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StandardsJava is case sensitive, so thisCustomer is different to ThisCustomer.
In OO languages, of which Java is one, classes are usually capitialized (first letter is upper case), and method names started with a lower case character.
Objects (instance of classes) names also start with a lower case letter.
In Java, classes are packaged in packages are these must be imported into an application via import statement.
For example:
import java.awt.*;
This would make available to this program all packages starting with java.awt, and therefore all classes within those packages.
import java.awt.Point; // imports only one Package
The import statement is at the start of a Java application, and of course several import statements are possible:
import java.awt.*; import java.io.*;
The books recommended at this site by Patrick Chan, give a description, in over 2500 pages, of several packages in Java.