Type Erasure

intermediate13 min readLesson 73 of 180

Why List<String> and List<Integer> are one class at runtime, and the compile rules erasure forces on arrays, overloads, and instanceof.

Type erasure — what generics really are

Generics exist at compile time only. List<String> and List<Integer> are the same class at runtime — both erase to List. Consequences:

List<String> a = new ArrayList<>();
List<Integer> b = new ArrayList<>();
a.getClass() == b.getClass()   // true!

// can't do these:
// new T[10]                       — no T at runtime
// if (x instanceof List<String>)  — erased
// catch (MyEx<String> e)          — generics in catch are banned

Practical rules:

Arrays of generic type are unsafe (new T[10] won't compile; use ArrayList<T>)
You cannot overload on erased parameters: f(List<String>) + f(List<Integer>) — same erasure, compile error
Need runtime type info? Pass a Class<T> token: <T> T parse(String s, Class<T> type)

Erasure explains why wildcards exist: flexibility must be expressible in the type system without runtime cost.

Now practice

Generics LabPECS signatures in action, a Comparable-bounded max, and the Class-token erasure workaround.3 challenges · · ~40 min