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Collections Practice Programs
Last Updated : 26 May, 2026
title: Collections Programs
Collections Framework34 words5 headingsExamples included
title: Collections Programs description: Collections ke important practice programs
1. Frequency of elements
List<String> list = Arrays.asList("java", "python", "java", "c++", "python", "java");
Map<String, Long> freq = list.stream()
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
freq.forEach((k, v) -> System.out.println(k + ": " + v));
2. Group elements by property
java exampleWoHoTech
List<String> words = Arrays.asList("apple", "bat", "cat", "ant", "ape");
Map<Integer, List<String>> byLength = words.stream()
.collect(Collectors.groupingBy(String::length));3. Sort Map by value
java exampleWoHoTech
Map<String, Integer> scores = new HashMap<>();
scores.put("Ram", 85); scores.put("Shyam", 92); scores.put("Geeta", 78);
scores.entrySet().stream()
.sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
.forEach(e -> System.out.println(e.getKey() + ": " + e.getValue()));4. Merge two Maps
java exampleWoHoTech
Map<String, Integer> m1 = new HashMap<>(Map.of("a", 1, "b", 2));
Map<String, Integer> m2 = new HashMap<>(Map.of("b", 3, "c", 4));
m2.forEach((k, v) -> m1.merge(k, v, Integer::sum));
// {a=1, b=5, c=4}5. Flatten nested List
List<List<Integer>> nested = Arrays.asList(
Arrays.asList(1, 2), Arrays.asList(3, 4), Arrays.asList(5)
);
List<Integer> flat = nested.stream()
.flatMap(Collection::stream)
.collect(Collectors.toList());
// [1, 2, 3, 4, 5]
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Collections Practice Programs.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Java topic.
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Collections FrameworkJava Collections Frameworktitle: Collections Framework OverviewCollections FrameworkComparable vs Comparatortitle: Comparable vs ComparatorCollections FrameworkGenericstitle: Generics in JavaCollections FrameworkIteratortitle: Iterator in JavaCollections FrameworkArrayListtitle: ArrayList in JavaCollections FrameworkLinkedListtitle: LinkedList in Java