Hashtable

The Hashtable Class in Java

The Hashtable class is one of the earliest implementations of the Map interface in Java.
It stores data as key-value pairs, similar to HashMap, but it is synchronized, which makes it thread-safe.

However, Hashtable is considered a legacy class and is generally replaced by:

• HashMap (for non-threaded use)
• ConcurrentHashMap (for multi-threaded use)

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Key Characteristics

• Unique Keys -- Duplicate keys overwrite the previous value
• No Guaranteed Order -- Iteration order is unpredictable
• No Null Allowed -- Neither keys nor values can be null
• Thread Safe -- All methods are synchronized
• Legacy Class -- Introduced before Java Collections Framework (Java 1.0)

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Common Use Cases

• Legacy applications that require thread safety
• Older APIs that depend on Hashtable
• Rare cases where full method-level synchronization is required

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Important Methods

Method	Description
put(K key, V value)	Inserts key-value pair
get(Object key)	Retrieves value by key
remove(Object key)	Removes entry
containsKey(Object key)	Checks key existence
containsValue(Object value)	Checks value existence
keySet()	Returns set of keys
values()	Returns collection of values
entrySet()	Returns key-value entries

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Example 1: Basic Operations

import java.util.Hashtable;
import java.util.Map;

public class HashtableExample {

    public static void main(String[] args) {

        Map<String,Integer> fruitCounts = new Hashtable<>();

        fruitCounts.put("Apple",3);
        fruitCounts.put("Banana",5);
        fruitCounts.put("Cherry",7);

        System.out.println(fruitCounts);

        System.out.println(fruitCounts.get("Apple"));

        System.out.println(fruitCounts.containsKey("Banana"));

        fruitCounts.remove("Cherry");

        System.out.println(fruitCounts);
    }
}

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Example 2: Null Values Not Allowed

import java.util.Hashtable;

public class HashtableNullExample {

    public static void main(String[] args) {

        Hashtable<String,String> map = new Hashtable<>();

        // map.put(null,"Unknown");  // Throws NullPointerException
        // map.put("Blue",null);     // Throws NullPointerException

    }
}

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Example 3: Iteration

import java.util.Hashtable;
import java.util.Map;

public class HashtableIterationExample {

    public static void main(String[] args) {

        Map<String,Integer> scores = new Hashtable<>();

        scores.put("Alice",95);
        scores.put("Bob",87);

        for(Map.Entry<String,Integer> entry : scores.entrySet()){
            System.out.println(entry.getKey()+" : "+entry.getValue());
        }

    }
}

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Performance Characteristics

Operation	Complexity
put()	O(1)
get()	O(1)
remove()	O(1)
iteration	O(n)

Because methods are synchronized, performance is slower than HashMap.

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When to Use Hashtable

Use Hashtable only when:

• Maintaining legacy systems
• Full method-level synchronization is required

Avoid when:

• High-performance concurrent systems → use ConcurrentHashMap
• Single-threaded applications → use HashMap

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Comparison: Hashtable vs HashMap vs ConcurrentHashMap

Feature	Hashtable	HashMap	ConcurrentHashMap
Thread Safety	Yes (synchronized)	No	Yes
Performance	Slower	Fast	Very fast in concurrent systems
Null Keys	Not allowed	Allowed (1)	Not allowed
Null Values	Not allowed	Allowed	Not allowed
Era	Legacy	Modern	Modern