what is java API?

Answer 1

In simple words
Api have pre-written class and method that helps programmer to write efficient code and reduce boilerplate code.

Answer 2

Java application programming interface(API) is a list of all classes that are part of the Java development kit(JDK). It includes all Java packages, classes, and interfaces,along with their methods, fields,and constructors.These prewritten classes provide a tremendous amount of functionality to a programmer. A programmer should be aware of these classes and should know how to use them.

More details,
https://resources.saylor.org/wwwresources/archived/site/wp-content/uploads/2013/02/CS101-1.3.5.3-Java-Application-Programming-Interface-API-FINAL.pdf