A constructor is code that is invoked when an object is created from the class blueprint. You do not need to write a constructor for every class. If a class does not have a user-defined constructor, a default, no-argument, public constructor is used.
The syntax for a constructor is similar to a method, but it differs from a method definition in that it never has an explicit return type and it is not inherited by the object created from it.
public class TestObject { // The no argument constructor public TestObject() { // more code here } }
TestObject myTest = new TestObject();
If you write a constructor that takes arguments, you can then use that constructor to create an object using those arguments.
If you create a constructor that takes arguments, and you still want to use a no-argument constructor, you must create your own no-argument constructor in your code. Once you create a constructor for a class, you no longer have access to the default, no-argument public constructor.
public class TestObject2 { private static final Integer DEFAULT_SIZE = 10; Integer size; //Constructor with no arguments public TestObject2() { this(DEFAULT_SIZE); // Using this(...) calls the one argument constructor } // Constructor with one argument public TestObject2(Integer ObjectSize) { size = ObjectSize; } }
TestObject2 myObject1 = new TestObject2(42); TestObject2 myObject2 = new TestObject2();
public class Leads { // First a no-argument constructor public Leads () {} // A constructor with one argument public Leads (Boolean call) {} // A constructor with two arguments public Leads (String email, Boolean call) {} // Though this constructor has the same arguments as the // one above, they are in a different order, so this is legal public Leads (Boolean call, String email) {} }
When you define a new class, you are defining a new data type. You can use class name in any place you can use other data type names, such as String, Boolean, or Account. If you define a variable whose type is a class, any object you assign to it must be an instance of that class or subclass.