Look at Library Source Code
All of the software stack that I use is open source, so I can hit F4 in my IDE, and see the inner workings of every library.
However, you have to go to the effort of downloading the source code, and setting up your IDE so that it knows where the source code is. Without this extra step, my IDE will decompile class files into Java. Don't do it! The extra effort is worth it. Reading the Javadocs is useful, and decompiled class files don't have meaningful names for variables.
Looking at the inner workings of libraries is worthwhile. Not only do you get a better understanding of how they work, but as a bonus, you will immerse yourself in other people's code.
You will learn new things. You can compare your programming style with others. If you find code that is hard to read, make sure you don't make the same mistake!