Difference between WeakReference vs SoftReference vs PhantomReference vs Strong reference in Java

WeakReference and SoftReference were added into Java API from long time but not every Java programmer is familiar with it. Which means there is a gap between where and how to use WeakReference and SoftReference in Java. Reference classes are particularly important in context of How Garbage collection works. As we all know that Garbage Collector reclaims memory from objects which are eligible for garbage collection, but not many programmer knows that this eligibility is decided based upon which kind of references are pointing to that object. This is also main difference between WeakReference and SoftReference in Java. Garbage collector can collect an object if only weak references are pointing towards it and they are eagerly collected, on the other hand Objects with SoftReference are collected when JVM absolutely needs memory. These special behaviour of SoftReference and WeakReference makes them useful in certain cases e.g. SoftReference looks perfect for implementing caches, so when JVM needs memory it removes object which have only SoftReference pointing towards them. On the other hand WeakReference is great for storing meta data e.g. storing ClassLoader reference. If no class is loaded then no point in keeping reference of ClassLoader, a WeakReference makes ClassLoader eligible for Garbage collection as soon as last strong reference removed. In this article we will explore some more about various reference in Java e.g. Strong reference and Phantom reference.

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How to Clone Collection in Java - Deep copy of ArrayList and HashSet

Programmer often mistook copy constructors provided by various collection classes, as a mean to clone Collection e.g. List, Set, ArrayList, HashSet or any other implementation. What is worth remembering is that, copy constructor of Collection in Java only provides shallow copy and not deep copy, which means objects stored in both original List and cloned List will be same and point to same memory location in Java heap. One thing, which adds into this misconception is shallow copy of Collections with Immutable Objects. Since Immutable can't be changed, It's Ok even if two collections are pointing to same object. This is exactly the case of String contained in pool, update on one will not affect other. Problem arise, when we use Copy constructor of ArrayList to create a clone of List of Employees, where Employee is not Immutable. In this case, if original collection modifies an employee, that change will also reflect into cloned collection. Similarly if an employee is modified in cloned collection, it will also appeared as modified in original collection. This is not desirable, in almost all cases, clone should be independent of original object. Solution to avoid this problem is deep cloning of collection, which means recursively cloning object, until you reached to primitive or Immutable. In this article, we will take a look at one approach of deep copying Collection classes e.g. ArrayList or HashSet in Java. By the way, If you know difference between shallow copy and deep copy, it would be very easy to understand how deep cloning of collection works.

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