length vs length() vs size(): All the three length, length() and size() give the number of elements present but in different contexts.
A) length variable:
In Java, array (not java.util.Array) is a predefined class in the language itself. To find the elements of an array, designers used length variable (length is a field member in the predefined class). They must have given length() itself to have uniformity in Java; but did not. The reason is by performance, executing length variable is speedier than calling the method length(). It is like comparing two strings with == and equals(). equals() is a method call which takes more time than executing == operator.
int student = new int;
System.out.println(student.length); // prints 5
B) length() method:
It is a method defined in String class. It gives the number of characters present in the string.
String str = "hello";
System.out.println(str.length()); // prints 5
C) size() method:
It is used to find the number of elements present in collection classes. It is defined in java.util.Collection interface.
ArrayList al = new ArrayList();
System.out.println(al.size()); // prints 0 as no elements are added yet
A fresher should be careful while using length vs length() vs size().