How can I return multiple values from a Java function?

[sbasak]

How can I return multiple values from a Java function?

In C we can use pointer to return multiple value from a function. But how do I achive the same in Java?

For example, how do write the following C code in Java?

prototype -> int solve_quad(int p, int q, int r, float *q1, float *q2)

while calling -> status = solve_quad(a,b,c,&root1,&root2);

Thanx

[crptcblade]

All objects are passed byref in Java. So instead of using basic types like float or long, use their wrapper classes...

int solve_quad(int p, int q, int r, Float q1, Float q2)

When you change the value inside the function, the change is reflected in the calling method.

[sbasak]

Thanx but what do these wrapper classes do actually?

[crptcblade]

Since everything in Java is supposed to be object-oriented, they provide a way to make the basic types into Objects. Mostly, I use them to be able to put numbers into Collections, which only take in Objects.

[sbasak]

Hi, I tried to return a value by reference but it didn't work. Can you please help?

[java]
import java.lang.*;

public class Test
{
public static void main(String [] args)
{
float x=5,y=0;
SquareIt(x,y);
System.out.println("Square of 5 is " +y);
}

void SquareIt(float a, Float b)
{
b=a*a;
}
}
[/java]

[crptcblade]

Hmmm, it seems that with the Number classes, all you can do is reinitialize the class...

public class Test 
{ 
    public static void main(String [] args) 
    { 
        float x=5,y=0; 
        SquareIt(x,y); 
        System.out.println("Square of 5 is " +y); 
    } 

    void SquareIt(float a, Float b) 
    { 
        b = new Float(a*a); 
    } 
}

Not the best solution, but it should work.

[sbasak]

Gives error as follows

Test.java:8: Incompatible type for method. Can't convert float to java.lang.Float.
SquareIt(x,y);
^

[crptcblade]

public class Test 
{ 
    public static void main(String [] args) 
    { 
        float x=5;
        Float y = new Float(0); 
        SquareIt(x,y); 
        System.out.println("Square of 5 is " +y.floatValue()); 
    } 

    void SquareIt(float a, Float b) 
    { 
        b = new Float(a*a); 
    } 
}

Try that. I don't have docs at my disposal right now, but that should work.

[sbasak]

sorry again, your earlier version didn't compile giving error
can't make static reference to method void SquareIt

Anyway, I tried to make the above program as an applet with following code

[javacode]
import java.awt.*;
import java.applet.*;
import java.text.*;
import java.lang.*;

public class Test extends Applet
{
public void init()
{
resize(200,200);
}

public void paint(Graphics g)
{
float x=5;
Float y = new Float(0.0);
SquareIt(x,y);
g.drawString("Square of 5 is " +y.floatValue(),10,10);
}

void SquareIt(float a, Float b)
{
b = new Float(a*a);
}
}


[/javacode]

Now evething is fine except the answer shows
Square of 5 is 0.0 instead of 25... !!!!
That means the value wasn't passed by reference??

[crptcblade]

If you can wait a while, I don't have access to anything Java here at work, but when I get home I'll make it work correctly.

[crptcblade]

It seems that it will not work the way I thought (its been a while since I've done Java). If you could change the value of the Float object, then it would work, but it seems that the Number classes are immutable.

Can you give an actual instance of when you would want to do this, since your SquareIt example doesn't really qualify.

[sbasak]

Take the original function of returning two roots of a quadratic equation

int solve_quad(int p, int q, int r, float *q1, float *q2)

I myself wrote a solution, but persoanlly I feel there should be a better alternative.

Anyway, I attach my version of the applet.

-----------------------------------------------------

// a quadratic equation solver
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.applet.*;
/* <applet code= "QuadSolverApp" width=160 height=250>
</applet>
*/
public class QuadSolverApp extends JApplet implements ActionListener
{
JTextField jtfa,jtfb,jtfc,jtfr1,jtfr2;
JLabel jlt,jla,jlb,jlc,jlr1,jlr2;
JButton jb;
public void init()
{
resize(160,250);
Container contentPane = getContentPane();
contentPane.setLayout(new FlowLayout());
// add text fields & lables
jlt = new JLabel("Quadratic Equation Solver");
contentPane.add(jlt);
jla = new JLabel("Enter a");
contentPane.add(jla);
jtfa = new JTextField(8);
contentPane.add(jtfa);
jlb = new JLabel("Enter b");
contentPane.add(jlb);
jtfb = new JTextField(8);
contentPane.add(jtfb);
jlc = new JLabel("Enter c");
contentPane.add(jlc);
jtfc = new JTextField(8);
contentPane.add(jtfc);
jlr1 = new JLabel("Root 1");
contentPane.add(jlr1);
jtfr1 = new JTextField(8);
contentPane.add(jtfr1);
jlr2 = new JLabel("Root 2");
contentPane.add(jlr2);
jtfr2 = new JTextField(8);
contentPane.add(jtfr2);
// add button
JButton jb = new JButton("Solve"); // button label
jb.setActionCommand("SolveEquation"); // button action command
jb.addActionListener(this);
contentPane.add(jb);
}
public void actionPerformed(ActionEvent ae)
{
if(ae.getActionCommand()=="SolveEquation") // button action command
{
// create a new instance of SolveClass
SolveClass sc = new SolveClass();
SolveClass sc2;
// call Solve method of SolveClass
sc2 =
sc.Solve(Float.parseFloat(jtfa.getText()),Float.parseFloat(jtfb.getText()),Float.parseFloat(jtfc.get Text()));
if(sc2.root1==0)
{
jtfr1.setText("No real root");
jtfr2.setText("No real root");
}
else
{
jtfr1.setText(String.valueOf(sc2.root1));
jtfr2.setText(String.valueOf(sc2.root2));
}
}
}
}
class SolveClass
{
static double root1,root2;
SolveClass Solve(double a, double b, double c)
{
double determinant;
SolveClass scf = new SolveClass();
determinant = b*b-4*a*c;
if(determinant > 0)
{
System.out.println("Two real roots");
scf.root1 = (-b + Math.sqrt(determinant))/(2*a);
scf.root2 = (-b - Math.sqrt(determinant))/(2*a);
}
else
{
scf.root1 = 0;
scf.root2 = 0;
System.out.println("No real root");
}
return scf;
} // end of method Solve
} // end of class SolveClass
-------------------------------------------------------------