System is a predefined final class, out is a PrintStream object and println is a built-in overloaded method in the out object.

A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement (post test) checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the loop body at least once.

A native method is a method that is implemented in a language other than Java.

The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. 
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.

A class containing abstract method is called Abstract class. An Abstract class can't be instantiated. 
Example of Abstract class:
abstract class testAbstractClass { 
    protected String myString; 
    public String getMyString() { 
        return myString; 
        } 
    public abstract string anyAbstractFunction();
}

In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Emaple of Interface:

public interface sampleInterface {
    public void functionOne();

    public long CONSTANT_ONE = 1000;
}

There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.

There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.