What is the garbage collector in Java?

Garbage Collector is part of JRE that makes sure that object that are not referenced will be freed from memory. Garbage collector can be viewed as a reference count manager. if an object is created and its reference is stored in a variable, its reference count is increased by one. during the course of execution if that variable is assigned with NULL. reference count for that object is decremented. so the current reference count for the object is 0. Now when Garbage collector is executed, It checks for the objects with reference count 0. and frees the resources allocated to it.

Advantage of Garbage Collection
  • It makes java memory efficient because garbage collector removes the unreferenced objects from heap memory.
  • It is automatically done by the garbage collector(a part of JVM) so we don't need to make extra efforts.

How can an object be unreferenced?
There are many ways:
  • By nulling the reference
    Student s=new Student();

  • By assigning a reference to another
    Student s1=new Student();
    Student s2=new Student();
    s1=s2;//now the first object referred by s1 is available for garbage collection

  • By annonymous object etc.
    new Student();
finalize() method

The finalize() method is invoked each time before the object is garbage collected. This method can be used to perform cleanup processing. This method is defined in Object class as:
protected void finalize(){}

The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created any object without new, you can use finalize method to perform cleanup processing (destroying remaining objects).

gc() method

The gc() method is used to invoke the garbage collector to perform cleanup processing. The gc() is found in System and Runtime classes.

public static void gc(){}

Many people think garbage collection collects and discards dead objects.
In reality, Java garbage collection is doing the opposite! Live objects are tracked and everything else designated garbage.

When an object is no longer used, the garbage collector reclaims the underlying memory and reuses it for future object allocation. This means there is no explicit deletion and no memory is given back to the operating system. To determine which objects are no longer in use, the JVM intermittently runs what is very aptly called a mark-and-sweep algorithm.

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