How to pass only some parameters to the constructor?

Programming Questions

thiezar

How to pass only some parameters to the constructor?

in Programming Questions • 6 months ago

I have a class with a lot of attributes. Each of those attributes have a default value.

class MyClass{
int a1=1;
int a2=2;
int a3=3;
...
MyClass(){
//in this constructor no parameters are passed so every attribute keeps it's default value
}
}

Now I want to instantiate an object of this class by passing to the constructor only the parameters I want to be different from the default value.

MyClass mc = new MyClass(a1=100, a5=500);
// I would like a constructor like this where I set the value to only some attributes of the class while others keep their default value

How should I do this? I hope I don't have to write a constructor for every combination of parameters :-/

Replies(10)

PhiLho

Re: How to pass only some parameters to the constructor?

6 months ago

I fear you have to, Java is a quite verbose language with few shortcuts, and named parameters aren't part of its features!
An alternative is to assign the parameters after calling the default constructor.
In classical Java, it means making a setter for each of them. Or making a builder for the class, if you want to ensure the right parameters are set before using the class.

In the less stringent Processing, just assign them:

MyClass mc = new MyClass();
mc.a1 = 100;
mc.a5 = 500;

Of course, I suppose this is only an example and your parameters are more differentiated, otherwise an array of parameters would do the job, too.

thiezar

Re: Re: How to pass only some parameters to the constructor?

6 months ago

otherwise an array of parameters would do the job, too.

Could you please explain how an array of parameters works? Maybe this is just what I'm looking for...

GoToLoop

Re: How to pass only some parameters to the constructor?

6 months ago

Perhaps you can explore polymorphism, setting constructors w/ diff. number of arguments!
This works better when fields of a class are of mixed datatypes.
However, even when mixed types aren't possible, polymorphism can be used w/ diff. # of parameters!

See my homemade example below:

void setup() {
  MyClass z = new MyClass(new int[] {6, -100, 3, 9, 53} );
  println(z);
  
  z = new MyClass(new int[] {-1, -2} );
  println(z);
  
  z = new MyClass(7);
  println( z.toArray() );
  
  exit();
}

final class MyClass {
  int a1 = 10, a2 = 20, a3 = 30;

  MyClass() { // uses default values
  }

  MyClass(int a) { // makes all of them = parameter
    a1 = a2 = a3 = a;
  }

  MyClass(int a, int b) { // changes 1st & 2nd, keeps 3rd
    a1 = a;
    a2 = b;
  }

  MyClass(int a, int b, int c) { // changes every1
    a1 = a;
    a2 = b;
    a3 = c;
  }

  MyClass(int[] a) { // extracts from array as much as possible
    switch(a.length) {
    case 0:    break;
    default:   a3 = a[2];
    case 2:    a2 = a[1];
    case 1:    a1 = a[0];
    }
  }

  int[] toArray() {
    return new int[] {a1, a2, a3};
  }

  @Override String toString() {
    return "a1 = " + a1 + "\t a2 = " + a2 + "\t a3 = " + a3;
  }
}

thiezar

Re: Re: How to pass only some parameters to the constructor?

6 months ago

I was hoping not to use a constructor for every combination of passed parameters because, as I said, there are a lot of them.

PhiLho

Re: How to pass only some parameters to the constructor?

6 months ago

For example:

class OneClass {
// Array of values with defaults
int[] fields = { 1, 2, 3 };
OneClass(int... params) {
for (int i = 0; i < min(fields.length, params.length); i++) {
fields[i] = params[i];
}
}
public String toString() { return java.util.Arrays.toString(fields); }
}
void setup() {
OneClass c1 = new OneClass();
println(c1);
OneClass c2 = new OneClass(5, 7);
println(c2);
OneClass c3 = new OneClass(11, 13, 17, 42);
println(c3);
}

You can pass some special value, like -1 if your values cannot be negative, or Integer.MAX_VALUE or similar, to tell not to change the default value on a given slot.

thiezar

Re: Re: How to pass only some parameters to the constructor?

6 months ago

This is useful. Thanks :-)

quarks

Re: How to pass only some parameters to the constructor?

6 months ago

It is not perfect but the code below allows the user to call the constructor with name/value pairs in an array.

Foo f1, f2;
public void setup() {
size(200, 200);
f1 = new Foo(new Object[] {
"name", "Boris", "age", 22
}
);
println(f1);
f2 = new Foo(new Object[] {
"name", "Marko", "salary", 312345.99
}
);
println(f2);
}
class Foo {
String name = "";
int age = 0;
float salary = 0.0;
Foo(Object... args) {
int nbrArgs = args != null ? args.length : 0;
for (int a = 0; a < nbrArgs; a += 2) {
String id = args[a].toString();
if (id.equalsIgnoreCase("name"))
name = args[a+1].toString();
else if (id.equalsIgnoreCase("age"))
age = Integer.parseInt(args[a+1].toString());
else if (id.equalsIgnoreCase("salary"))
salary = Float.parseFloat(args[a+1].toString());
else
println("Error in parameters - can't find " + id);
}
}
public String toString() {
String s = "Foo called " + name + " age " + age + " salary " + salary;
return s;
}
}

PhiLho

Re: How to pass only some parameters to the constructor?

6 months ago

Or one can use a HashMap...

Nice trick, anyway.

kevinwork..

Re: How to pass only some parameters to the constructor?

6 months ago

Yeah, just use a Map, or a builder like somebody else already said:

public class Obj{
int x;
int y;
int y
public Obj(){
//whatever
}
public Obj setX(int x){
this.x = x;
return this;
}
public Obj setY(int y){
this.y = y;
return this;
}
public Obj setZ(int z){
this.z = z;
return this;
}
}
public void setup(){
Obj obj = new Obj().setX(1).setY(2).setZ(3);
}

quarks

Re: How to pass only some parameters to the constructor?

6 months ago

I had thought of using a HashMap but I wanted it to be a Foo class (static) attribute and Processing dos not like static variables.

Having said that using a HashMap mades the coding neater- just one line per field. The code below uses the HashMap approach.

import java.util.*;
HashMap<String, Object> cmap = new HashMap<String, Object>();
Foo f1, f2;
public void setup() {
size(200, 200);
f1 = new Foo(new Object[] {
"name", "Boris", "age", 22
}
);
println(f1);
f2 = new Foo(new Object[] {
"name", "Marko", "salary", 3123.99
}
);
println(f2);
}
class Foo {
String name = "";
int age = 0;
float salary = 0.0;
Foo(Object... args) {
int nbrArgs = args != null ? args.length : 0;
Object value;
// Add the arguments to an empty map
cmap.clear();
for (int a = 0; a < nbrArgs; a += 2)
cmap.put(args[a].toString(), args[a+1]);
name = (value = cmap.get("name")) == null ? "" : value.toString();
age = (value = cmap.get("age")) == null ? 0 : Integer.parseInt(value.toString());
salary = (value = cmap.get("salary")) == null ? 0.0 : Float.parseFloat(value.toString());
}
public String toString() {
String s = "Foo called " + name + " age " + age + " salary " + salary;
return s;
}
}

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