The object nothing is left uninitialized. They are used to initialize the object's data. What this implies is that objects can be redirected to point at different data, and data can be pointed at by multiple objects. In addition to constructors, classes usually have methods defined for them.
Methods are functions or procedures which manipulate an object's data.
For example, for the String class we have many different predefined methods. It should be noted that these methods are predefined as part of the Java language, but later we will see that we have the capability to define our own classes, and the methods to manipulate them.