In Java, references are used to refer to objects in memory. The Java language specification defines four different types of references: strong, soft, weak, and phantom references.
Strong reference: A strong reference is the most common type of reference in Java. It is created by assigning an object to a variable or passing an object as a parameter to a method. As long as a strong reference to an object exists, the object is considered to be in use and cannot be garbage collected.
Soft reference: A soft reference is used to refer to objects that should be garbage collected only when memory is low. Soft references are created using the SoftReference class, and they are typically used for caching or other memory-sensitive applications. When memory is low, the garbage collector will attempt to reclaim memory used by objects with soft references.
Weak reference: A weak reference is used to refer to objects that should be garbage collected as soon as they are no longer in use. Weak references are created using the WeakReference class. When the garbage collector runs, it will reclaim memory used by objects with weak references.
Phantom reference: A phantom reference is used to track when an object is about to be garbage collected. Phantom references are created using the PhantomReference class. When an object with a phantom reference is about to be garbage collected, the phantom reference is added to a queue. The application can then process the queue to perform any necessary cleanup operations before the object is garbage collected.
These types of references are used for different purposes in Java. Strong references are used for regular object referencing, whereas soft and weak references are used for caching and memory-sensitive applications. Phantom references are typically used for resource management and cleanup operations.