeMap

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 10; i++) map.put(i * 10, "item" + i);

var view = map.subMap(30, 70); // [30, 70)
System.out.println(view); // {30=item3, 40=item4, 50=item5, 60=item6}

map.put(45, "new"); // also visible through view!
System.out.println(view.containsKey(45)); // true

headMap: Schlüssel unterhalb einer Grenze

TreeMap<String, Integer> words = new TreeMap<>();
"banana cherry apple date elderberry".chars()
    .mapToObj(c -> String.valueOf((char)c)).distinct()
    .forEach(w -> words.put(w, w.length()));
// Actually let's use real words:
TreeMap<String, Integer> wc = new TreeMap<>();
wc.put("apple",5); wc.put("banana",6); wc.put("cherry",6); wc.put("date",4);

System.out.println(wc.headMap("cherry")); // {apple=5, banana=6}

tailMap: Schlüssel ab einer Grenze

TreeMap<Integer, String> grades = new TreeMap<>();
grades.put(50,"F"); grades.put(60,"D"); grades.put(70,"C"); grades.put(80,"B"); grades.put(90,"A");

// All passing grades (>= 60)
var passing = grades.tailMap(60);
System.out.println(passing); // {60=D, 70=C, 80=B, 90=A}

subMap mit inklusiven Grenzen

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 10; i <= 100; i += 10) map.put(i, "v"+i);

// [30, 60] — both inclusive
System.out.println(map.subMap(30, true, 60, true));
// {30=v30, 40=v40, 50=v50, 60=v60}

// (30, 60) — both exclusive
System.out.println(map.subMap(30, false, 60, false));
// {40=v40, 50=v50}

Ändern von Daten über eine Ansicht

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 1; i <= 5; i++) map.put(i * 10, "v" + i);

var view = map.subMap(20, 40); // [20, 40)
view.remove(20); // removes from both view and original map
System.out.println(map.containsKey(20)); // false

// This would throw IllegalArgumentException:
// view.put(50, "out of range");

Anwendungsfall: Protokoll-Bereichsabfrage

import java.time.*;
TreeMap<LocalDateTime, String> logs = new TreeMap<>();
logs.put(LocalDateTime.of(2024,1,1,8,0), "Server start");
logs.put(LocalDateTime.of(2024,1,1,10,0), "Request spike");
logs.put(LocalDateTime.of(2024,1,1,14,0), "Maintenance");
logs.put(LocalDateTime.of(2024,1,1,18,0), "Server stop");

var morning = logs.subMap(
    LocalDateTime.of(2024,1,1,8,0), true,
    LocalDateTime.of(2024,1,1,12,0), false
);
morning.forEach((t,m) -> System.out.println(t+" : "+m));

Anwendungsfall: Suche in einem Preisbereich

TreeMap<Double, String> products = new TreeMap<>();
products.put(9.99, "Pen");
products.put(24.99, "Book");
products.put(49.99, "Headphones");
products.put(299.99, "Tablet");

double min = 10.0, max = 100.0;
var affordable = products.subMap(min, true, max, true);
affordable.forEach((p,n) -> System.out.println(n+" $"+p));
// Book $24.99, Headphones $49.99

Absteigende subMap

TreeMap<Integer, String> map = new TreeMap<>();
for (int i = 10; i <= 100; i += 10) map.put(i, "v"+i);

// Get [40, 80] in descending order
map.subMap(40, true, 80, true)
   .descendingMap()
   .forEach((k,v) -> System.out.println(k + "=" + v));
// 80=v80, 70=v70, 60=v60, 50=v50, 40=v40

NavigableMap-Interface

Performance von Ansichten

Stolperfalle: Veraltete Ansichten

TreeMap<Integer, String> map = new TreeMap<>();
map.put(10, "a"); map.put(20, "b"); map.put(30, "c");

var view = map.subMap(10, 30);
map.clear(); // view becomes empty
System.out.println(view.size()); // 0 — but no exception

Kurzer Test

Zusammenfassung: Submaps und Bereichsansichten