Java Performance Tips

This is an evolving tips list (or a cheatsheet) for common Java performance optimizations.

Time Optimization

• Skip the Stream Pipeline for Conversions: Don’t build a complex pipeline just to pour a bucket of water.

// ⚠️
return validTeams.stream().toList();
// Spins up a full Stream pipeline (Spliterator, ReferencePipeline, etc.) just to copy data.
// - 🐢 High Overhead: Allocates temporary objects that increase Garbage Collection pressure.
// - 🔒 Rigid: Returns an Immutable List (read-only), which creates runtime errors if modified.

// ✅
return new ArrayList<>(validTeams);
// "Teleports" data using System.arraycopy internally.
// - ⚡ Low Overhead: Zero temporary object allocation; blazing fast memory copy.
// - 🛠️ Flexible: Returns a standard Mutable List that is compatible with all Java versions.

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Space Optimization

• Optimize Sorting with In-Place Implementation

// ⚠️
final int[] sorted = Arrays.stream(nums).sorted().toArray(); 
// This spikes space usage to O(N) because it allocates a new array and creates stream objects.

// ✅
Arrays.sort(nums);
// Sorts in-place, avoiding the O(N) heap allocation.