Moving a Cast From the Moment It Runs to the Moment It Compiles

Before generics, every collection held Object. Taking something out meant casting it back, and the cast was a promise nobody checked until the line ran.

java

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

String first = (String) refs.get(1);

compiles cleanly, throws on line 5

A type parameter moves that failure forward. List<String> tells the compiler what belongs in the list, and it refuses the line that put the wrong thing in rather than the line that took it out.

java

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

String first = refs.get(0);

line 3 no longer compiles, and line 5 needs no cast

What changes when the type is a parameter
Compared on	`List`	`List<String>`
Wrong thing added	Accepted	Refused by the compiler
Taking something out	Needs a cast	Needs nothing
Where a mistake is found	At the line that reads it	At the line that wrote it

So generics do not make anything faster and do not change what is stored. They move a failure from the machine of whoever runs your code to the machine of whoever writes it, which is the only place a failure is cheap.