CODEWITHSUNDEEP
java
user468687
user468687

Reputation:

In Java why does Arrays.copyOf source code sometimes use reflection?

I'm having trouble understanding this bit of code in java.util.Arrays in openjdk. I'm curious why this code uses reflection (Array.newInstance) and why it checks to see if the types are the same? Why can't it just do T[] copy = (T[]) new Object[newLength] even if the type T and U differ?

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

Upvotes: 1

Views: 857

Answers (2)

Axel
Axel

Reputation: 14169

You cannot instantiate an Array from a generic type like this new T[newLength]. So all you can do is new Object[newLength]. But Arrays do store information about the item type in order to be able to throw ArrayStoreException, which would be lost in this case.

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1502686

It can - but then you'd end up with an Object[] even if you'd asked for a String[]. So this should fail:

String[] strings = Arrays.copyOf(existingArray, 10, String[].class);
Object[] objects = strings;
objects = new Object();

... but it wouldn't if Array.copyOf always actually returned an Object[].

Upvotes: 4

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