> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/core_java_interview_questions/sec_6_garbage_collection.md).

# Sec 6: Garbage Collection and I\\/O Questions

## 117) What is Garbage Collection?

Garbage collection is a process of **reclaiming the runtime unused objects**.It is performed for memory management.

## 118) What is gc()?

gc() is a daemon (守护进程) thread.gc() method is defined in System class that is used to send request to JVM to perform garbage collection.

## 119) What is the purpose of finalize() method?

finalize() method is invoked just before the object is garbage collected.It is used to **perform cleanup processing**.

## 120) Can an unrefrenced objects be refrenced again?

Yes.

## 121) What kind of thread is the Garbage collector thread?

Daemon thread.

## 122) What is difference between final, finally and finalize?

![](https://1241747088-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lpv9LvBSlFaukf_ALqh%2F-Lpv9NPw3Ji1X5Vk8CES%2F-Lpv9yNxOyUxgHbdQCy3%2Fsec2_29.png?generation=1569729545443453\&alt=media)

**-final**: final is a keyword, final can be variable, method or class.You, can't change the value of final variable, can't override final method, can't inherit final class.\
**-finally**: finally block is used in exception handling. finally block is always executed.\
**-finalize()**: finalize() method is used in garbage collection.finalize() method is invoked just before the object is garbage collected.The finalize() method can be used to perform any cleanup processing.

## 123) What is the purpose of the Runtime class?

The purpose of the Runtime class is to provide access to the Java runtime system.

## 124) How will you invoke any external process in Java?

By Runtime.getRuntime().exec(?) method.

## 125) What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

The Reader/Writer class hierarchy is **character-oriented**, and the InputStream/OutputStream class hierarchy is **byte-oriented**.

## 126) What an I/O filter?

An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.