A List of Strings That Cannot Be Handed an Integer

One declaration, and the error that arrives before anything runs.

java

List<String> refs = new ArrayList<>();
refs.add("AB-1");
refs.add(42);

text

Refs.java:3: error: incompatible types: int cannot be converted to String
        refs.add(42);
                 ^

The error names the line that made the mistake. Without the type parameter this would have compiled, run, and thrown a ClassCastException somewhere else entirely — in a method that reads the list, possibly in a different file.

What to notice is the <> on the right. The compiler infers the type argument from the declaration, so writing new ArrayList<String>() is allowed and never necessary.

Java
Generics