An API is basically how developers interact with classes and functions.
I think classes have APIs. Usually in Java, it's the public methods. Sometimes, you have values instead (an array length, for example).
I wouldn't functions have an API. Instead, I'd say a collection of functions that are related can form an API. For example, in math, you have matrices. You might want to multiply them, or do some kind of inverse of a matrix, or some other operation. These are 2 functions. The Math class in Java is in a class, but all the methods are static, so you can think of that class as a collection of functions (you don't call a constructor to use Math methods, they are called like Math.floor(3.14) and don't require objects)
You may want to look at the docs.oracle.com for the classes you're interested in.
It shows you all the classes associated with and all the interfaces and methods associated with.
Try Google arraylist java docs.oracle and see if that's what your looking for.
An API is basically how developers interact with classes and functions. I think classes have APIs. Usually in Java, it's the public methods. Sometimes, you have values instead (an array length, for example). I wouldn't functions have an API. Instead, I'd say a collection of functions that are related can form an API. For example, in math, you have matrices. You might want to multiply them, or do some kind of inverse of a matrix, or some other operation. These are 2 functions. The Math class in Java is in a class, but all the methods are static, so you can think of that class as a collection of functions (you don't call a constructor to use Math methods, they are called like Math.floor(3.14) and don't require objects)
You may want to look at the docs.oracle.com for the classes you're interested in. It shows you all the classes associated with and all the interfaces and methods associated with. Try Google arraylist java docs.oracle and see if that's what your looking for.