Definition. Class Member

[comp1110-7B]
  • Fabian Muehlboeck

Member is the generic term for parts of a class, such as fields or methods.

Context

    More on classes

    [comp1110-7E]
    • Fabian Muehlboeck

    So far, we have seen class definitions with fields and methods, but always had to initialise the fields of a new object explicitly. We can make this more convenient.

    Definition. Class Member

    [comp1110-7B]
    • Fabian Muehlboeck

    Member is the generic term for parts of a class, such as fields or methods.

    Construction. Constructors

    [comp1110-7C]
    • Fabian Muehlboeck

    A constructor is a class member that is used to initialise objects of a class when they are created. By default, every class has a parameterless constructor that does nothing, and for a class named MyClass, can be called using:

    new MyClass()

    You can define your own constructors. As soon as you do, the default parameterless constructor does not exist anymore. Constructors are special because, unlike normal methods, they do not have a name or return type. In Java, you specify them using the class type, as in:

    class MyClass { /** * An empty, parameterless constructor for MyClass */ MyClass() { } }

    Example. Initialising Objects

    [comp1110-7D]
    • Fabian Muehlboeck

    The most common use of constructors is to initialise the fields of an object:

    class Person { String name; int age; /** * Constructs a Person with a given age and name * @param n the name of the new Person * @param a the age of the new Person */ Person(String n, int a) { name = n; age = a; } }

    This way, new instances of the above class can be created using:

    new Person("Julia", 22)

    Backlinks

      Construction. Access Modifiers

      [comp1110-7W]
      • Fabian Muehlboeck

      Data type definitions (e.g. Enums and Class Definitions) and members may be given an access modifier. The following access modifiers exist in Java:

      • public - this means that the data type or member should be accessible by everyone, from anywhere.
      • private - this means that the data type or member should only be accessible from within the scope of the outermost enclosing context.
      • protected - this will be explained later and is not important for now.

      In addition to these modifiers, the meaning of not having a modifier at all is called package private. These data types or members are accessible from everywhere in the same package, as if they were public, but inaccessible from outside of that package, as if they were private.

      Access modifiers are about syntactic scope, that is, about how code is written and nested. In particular, any private field can be accessed from any code within the same class, not just from within the same object.

      Construction. Constructors

      [comp1110-7C]
      • Fabian Muehlboeck

      A constructor is a class member that is used to initialise objects of a class when they are created. By default, every class has a parameterless constructor that does nothing, and for a class named MyClass, can be called using:

      new MyClass()

      You can define your own constructors. As soon as you do, the default parameterless constructor does not exist anymore. Constructors are special because, unlike normal methods, they do not have a name or return type. In Java, you specify them using the class type, as in:

      class MyClass { /** * An empty, parameterless constructor for MyClass */ MyClass() { } }
      bars magnifying-glass caret-down plus minus arrow-right xmark