In java, that string is an object. That's a design choice.

Object references can be null and are null if there is no object assigned to it. If you call such an object, a NullPointerException is thrown. That's standard oo-behaviour.

Java designers made the choice of having strings as object and having those methods throwing null-pointer exceptions is what you get if you want strings to be objects.

Why didn't Java designers think of this when they designed the String class in Java 1 or Java 2, or even add such a functionality in a later Java version?

They probably did think about it and chose to incorporate oo-behaviour.

In java, that string is an object. That's a design choice.

Object references can be null and are null if there is no object assigned to it. If you call such an object, a NullPointerException is thrown. That's standard oo-behaviour.

Java designers made the choice of having strings as object and having those methods throwing null-pointer exceptions is what you get if you want strings to be objects.

Why didn't Java designers think of this when they designed the String class in Java 1 or Java 2, or even add such a functionality in a later Java version?

They probably did think about it and chose to incorporate oo-behaviour.