A good reference helps you answer a specific question and tells you which Java version its answer covers. Use a book for connected explanation, the versioned API documentation for a library contract, and a small program to check your interpretation. This guide demonstrates that routine with a list-copy question.
Choose a reference for the task
Write down the project JDK, build target and question first. Run java --version and javac --version, then check the build configuration: a project running on one JDK may compile for an older release. Use the setup guide if those disagree.
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| Question | Best starting point | Check before relying on it |
|---|---|---|
| How do these concepts fit together? | A well-structured book or official lesson | Edition, prerequisites, exercises and errata. |
| What does this method promise? | Versioned API documentation | Parameters, return value, exceptions and since/deprecation notes. |
| Is this syntax valid? | The Java Language Specification for the target release | Whether preview features are involved. |
| Why did a release change behavior? | Migration guidance and the relevant JEP | Delivered release and documented compatibility impact. |
Before buying a book, inspect a sample chapter and table of contents. Compile one available example for your target release, check whether the author maintains errata, and decide whether its focus matches your work. An older edition can explain enduring ideas well; keep a separate list of version-sensitive examples to verify. Coverage centered on applet deployment or an obsolete exam needs historical context.
Follow one API question to its answer
Suppose a method receives a mutable list, and you want to retain its current membership. Open the Java 25 List documentation, find copyOf, and read its complete contract. Record three questions: can the resulting list be structurally changed, can it contain nulls, and what happens if an element itself is mutable?
List.copyOf returns an unmodifiable list, rejects null elements, and reflects the source elements at the time of copying. The elements themselves are shared objects. This distinguishes a membership snapshot from a deep copy. Do not infer that every call creates a distinct object: the contract permits reuse in some circumstances.
Download the source, checks and README (ZIP)
javac ReferenceCheck.java
java ReferenceChecksource=3, snapshot=2
snapshot mutation rejected
shared element=oak!
null element rejectedThe program adds a third item to the source after copying, then attempts to add to the snapshot. It also copies a list containing a mutable StringBuilder and changes that builder. Seeing oak! demonstrates shared element identity. The final case checks the documented null restriction. These are observations of named cases, not proof of every possible property.
Separate a library contract from language rules
An unfamiliar API and unfamiliar syntax often appear together. Isolate them. Replace a complex pipeline with a few named variables, or try the syntax in a tiny file. Use the Java SE 25 Language Specification for language rules, and the javac manual for compiler options such as --release. Check the specification for the project version rather than assuming the newest syntax is accepted by its build.
For example, a successful run on Java 25 does not establish that an API exists in Java 8. Compile against the intended target and resolve any unavailable symbols. Keep preview features explicit in teaching notes so that a normal compile command remains reproducible.
Keep a short evidence note
Save the question, documentation URL and version, relevant method name, smallest check, expected output and actual result. Include a counterexample you tried. When the project upgrades, that note makes a focused recheck possible.
For the list example, the useful conclusion is: “Store a membership snapshot; retain awareness that mutable elements are shared.” If the program needs independent element state, define and test an element-copy policy. The reference supplied the contract, while the example exposed the practical consequence for your design.
Use the downloadable README as a lookup checklist. When asking a colleague for help, include the small program and version output instead of a large screenshot of the entire application.