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() { } }

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

      Example. Bank account class

      [comp1110-3E]
      • Liam O'Connor, Fabian Muehlboeck
       class BankAccount { //Data invariant: balance >= 0 private int balance; /** * @param initialDeposit >= 0 */ public BankAccount(int initialDeposit) { this.balance = initialDeposit; } public void deposit(int amount) { if (amount >= 0) { this.balance = this.balance + amount; } } public int getBalance() { return this.balance; } }

      In this snippet, the fiekd balance store the object's data, while the constructor defines how that data is initialised. The balance field uses the private access modifier, while the constructor and the two other methods are public.

      Related

        Definition. Class Member

        [comp1110-7B]
        • Fabian Muehlboeck

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

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