| 16. Class and object |
| What it is | A blueprint, a user-defined data type | An instance made from that blueprint |
| Memory | No memory for data until objects are made | Takes memory for its own data members when made with new |
| 17. Primitive and composite data types |
| Built from | Built into Java | Made from primitive and other types |
| Example and size | int, char, double, with fixed sizes | String, arrays, classes, sizes vary |
| 18. Call by value and call by reference |
| What is passed | A copy of the value (primitives) | A reference to the same object (arrays, objects) |
| Effect on caller | Changes inside the method do not reach the caller | Changes to the object inside the method are seen by the caller |
| 19. Pure and impure method |
| Changes state | No: it changes nothing outside itself | Yes: it changes a data member or a passed object |
| Same input | Always gives the same result | Its effect may differ as the state changes |
| 20. Actual and formal parameters |
| Found in | The method call | The method definition |
| Example | 5 in area(5) | s in int area(int s) |
| 21. Static and non-static method |
| Called by | The class name, with no object needed | An object of the class |
| Can use | Only static members directly | Both static and instance members |
| 22. void method and method with a return type |
| Returns | Nothing | One value of the stated type |
| Call used as | A statement on its own | A value inside an expression or assignment |
| 23. Constructor and method |
| Name and return type | Same name as the class, no return type | Any name, must declare a return type or void |
| How it runs | Automatically, when new makes an object | Only when called by name |
| 24. Default and parameterised constructor |
| Parameters | None | One or more |
| Initial values | Fixed or default values for every object | Values passed in, so each object can differ |
| 25. Autoboxing and unboxing |
| Converts | A primitive into its wrapper object | A wrapper object back into its primitive |
| Example | Integer k = 25; | int j = k; |
| 26. Primitive type and wrapper class |
| Nature | A plain value such as int | An object such as Integer holding that value |
| Methods | Has none | Has methods such as parseInt and toString |
| 27. Integer.parseInt() and Integer.valueOf() |
| Returns | A primitive int | An Integer object |
| Example | int a = Integer.parseInt("15"); | Integer b = Integer.valueOf("15"); |
| 28. Character.isUpperCase() and Character.toUpperCase() |
| Returns | boolean: is it a capital? | char: the capital form |
| Example | isUpperCase('k') is false | toUpperCase('k') is 'K' |
| 29. private and public |
| Visible in | Its own class only | Every class |
| Typical use | Data members, to hide them | Methods that other classes must call |
| 30. Local and instance variable |
| Declared | Inside a method or block | Inside the class, outside every method |
| Lifetime | Only while the method or block runs | As long as the object exists |
| 31. Instance and class (static) variable |
| Copies | One per object | One shared by every object |
| Declared with | No static keyword | The static keyword |
| 32. Encapsulation and abstraction |
| Means | Binding data with its methods and hiding the data | Showing only essential features, hiding details of how |
| Achieved by | A class with private data and public methods | Exposing what an object does, not how it does it |
