Let's start with SVN, because 80% of projects on sourceforge google code are
managed with SVN version control system, and github projects can be downloaded directly

Simple intructions on downloading projects from sourceforge, and google code with SVN, and github.

I: find svn command
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Sourceforge:

1. go to sourceforge.net, press "search" button without input anything.

2. click on "Advanced" at the right of the search input, and select "java" as the  programming language.

3. in the generated list, go into any project. 

4. goto the "code" section in the navigation bar

5. in the code page, try to find something starting with "svn checkout/co ...", it is a command for downloading the code of the project.

6. the command has four parts "svn", "co" or "checkout", a url, and a local folder name.

Google Code:

1. goto code.google.com, press "Java" button, and a list of java projects will appear

2. goto any project in the list

3. goto the "source" part in the navigation bar

4. in the code page, try to find something starting with "svn checkout/co ...", it is a command for downloading the code of the project.

5. the command has four parts "svn", "co" or "checkout", a url, and a local folder name.

Github:

1. goto https://github.com/explore

2. type "java" the search input, and enter, a list of projects will show up. 

3. on the left side, choose Java as the programming language

4. goto any project in the list

5. on the right side, click on "Download ZIP"

6. extract the zip file to a desired place, then there is no need for using svn tool, you already get the code


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II: work with svn tool

linux:

1. run sudo apt install svn

2. make a diretory at a suitable place, say /home/xwang/projects/aaa

3. under the directory in 2, run the svn command.

windows:

1. download and install Tortoise SVN: from http://tortoisesvn.net/

2. after you installed Tortoise svn, make a directory at a suitable place, say D:\projects\aaa.

3. right click on the folder "aaa", click on svn checkout, and intput the url in the svn command, then click ok.