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How to make a constructor that takes 2 if there are 2 parameters, or 3 if there are 3

New to Java ...

I have a name class that has:

private String firstName; private String middleInitial; private String lastName;

as their instance variables.

If I had certain data that only had firstName and lastName, no middleInitial, how could I make the constructor so that instead of three? instead of <3>

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9 answers

You just write a constructor with two parameters and a constructor with three

 public YourClass(String firstName, String lastName) { ... } public YourClass(String firstName, String middleInitial, String lastName) { ... }

Then callers can use the appropriate constructor depending on their needs.

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Well, two options:

  • Just create a constructor with three parameters and call it using null or an empty string for middleInitial
  • Overload the constructors, perhaps by calling them from one another.

As an example, the last one uses an empty string as the initial default average:

 public Person(String firstName, String middleInitial, String lastName) { this.firstName = firstName; this.middleInitial = middleInitial; this.lastName = lastName; } public Person(String firstName, String lastName) { this(firstName, "", lastName); }

However, the compiler will need to know which one you are calling from the call site. So you can do:

 new Person("Jon", "L", "Skeet");

or

 new Person("Jon", "Skeet");

... but you cannot do:

 // Invalid new Person(firstName, gotMiddleInitial ? middleInitial : ???, lastName);

and expect the compiler to decide to use the two-name option.

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In Java, constructors cannot have default arguments. Your only option is to write two constructors. Fortunately, Java allows you to call constructors from other constructors. You can do something like:

 public class MyClass { private String firstName; private String middleInitial; private String lastName; public MyClass(String firstName, String middleInitial, String lastName) { this.firstName = firstName; this.middleInitial = middleInitial; this.lastName = lastName; } public MyClass(String firstName, String lastName) { this(firstName, "", lastName); } ... }
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You can use two constructors:

 public Person(String firstName, String lastName) { this(firstName, null, lastName); } public Person(String firstName, String middleInitial, String lastName) { this.firstName = firstName; this.middleInitial = middleInitial; this.lastName = = lastName; }
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Define 2 constructors, one with two parameters and one with three parameters.

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You can write two constructors.

 public Person( String firstName, String lastName, String middleName ) { ... } public Person( String firstName, String lastName ) { ... }
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 public void myBike(String name, String color) { System.out.println("My bike name is " + name + " and is " + color + "."); } public void myBike(String name, String color, float height) { System.out.println("My bike name is " + name + " and is " + color + "."); System.out.println("My bike is also " + height + " inches tall."); }
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 public Class Name{ private String first; private String middle; private String last; public Name(String first, String middle, String last){ this.first = first; this.middle = middle; this.last = last; } public Name(String first, String last){ this.first = first; this.last = last; } }
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Figure Builder ...

 class Name { Builder builder; public String getSurname() { return builder.surname; } // getter... public Name(Builder builder) { this.builder = builder; } class Builder { String surname = ""; String middleName = ""; String name = ""; Builder surname(String surname) { this.surname = surname; return this; } Builder middleName(String middleName) { this.middleName = middleName; return this; } Builder name(String name) { this.name = name; return this; } Name build() { return new Name(this); } } }
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