Have we seen Objects before?

[comp1110-85]
  • Fabian Muehlboeck

Yes! Every string and array is an object. When we wrote guess.charAt(5), we were calling a method of the String class on the particular String object referred to by guess.

Similarly, the difference between arr.length for arrays and str.length() for strings is that the former is a field that we can access, and the latter is a method we have to call.

Context

    Construction. Class Definitions

    [comp1110-80]
    • Fabian Muehlboeck

    In Java, objects are instances of classes. A class definition is a kind of data definition, similar to what we have seen before for enums. However, classes are much more powerful, and they are used to model pretty much everything in our programs that cannot be modeled by a simpler data type. The core parts we will deal with for now are fields, which contain an object's data, and methods, which define an object's behaviour. The basic definition of a class consists of the keyword class and a class name, followed by curly braces, like so:

    class Person { //this is where the fields and methods of a class go }

    This does define a new type Person. To create a new instance of it (i.e. an object), you could write something simliar to what we did for arrays, using the keyword new:

    Person p = new Person();

    This is not very useful, because the Person class above has no data or behaviour, but you can add those.

    By convention, class names in Java should be nouns that start with a capital letter, and use capital letters to mark the start of any word internal to the name. For example: LectureNote .

    Construction. Fields

    [comp1110-81]
    • Fabian Muehlboeck

    Every class can have arbitrarily many fields. At the very least, each field has a type and name, just like a local variable. There are some other properties that fields can have, which we'll see later. To define a field, you declare it just like a local variable:

    String name;

    You can also give the field a default value, just like a local variable definitions:

    String name = "Unknown";

    By convention, in Java, field names are camelCased. That is, they start with a lower-case letter and otherwise contain only letters and digits, with uppercase letters indicating the start of a new combined word, as in sumOfAllParts.

    In principle, you can read and write the fields of an object similar to how you can read and write individual array elements, though the syntax is more like when you ask an array for its length:

    String oname = obj.name; if(oname.equals("Unknown")) { obj.name = "Timmy"; }

    Warning. 

    • Fabian Muehlboeck
    Note that array.length has the special property that you cannot assign values to it.

    Example. Fields

    [comp1110-82]
    • Fabian Muehlboeck

    Say we want our Person class from earlier to have two fields: one representing the name of the person, and one representing their age. Names would typically be represented as Strings, and in our case, we only care about completed years of age, so we use int as the type for that field.

    class Person { int age; String name; }

    We can then create a new Person value and write to its fields:

    Person p = new Person(); p.name = "Timmy"; p.age = 5;

    Warning. 

    • Fabian Muehlboeck
    This is not how you would typically do this; we'll learn about the right way to do this later.

    Construction. Methods

    [comp1110-83]
    • Fabian Muehlboeck

    So far, we have sometimes written code in separate functions from the main function. But in every case so far, we had to give those functions all the arguments that they were supposed to work with.

    Methods defined in a class implicitly have access to the fields defined in that class. This is because methods always need to be called on a specifc object, whose field values can then be used within the method.

    So in principle, writing a method is just like writing a function, except inside a class. Within the class, you can all other methods within the class just like we called functions before.

    By convention, in Java, method names are camelCased. That is, they start with a lower-case letter and otherwise contain only letters and digits, with uppercase letters indicating the start of a new combined word, as in sumOfAllParts.

    class MyClass { int myMethod() { return 42; } }

    Example. Methods

    [comp1110-84]
    • Fabian Muehlboeck

    Building on our example from Example [comp1110-82], imagine we want to just have a way to print a nice description for any of our basic Person objects.

    To do this, we'll add a printDescription method to the class:

    class Person { int age; String name; void printDescription() { IO.println(name + " (Age " + age + ")"); } }

    We can try this with the following main function:

    void main() { Person p = new Person(); p.name = "Timmy"; p.age = 5; p.printDescription(); }

    This will print:

    Timmy (Age 5)

    Have we seen Objects before?

    [comp1110-85]
    • Fabian Muehlboeck

    Yes! Every string and array is an object. When we wrote guess.charAt(5), we were calling a method of the String class on the particular String object referred to by guess.

    Similarly, the difference between arr.length for arrays and str.length() for strings is that the former is a field that we can access, and the latter is a method we have to call.

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