How do I call one constructor from another in Java?
Yes, it is possible:
public class Foo { private int x; public Foo() { this(1); } public Foo(int x) { this.x = x; }}
To chain to a particular superclass constructor instead of one in the same class, use super
instead of this
. Note that you can only chain to one constructor, and it has to be the first statement in your constructor body.
See also this related question, which is about C# but where the same principles apply.
Using this(args)
. The preferred pattern is to work from the smallest constructor to the largest.
public class Cons { public Cons() { // A no arguments constructor that sends default values to the largest this(madeUpArg1Value,madeUpArg2Value,madeUpArg3Value); } public Cons(int arg1, int arg2) { // An example of a partial constructor that uses the passed in arguments // and sends a hidden default value to the largest this(arg1,arg2, madeUpArg3Value); } // Largest constructor that does the work public Cons(int arg1, int arg2, int arg3) { this.arg1 = arg1; this.arg2 = arg2; this.arg3 = arg3; }}
You can also use a more recently advocated approach of valueOf or just "of":
public class Cons { public static Cons newCons(int arg1,...) { // This function is commonly called valueOf, like Integer.valueOf(..) // More recently called "of", like EnumSet.of(..) Cons c = new Cons(...); c.setArg1(....); return c; }}
To call a super class, use super(someValue)
. The call to super must be the first call in the constructor or you will get a compiler error.
[Note: I just want to add one aspect, which I did not see in the other answers: how to overcome limitations of the requirement that this() has to be on the first line).]
In Java another constructor of the same class can be called from a constructor via this()
. Note however that this
has to be on the first line.
public class MyClass { public MyClass(double argument1, double argument2) { this(argument1, argument2, 0.0); } public MyClass(double argument1, double argument2, double argument3) { this.argument1 = argument1; this.argument2 = argument2; this.argument3 = argument3; }}
That this
has to appear on the first line looks like a big limitation, but you can construct the arguments of other constructors via static methods. For example:
public class MyClass { public MyClass(double argument1, double argument2) { this(argument1, argument2, getDefaultArg3(argument1, argument2)); } public MyClass(double argument1, double argument2, double argument3) { this.argument1 = argument1; this.argument2 = argument2; this.argument3 = argument3; } private static double getDefaultArg3(double argument1, double argument2) { double argument3 = 0; // Calculate argument3 here if you like. return argument3; }}