In Eclipse, when you press Ctrl button and click on any Class names, the IDE will take you to the source file for that class. This is the normal behavior for the classes you have in your project.

But, in case you want the same behavior for Java’s core classes too, you can have it by attaching the Java source with the Eclipse IDE. Once you attach the source, thereafter when you Ctrl+Click any Java class names (String for example), Eclipse will open the source code of that class.

To attach the Java source code with Eclipse,

1. When you install the JDK, you must have selected the option to install the Java source files too. This will copy the src.zip file in the installation directory.
2. In Eclipse, go to Window -> Preferences -> Java -> Installed JREs -> Add and choose the JDK you have in your system.
3. Eclipse will now list the JARs found in the dialog box. There, select the rt.jar and choose Source Attachment. By default, this will be pointing to the correct src.zip. If not, choose the src.zip file which you have in your java installation directory.
4. Similarly, if you have the javadoc downloaded in your machine, you can configure that too in this dialog box.

 
Done! Here after, for all the projects for which you are using the above JDK, you’ll be able to browse the Java’s source code just like how you browse your own code.

Reference: Attaching Java source with Eclipse IDE from Veera Sundar one of our JCG partners.

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