Java Collections Utility — Free Java Tutorial

TL;DR: Learn Java Collections Utility in Java with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Java Collections Utility in Java

The java.util.Collections class provides static utility methods for working with collections: sorting, searching, shuffling, filling, copying, finding min/max, and creating immutable/synchronized wrappers.

Key methods: sort(), reverse(), shuffle(), min(), max(), frequency(), disjoint(), fill(), copy(), nCopies(), singletonList(), unmodifiableList(), synchronizedList().

Java 9+ introduced factory methods on collection interfaces: List.of(), Set.of(), Map.of() — these create immutable collections directly without needing Collections utility.

unmodifiableList() wraps a list in a read-only view — the underlying list can still be modified through the original reference. Use List.copyOf() for a truly immutable copy.

List<Integer> list = new ArrayList<>(Arrays.asList(3,1,4,1,5));
Collections.sort(list);          // [1,1,3,4,5]
Collections.reverse(list);       // [5,4,3,1,1]
Collections.shuffle(list);       // random order
Collections.max(list);           // 5
Collections.min(list);           // 1
Collections.frequency(list, 1);  // 2

List<String> immutable = List.of("a","b","c");  // Java 9+
Map<String,Integer> map = Map.of("k1",1,"k2",2);

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