How can constructors call one another and copy objects?

"In both of these examples, we repeated a lot of code. One shortcut is to make all your constructors call the most general constructor. To invoke another constructor, we use the this keyword as a method call. For example:\n\n1 public Student() {\n\n2 this(null, null, 0, 0.0);\n\n3 }\n\n4\n\n5 public Student(String firstName, String lastName) {\n\n6 this(firstName, lastName, 0, 0.0);\n\n7 }\n\nAnother, very useful type of constructor is the copy constructor that allows you to make a copy of an instance by passing it to a constructor. The following example copies each of the member variables of s into this.\n\n1 public Student(Student s) {\n\n2 this(s.firstName, s.lastName, s.id, s.gpa);\n\n3 }"