HashTable | Learn for Master

Java HashMap Examples

August 1, 2016 Author: david

In this post, I walk through the concept of Java HashMap, and use examples to describe its usage cases.

What is HashMap

Java HashMap is a Hash table based implementation of the Java Map interface. It provides all of the optional map operations, and permits null values and the null key. The order of Keys in a HashMap is not guaranteed. So we should not reply on the order of the keys in a HashMap.

HashMap vs Hashtable

Based on java doc, Java HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.

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Java Map vs HashMap vs TreeMap vs LinkedHashMap

August 1, 2016 Author: david

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Java Map

Java Map is an interface with the following signature.

public interface Map<K,V>

Here are some properties of Java Map:

It defines an operation to map keys to values.
A map cannot contain duplicate keys; each key can map to at most one value.
The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings.
The order of a map is dependent on its implementations.

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Count word frequency

July 30, 2016 Author: david

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Count word frequency is a popular task for text analysis. In this post, I describe how to count word frequency using Java HashMap, python dictionary, and Spark.

Use Java HashMap to Count Word frequency

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class JavaHashMapExample {
    public Map<String, Integer> countFreq(List<String> list){
        Map<String, Integer> histogram = new HashMap<>();
        for(String word: list) {

            if (!histogram.containsKey(word)) {
                histogram.put(word, 1);
            } else {
                histogram.put(word, histogram.get(word) + 1);
            }
        }
        return histogram;
    }

    public static void main(String[] args) {
        String[] a = {"a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"};
        List<String> wordList = Arrays.asList(a);
        System.out.println(new JavaHashMapExample().countFreq(wordList));
     }
}

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import java.util.Arrays;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

public class JavaHashMapExample {

public Map<String, Integer> countFreq(List<String> list){

Map<String, Integer> histogram = new HashMap<>();

for(String word: list) {

if (!histogram.containsKey(word)) {

histogram.put(word, 1);

} else {

histogram.put(word, histogram.get(word) + 1);

}

return histogram;

}

public static void main(String[] args) {

String[] a = {"a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"};

List<String> wordList = Arrays.asList(a);

System.out.println(new JavaHashMapExample().countFreq(wordList));

}

{a=5, b=2, c=6, d=3}

Use Python Dict to count word frequency

words = ["a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"]

freq = {}

for w in words:
    if w in freq:
        freq[w] += 1
    else:
        freq[w] = 1

print freq

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words = ["a", "b", "c", "d", "a", "b", "a", "d", "a", "c", "c", "d", "a", "c", "c", "c"]

freq = {}

for w in words:

if w in freq:

freq[w] += 1

else:

freq[w] = 1

print freq

The output:

{‘a’: 5, ‘c’: 6, ‘b’: 2, ‘d’: 3}

Use Spark to count word Frequency

The above method works well for small dataset. However, if you have a huge dataset, the hashTable based method will not work. You will need to develop a distributed program to accomplish this task.

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Java for Loop

July 30, 2016 Author: david

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In this post, I describe how to do for loop on an Java ArrayList. Array, and HashMap.

Java Array for loop

In the following example, we create an int array, then loop it to add numbers from 1 to 100. Then we loop it again to calculate the sum of the numbers in the array.

// Create an array with room for 100 integers
int[] nums = new int[100];

// Fill it with numbers using a for-loop
for (int i = 0; i < nums.length; i++)
    nums[i] = i + 1;  // +1 since we want 1-100 and not 0-99

// Compute sum
int sum = 0;
for (int n : nums)
    sum += n;

// Print the result (5050)
System.out.println(sum);

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// Create an array with room for 100 integers

int[] nums = new int[100];

// Fill it with numbers using a for-loop

for (int i = 0; i < nums.length; i++)

nums[i] = i + 1; // +1 since we want 1-100 and not 0-99

// Compute sum

int sum = 0;

for (int n : nums)

sum += n;

// Print the result (5050)

System.out.println(sum);

Java List for loop

We provide four ways to show how to loop a java List.

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class JavaLoopExample {
    public static void main(String[] argv) {

        String sArray[] = new String[] { "Value 1", "Value 2", "Value 3" };

        // convert array to list
        List<String> list = Arrays.asList(sArray);

        // iterator loop
        System.out.println("iterator.hasNext()");
        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

        // for loop
        System.out.println("for loop");
        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }

        // for loop advance
        System.out.println("for loop advance");
        for (String temp : list) {
            System.out.println(temp);
        }

        // while loop;
        System.out.println("while advance");
        int j = 0;
        while (j < list.size()) {
            System.out.println(list.get(j));
            j++;
        }
    }
}

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import java.util.Arrays;

import java.util.Iterator;

import java.util.List;

public class JavaLoopExample {

public static void main(String[] argv) {

String sArray[] = new String[] { "Value 1", "Value 2", "Value 3" };

// convert array to list

List<String> list = Arrays.asList(sArray);

// iterator loop

System.out.println("iterator.hasNext()");

Iterator<String> iterator = list.iterator();

while (iterator.hasNext()) {

System.out.println(iterator.next());

}

// for loop

System.out.println("for loop");

for (int i = 0; i < list.size(); i++) {

System.out.println(list.get(i));

}

// for loop advance

System.out.println("for loop advance");

for (String temp : list) {

System.out.println(temp);

}

// while loop;

System.out.println("while advance");

int j = 0;

while (j < list.size()) {

System.out.println(list.get(j));

j++;

}

iterator.hasNext()
Value 1
Value 2
Value 3
for loop
Value 1
Value 2
Value 3
for loop advance
Value 1
Value 2
Value 3
while advance
Value 1
Value 2
Value 3

Java HashMap for loop

If you’re only interested in the keys,

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