... is there a way for a function to take a type as input?

For object types, I believe there are advanced techniques for it. But it's beyond my knowledge for now!

For primitive types, it's much easier to take advantage of Java's method signature.
That is, you can have many methods sharing the same name,
but with different data-types and/or number of parameters.

Take a look at the class implementation below, and pay attention on the many set() methods.
Also to its many constructors. Take note on how each version has different types of parameters:

/** 
 * Mini PVector Class Storage for Doubles (v1.01)
 * by GoToLoop (2013/Aug)
 * 
 * http://forum.processing.org/topic/can-pvector-work-with-doubles
 */

class PVectorD {
  double x, y, z;

  PVectorD(double xx, double yy) {
    set(xx, yy);
  }

  PVectorD(double xx, double yy, double zz) {
    set(xx, yy, zz);
  }

  PVectorD(PVector p) {
    set(p);
  }

  PVectorD(int[] a) {
    set(float(a));
  }

  PVectorD(float[] a) {
    set(a);
  }

  PVectorD(double[] a) {
    set(a);
  }

  void clear() {
    x = y = z = 0;
  }

  void set(double xx, double yy) {
    x = xx;
    y = yy;
  }

  void set(double xx, double yy, double zz) {
    x = xx;
    y = yy;
    z = zz;
  }

  void set(PVector p) {
    x = p.x;
    y = p.y;
    z = p.z;
  }

  void set(int[] a) {
    set(float(a));
  }

  void set(float[] a) {
    final int len = a.length;

    if (len > 0)  x = a[0];
    if (len > 1)  y = a[1];
    if (len > 2)  z = a[2];
  }

  void set(double[] a) {
    final int len = a.length;

    if (len > 0)  x = a[0];
    if (len > 1)  y = a[1];
    if (len > 2)  z = a[2];
  }

  PVectorD get() {
    return new PVectorD(x, y, z);
  }

  PVector toPVector() {
    return new PVector((float) x, (float) y, (float) z);
  }

  double[] toArray() {
    return new double[] {
      x, y, z
    };
  }

  String toString() {
    return "[ " + x + ", " + y + ", " + z + " ]";
  }
}

void setup() {
  println(new PVector());

  final int[] arr = {
    4, -5, 1, 56
  };

  final PVectorD pvd = new PVectorD(arr);
  println(pvd);

  pvd.set(-3.8, .5e4);
  pvd.set(new PVector(99, .7e-4, -2));
  println(pvd);

  println(pvd.get());
  println(pvd.toPVector());
  println(pvd.toArray());

  pvd.clear();
  println(pvd);

  exit();
}

My attempt of creating a 1D Array based on Nth column element of each row from a 2D Array:

/** 
 * Extract Nth Col of Each Row (v1.12)
 * by GoToLoop (2013/Aug)
 * 
 * http://forum.processing.org/topic/taking-type-as-function-input
 */

final static int ROWS = 80, COLS = 42, NTH = 3, MAX = 0200;
final static byte[][] arr2d = new byte[ROWS][COLS];
byte[] arr1d;

void setup() {
  random2dArrayFill(arr2d, MAX);
  println(arr2d[ROWS-1]);
  println();

  arr1d = extractAllNthElements(arr2d, NTH);
  println(arr1d);

  println("\n" + arr1d[ROWS-1]);
  println(arr1d[ROWS-1] == arr2d[ROWS-1][NTH]);

  exit();
}

void random2dArrayFill(byte[][] arr2D, int max) {
  final int rows = arr2D.length, cols = arr2D[0].length;

  for (int r=0; r!=rows; ++r) {
    final byte[] arr1D = arr2D[r];
    for (int c=0; c!= cols; arr1D[c++] = (byte) random(max));
  }
}

final static byte[] extractAllNthElements(byte[][] arr2D, int elem) {
  final int rows = arr2D.length;
  final byte[] arr1D = new byte[rows];

  for (int r=0; r!=rows; arr1D[r] = arr2D[r++][elem]);

  return arr1D;
}

i guess what you are searching for is java's generics, quite a vast topic.
but in fact, people use them all the time, e.g. when declaring an arraylist like that:

ArrayList<Type> name;

here is a short example:

1.   Integer[] int_arr    = {0,1,2,3,4,5,6,7,8,9};
2.   Float[]   float_arr  = {0f,1f,2f,3f,4f,5f,6f,7f,8f,9f};
3.   String[]  string_arr = "0 1 2 3 4 5 6 7 8 9".split("\\s+");
4.  
5.   public void setup() {
6.  
7.     {
8.       Integer[] cpy = new Integer[int_arr.length];
9.       int end = copyArray(int_arr, cpy, 3);
10.       System.out.println("--- Integer[] cpy ---");
11.       printArray(cpy, end);
12.     }
13.    
14.    
15.    
16.     {
17.       Float[] cpy = new Float[float_arr.length];
18.       int end = copyArray(float_arr, cpy, 3);
19.       System.out.println("--- Float[] cpy ---");
20.       printArray(cpy, end);
21.     }
22.    
23.    
24.    
25.     {
26.       String[] cpy = new String[string_arr.length];
27.       int end = copyArray(string_arr, cpy, 3);
28.       System.out.println("--- String[] cpy ---");
29.       printArray(cpy, end);
30.     }
31.    
32.    
33.     exit();
34.   }
35.  
36.   <T> void printArray(T[] a, int num) {
37.     for(int i = 0; i < num; i++){
38.       System.out.println("["+i+"] "+a[i]);
39.     }
40.   }
41.   <T> int copyArray(T[] src, T[] dst, int step) {
42.     int iter_dst = 0;
43.     int iter_src = 0;
44.     while(iter_src < src.length){
45.       dst[iter_dst] = src[iter_src];
46.       iter_dst++;
47.       iter_src+=step;
48.     }
49.     // returns required length of dst[] ... for resizing, etc.
50.     return iter_dst;
51.   }

additional information

http://docs.oracle.com/javase/tutorial/extra/generics/methods.html

You can also have a Class itself as a parameter, something like:

I want to know if it is possible for a function to take type (e.g. int, float, double, String[]) as an input so that the output type can vary depending on the input value.

The easiest way to do this is method overloadling. This is where you have several functions with the same name but have different parameter types e.g.


int doThis(int p){
  println("Return double the integer");
  return p * 2;
}

float doThis(float p){
  println("Return triple the float");
  return p * 3;
}

int doThis(String[] p){
  println("Return total number of charecters");
  int sum = 0;
  for(String s : p)
    sum += s.length();
  return sum;
}

The method call depends on what data you pass when you call the method

doThis(5);
doThis(3.142);

etc.

It appears that I have stumbled into quite an advanced topic. I will most likely just use a combination of GoToLoop's first recommendation and quarks' method "overloadling." (Did you mean " overlaoding"? * ) What others said kind of went over my head, but I will still look at implementing more advanced methods to see if they will make my code faster. In the meantime, though, I will post my method overloading code if there are any problems to make sure I did it right.

Although I didn't say that way, but my 1st example w/ custom PVectorI class
is actually using method/constructor overloading (also called polymorphism).
I said instead that Java uses method signature to identify which 1 to invoke back there!