Java - Trie (Prefix Tree)

Posted on by Kenny Cason
tags = [ java, tree, trie, prefix tree ]

A Java implmentation of a Trie (i.e. Prefix Tree). A definition of a Trie can be found This implementation (ObjectTrie.java) is capable of storing an Array of any generic data. i.e. Object[], or Integer[], etc. In the end I also include a wrapper Trie.java that wraps ObjectTrie.java providing String support.

Trie.java

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package krunch.lib.ds.trie;

public class Trie {
    
    private ObjectTrie<Character> trie;
    
    public Trie() {
        trie = new ObjectTrie<Character>(' ');
    }
    
    public void insert(String s) {
        trie.insert(toArray(s));
    }
    
    public boolean search(String s) {
        return trie.search(toArray(s));
    }
    
    public int numberEntries() {
        return trie.numberEntries();
    }
    
    private Character[] toArray(String s) {
        Character[] cArray = new Character[s.length()];
        for(int i = 0; i < cArray.length; i++) {
            cArray[i] = s.charAt(i);
        }
        return cArray;
    }
    
    public String toString() {
        return trie.toString();
    }

}

Node.java

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package krunch.lib.ds.trie;

import java.util.ArrayList;

public class Node<T> {
    
    private T value;
    
    private boolean endMarker;
    
    public ArrayList<Node<T>> children;
    
    
    public Node(T value) {
        this.value = value;
        this.endMarker = false;
        this.children = new ArrayList<Node<T>>();
    }
    
    public Node<T> findChild(T value) {
        if(children != null) {
            for(Node<T> n : children) {
                if(n.getValue().equals(value)) {
                    return n;
                }
            }
        }
        return null;
    }
    
    public T getValue() {
        return value;
    }
    
    public void setEndMarker(boolean endMarker) {
        this.endMarker = endMarker;
    }
    
    public boolean isEndMarker() {
        return endMarker;
    }

    public Node<T> addChild(T value) {
        Node<T> n = new Node<T>(value);
        children.add(n);
        return n;
    }
    
}

ObjectTrie.java

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package krunch.lib.ds.trie;

public class ObjectTrie<T> {

    private Node<T> root;

    private int numberEntries;

    public ObjectTrie(T rootNodeValue) {
        root = new Node<T>(rootNodeValue); // "empty value", usually some "null"  value or "empty string"
        numberEntries = 0;
    }

    public void insert(T[] values) {
        Node<T> current = root;
        if (values != null) {
            if (values.length == 0) { // "empty value"
                current.setEndMarker(true);
            }
            for (int i = 0; i < values.length; i++) {
                Node<T> child = current.findChild(values[i]);
                if (child != null) {
                    current = child;
                } else {
                    current = current.addChild(values[i]);
                }
                if (i == values.length - 1) {
                    if (!current.isEndMarker()) {
                        current.setEndMarker(true);
                        numberEntries++;
                    }
                }
            }
        } else {
            System.out.println("Not adding anything");
        }
    }

    public boolean search(T[] values) {
        Node<T> current = root;
        for (int i = 0; i < values.length; i++) {
            if (current.findChild(values[i]) == null) {
                return false;
            } else {
                current = current.findChild(values[i]);
            }
        }
        /*
         * Array T[] values found in ObjectTrie. Must verify that the "endMarker" flag
         * is true
         */
        if (current.isEndMarker()) {
            return true;
        } else {
            return false;
        }
    }

    public int numberEntries() {
        return numberEntries;
    }
    
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("number entries: ");
        sb.append(numberEntries);
        
        return sb.toString();
    }

}

TrieTest.java (Unit Test)

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package krunch.lib.utils.ds.trie;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import java.util.HashMap;

import krunch.lib.ds.trie.Trie;
import krunch.lib.ds.trie.ObjectTrie;

import org.junit.Test;

public class TrieTest {

    @Test
    public void testStringTrie() {
        Trie trie = new Trie();
        assertEquals(0, trie.numberEntries());
        trie.insert("HELLO");
        assertEquals(1, trie.numberEntries());
        trie.insert("HELLO");                    // duplicate words do not count
        assertEquals(1, trie.numberEntries());
        trie.insert("FROG");
        assertEquals(2, trie.numberEntries());    
        
        assertTrue(trie.search("HELLO"));        // should find it
        assertTrue(trie.search("FROG"));        // should find it
        assertFalse(trie.search("HEL"));        // not a full word
    }
    
    
    @Test
    public void testObjectTrie() {    
        ObjectTrie<Object> trie = new ObjectTrie<Object>(new Object());
        Object o1 = 12;
        Object o2 = "ASFAS";
        Object o3 = new Character('X');
        Object o4 = new HashMap<String, String>();
        
        Object[] oArray1 = new Object[] {o1, o1, o2, o3, o3, o4};
        Object[] oArray2 = new Object[] {o1, o2, o3, o4, o4, o1};
        Object[] oArray3 = new Object[] {o1, o1, o2, o2};
        
        assertEquals(0, trie.numberEntries());
        trie.insert(oArray1);
        assertEquals(1, trie.numberEntries());
        trie.insert(oArray1);                    // duplicate words do not count
        assertEquals(1, trie.numberEntries());
        trie.insert(oArray2);
        assertEquals(2, trie.numberEntries());    
        
        assertTrue(trie.search(oArray1));        // should find it
        assertTrue(trie.search(oArray2));        // should find it
        assertFalse(trie.search(oArray3));        // not a full word
    }

}
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