If you want to use Java SDK source code, you can download a copy of OpenJDK. Since OpenJDK and Oracle JDK have very similar code base and Oracle SDK source code are not available, this will be a good choice.
In this web page, you can find the source of Java SDK source code. After downloading and unzipping, the source code is located at the following location: /jdk/src/share/classes/java/.
Inside this directory, you can read various SDK packages such as awt, io, lang, math, util, sql, etc.
Here is a snapshot of the directory.
This is not all SDK packages, other packages can be found at other similar directories (e.g. /jdk/src/share/classes/javax/).
<pre><code> String foo = "bar"; </code></pre>