Why You Cannot Write new T[]

A generic class that needs an array of its own type parameter. The line looks unremarkable and the compiler refuses it outright.

java

class Box<T> {
    private T[] items = new T[10];
}

text

Box.java:2: error: generic array creation

T is gone by the time the program runs, and creating an array needs a real type at that moment — an array carries its element type with it and checks every write against it. There is nothing to carry.

What else erasure makes impossible on a type parameter
Written	Why it is refused
`new T[10]`	Array creation needs a type that still exists at runtime
`new T()`	There is no class to instantiate
`x instanceof T`	Nothing to test against
`T.class`	There is no class literal for something erased
`catch (T e)`	The handler is matched by type at runtime

The practical answer is almost always to use a collection instead. List<T> works because a list stores Object references and the compiler inserts the casts, which is exactly what an array cannot do.

java

class Box<T> {
    private final List<T> items = new ArrayList<>();
}