How do I call one constructor from another in Java? How do I call one constructor from another in Java? java java

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;  }}