Why class is known as composite data type?

Answer 1

A composite data type is one which is composed with various primitive data type. A class defined with various primitive data types such as int, double etc; so it is known as a composite data type; and it is used to create objects which hold similar types of values and behaviours (functions).
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Answer 2

Because it can have other data types inside. For example:

class Student {
private int number;
private String name;
}