Why a List<String> Is Not a List<Object>

A String is an Object, and every method taking an Object accepts one. A List<String> handed to a method taking List<Object> is refused, which reads like an inconsistency.

text

Report.java:8: error: incompatible types: List<String> cannot be converted to List<Object>

Imagine it were allowed. The method would be holding something it believes is a list of objects, and nothing would stop it from doing this.

java

void spoil(List<Object> anything) {
    anything.add(42);
}

List<String> refs = new ArrayList<>();
spoil(refs);

String first = refs.get(0);

Line 2 is legal for a list of objects. Line 8 would then fail with a ClassCastException on a list whose declaration promised it held only strings — which is exactly the failure generics exist to remove.

When a method genuinely only reads, say so with a wildcard: List<? extends Object>, usually written List<?>. That accepts a list of anything and refuses to add to it, which is the combination that makes it safe.