Что ему не нравится?
package com.javarush.task.task20.task2028;
import java.io.Serializable;
import java.util.AbstractList;
import java.util.Collection;
import java.util.List;
/*
Построй дерево(1)
*/
public abstract class CustomTree extends AbstractList<String> implements Cloneable, Serializable {
@Override
public String get(int index){
throw new UnsupportedOperationException();
}
@Override
public String set(int index, String element){
throw new UnsupportedOperationException();
}
@Override
public void add(int index, String element){
throw new UnsupportedOperationException();
}
@Override
public String remove(int index){
throw new UnsupportedOperationException();
}
@Override
public List<String> subList(int fromIndex, int toIndex){
throw new UnsupportedOperationException();
}
@Override
protected void removeRange(int fromIndex, int toIndex){
throw new UnsupportedOperationException();
}
@Override
public boolean addAll(int index, Collection<? extends String> c){
throw new UnsupportedOperationException();
}
}