Union Find Template (union by rank + path compression). Both techniques are used to optimize find method.
The below implementation is from LC disjoint set tutorial.

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// UnionFind.class
class UnionFind {
    private int[] root;
    // Use a rank array to record the height of each vertex, i.e., the "rank" of each vertex.
    private int[] rank;

    public UnionFind(int size) {
        root = new int[size];
        rank = new int[size];
        for (int i = 0; i < size; i++) {
            root[i] = i;
            rank[i] = 1; // The initial "rank" of each vertex is 1, because each of them is
                         // a standalone vertex with no connection to other vertices.
        }
    }

	// The find function here is the same as that in the disjoint set with path compression.
  // Its function is to return the root of a given node.
    public int find(int x) {
        if (x == root[x]) {
            return x;
        }
        return root[x] = find(root[x]);
    }

	// The union function with union by rank
    public void union(int x, int y) {
        int rootX = find(x);
        int rootY = find(y);
        if (rootX != rootY) {
            if (rank[rootX] > rank[rootY]) {
                root[rootY] = rootX;
            } else if (rank[rootX] < rank[rootY]) {
                root[rootX] = rootY;
            } else {
                root[rootY] = rootX;
                rank[rootX] += 1;
            }
        }
    }

    public boolean connected(int x, int y) {
        return find(x) == find(y);
    }
}

Below is my version of union find. Should remember, memorize it and use it directly and comfortably.
The below implementation is from LC 547. Number of Provinces, which is a classic Union Find problem.

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private static class UnionFind {
        private int[] cityParent;
        private int[] rank;
        private int count;

        private UnionFind(int size) {
            cityParent = new int[size + 1];
            rank = new int[size + 1];
            for (int i = 1; i < cityParent.length; i++) {
                cityParent[i] = i;
                rank[i] = 1;
            }
            count = size;
        }

        private int find(int city) {
            if (city == cityParent[city]) {
                return city;
            }
            return cityParent[city] = find(cityParent[city]);
        }

        private void union(int city1, int city2) {
            int city1Root = find(city1);
            int city2Root = find(city2);
            if (city1Root != city2Root) {
                if (rank[city1Root] < rank[city2Root]) {
                    cityParent[city1Root] = city2Root;
                } else if (rank[city1Root] > rank[city2Root]) {
                    cityParent[city2Root] = city1Root;
                } else {
                    cityParent[city2Root] = city1Root;
                    rank[city1Root] += 1;
                }
                count -= 1;
            }
        }

        private int getCount() {
            return count;
        }
}