...using methods that apply to only a single type.

The important thinking/ learning here concerns...

• What information is available to the compiler when it compiles a .java file?
• What information is available to the JVM when it runs the instructions that the compiler produces?

The assignment attempts to help people think about these questions by...

• isolating catElementsStartingWith, into a file (Accumulator.java) away from all the other code; and...
• asking you to compile that file separately, with a command like javac Accumulator.java.

Maybe this isolation can help people understand that the compiler only knows what it can infer from that file and any .class files it depends on.

For example the compiler knows nothing about the data in a list. That data can change when the programmer changes UserOfList.java, after compiling Accumulator.java. So the compiler can make no decisions that required knowing the data. The user of Accumulator could make a list that contained only Strings. But the compiler cannot know that. So the compiler's decisions on what Java code is ok cannot depend on the type of data in the list.

The compiler always compiles one file at a time, even when you issue a command like javac UserOfList.java that can result in its compiling compiling several in succession. So the thinking done in this assignment is broadly useful.