corejava-inheritance: How can you say that java does not support multiple inheritance?

First i think you should focus on the actual meaning of multiple inheritance - Multiple inheritance is about multiple-direct-inheritance. That is a class can have two immediate parents. In java no class is allowed to have two immediate parent classes.
A class that extends that other class, but it extends Object, too, so you are still in one line of inheritance, not multiple inheritance. A single class can´t have two immediate parent classes it can have grandparent class though. Let us make it better understandable by following example:

Note:
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If A extends B and B extends C, is not the same thing as A extends both B and C. In java a class that has no parent class by default its parent would be Object class. Consider following example:

class Animal {
}

class Cat extends Animal {
}

class Tiger extends Cat {
}

In the above example Tiger inherits from Cat which inherits from Animal which (by default) inherits from Object. In above case Object class is the parent of Animal and grandparent of Cat class. So in this way we says that Object class is the parent of all classes.