When and why do we use abstract classes or interfaces?

Question

I have found that interfaces are also called "contracts" and include the main elements (variables and methods) to be implemented in the program.
My question relates to the initial stage of designing a program, in which we plan the implementation of the tasks.
https://softwareengineering.stackexchange.com/questions/336543/is-design-by-contract-the-same-as-using-interfaces-to-create-a-contract

Answer 1

Have a look on the link in this thread. Hope it helps.

https://www.sololearn.com/Discuss/1780241/?ref=app

Answer 2

Interfaces are very useful to let the user to override methods. Interfaces are used a lot in event handling, and java.util.Function package.

Answer 3

Using the keyword interface, you can fully abstract a class’ interface from its implementation.
That is, using interface, you can specify what a class must do, but not how it does it. Interfaces are syntactically similar to classes, but they lack instance variables, and their methods are declared without any body. In practice, this means that you can define interfaces that don’t make assumptions about how they are implemented. Once it is defined, any number of classes can implement an interface. Also, one class can implement any number of interfaces.

https://gopinadhs.files.wordpress.com/2012/07/java-the-complete-reference-7th-edition.pdf