Synchronized collections

The synchronized collections are collections with the atomic operations. This means, only one thread can perform a collection operation at time.

You can make synchronized collection from any non-synchronized collection by Collections.synchronizedXXX() methods.

You must explicitly synchronize the collection when traversing it. Otherwise, you might get a ConcurrentModificationException if another thread modifies the collection.

List<String> lst = Collections.synchronizedList(new LinkedList<>());
    for(String el: lst){
        // ...
Map<String, String> syncMap = Collections.
    synchronizedMap(new HashMap<String, String>);
Set<String> keys = m.keySet();  // Needn't be in synchronized block

synchronized(syncMap) {  // Synchronizing on syncMap, not keys!
    Iterator i = s.iterator(); // Must be in synchronized block
    while (i.hasNext())


Synchronized list based on array. version of ArrayList. If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector.


Represents a last-in-first-out (LIFO) stack of objects. It extends class Vector with five operations that allow a vector to be treated as a stack.

method description
empty() Tests if this stack is empty.
peek() Looks at the object at the top of this stack without removing it from the stack.
pop() Removes the object at the top of this stack and returns that object.
push(item) Pushes an item onto the top of this stack.
search(o) Returns the 1-based position where an object is on this stack. Value -1 indicates that the object is not on the stack.


Synchronized implementation of Map. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.