Difference between size and length methods?

• .length is a field, containing the capacity (NOT the number of elements the array contains at the moment) of arrays.

• length() is a method used by Strings (amongst others), it returns the number of chars in the String; with Strings, capacity and number of containing elements (chars) have the same value.

• size() is a method implemented by all members of Collection (lists, sets, stacks,...). It returns the number of elements (NOT the capacity; some collections even don´t have a defined capacity) the collection contains.

size() is a method specified in java.util.Collection, which is then inherited by every data structure in the standard library. length is a field on any array (arrays are objects, you just don't see the class normally), and length() is a method on java.lang.String, which is just a thin wrapper on a char[] anyway.

Perhaps by design, Strings are immutable, and all of the top-level Collection subclasses are mutable. So where you see "length" you know that's constant, and where you see "size" it isn't.

length is constant which is used to find out the array storing capacity not the number of elements in the array

Example:

int[] a = new int[5]

a.length always returns 5, which is called the capacity of an array. But

number of elements in the array is called size

Example:

int[] a = new int[5]
a[0] = 10

Here the size would be 1, but a.length is still 5. Mind that there is no actual property or method called size on an array so you can't just call a.size or a.size() to get the value 1.

The size() method is available for collections, length works with arrays in Java.