[java] How to use an output parameter in Java?

Could someone please give me some sample code that uses an output parameter in function? I've tried to Google it but just found it just in functions. I'd like to use this output value in another function.

The code I am developing intended to be run in Android.

As a workaround a generic "ObjectHolder" can be used. See code example below.

The sample output is:

name: John Doe
dob:1953-12-17
name: Jim Miller
dob:1947-04-18

so the Person parameter has been modified since it's wrapped in the Holder which is passed by value - the generic param inside is a reference where the contents can be modified - so actually a different person is returned and the original stays as is.

/**
 * show work around for missing call by reference in java
 */
public class OutparamTest {

 /**
  * a test class to be used as parameter
  */
 public static class Person {
   public String name;
     public String dob;
     public void show() {
      System.out.println("name: "+name+"\ndob:"+dob);
   }
 }

 /**
  * ObjectHolder (Generic ParameterWrapper)
  */
 public static class ObjectHolder<T> {
    public ObjectHolder(T param) {
     this.param=param;
    }
    public T param;
 }

 /**
  * ObjectHolder is substitute for missing "out" parameter
  */
 public static void setPersonData(ObjectHolder<Person> personHolder,String name,String dob) {
    // Holder needs to be dereferenced to get access to content
    personHolder.param=new Person();
    personHolder.param.name=name;
    personHolder.param.dob=dob;
 } 

  /**
   * show how it works
   */
  public static void main(String args[]) {
    Person jim=new Person();
    jim.name="Jim Miller";
    jim.dob="1947-04-18";
    ObjectHolder<Person> testPersonHolder=new ObjectHolder(jim);
    // modify the testPersonHolder person content by actually creating and returning
    // a new Person in the "out parameter"
    setPersonData(testPersonHolder,"John Doe","1953-12-17");
    testPersonHolder.param.show();
    jim.show();
  }
}

Wrap the value passed in different classes that might be helpful doing the trick, check below for more real example:

  class Ref<T>{

    T s;

    public void set(T value){
        s =  value;
    }

    public T get(){
        return s;
    }

    public Ref(T value) {
        s = value;
    }
}


class Out<T>{

    T s;

    public void set(T value){
        s =  value;
    }
    public T get(){
        return s;
    }

    public Out() {
    }
}

public static void doAndChangeRefs (Ref<String> str, Ref<Integer> i, Out<String> str2){
    //refs passed .. set value
    str.set("def");
    i.set(10);

    //out param passed as null .. instantiate and set 
    str2 = new Out<String>();
    str2.set("hello world");
}
public static void main(String args[]) {
        Ref<Integer>  iRef = new Ref<Integer>(11);
        Out<String> strOut = null; 
        doAndChangeRefs(new Ref<String>("test"), iRef, strOut);
        System.out.println(iRef.get());
        System.out.println(strOut.get());

    }

This is not accurate ---> "...* pass array. arrays are passed by reference. i.e. if you pass array of integers, modified the array inside the method.

Every parameter type is passed by value in Java. Arrays are object, its object reference is passed by value.

This includes an array of primitives (int, double,..) and objects. The integer value is changed by the methodTwo() but it is still the same arr object reference, the methodTwo() cannot add an array element or delete an array element. methodTwo() cannot also, create a new array then set this new array to arr. If you really can pass an array by reference, you can replace that arr with a brand new array of integers.

Every object passed as parameter in Java is passed by value, no exceptions.

Java does not support output parameters. You can use a return value, or pass in an object as a parameter and modify the object.

Thank you. I use passing in an object as a parameter. My Android code is below

    String oPerson= null;
    if (CheckAddress("5556", oPerson))
    {
        Toast.makeText(this, 
                "It's Match! " + oPerson,                   
                Toast.LENGTH_LONG).show();
    }

    private boolean CheckAddress(String iAddress, String oPerson)
{
    Cursor cAddress = mDbHelper.getAllContacts();
    String address = "";        
    if (cAddress.getCount() > 0) {
        cAddress.moveToFirst();
        while (cAddress.isAfterLast() == false) {
            address = cAddress.getString(2).toString();
            oPerson = cAddress.getString(1).toString(); 
            if(iAddress.indexOf(address) != -1)
            {
                Toast.makeText(this, 
                        "Person : " + oPerson,                  
                        Toast.LENGTH_LONG).show();
                System.out.println(oPerson);
                cAddress.close();
                return true;                    
            }
            else cAddress.moveToNext();
        }
    }
    cAddress.close();
    return false;
}

The result is

Person : John

It's Match! null

Actually, "It's Match! John"

Please check my mistake.

You can either use:

• return X. this will return only one value.

• return object. will return a full object. For example your object might include X, Y, and Z values.

• pass array. arrays are passed by reference. i.e. if you pass array of integers, modified the array inside the method, then the original code will see the changes.

Example on passing Array.

void methodOne{
    int [] arr = {1,2,3};
    methodTwo(arr);
    ...//print arr here
}
void methodTwo(int [] arr){
    for (int i=0; i<arr.length;i++){
         arr[i]+=3;
    }
}

This will print out: 4,5,6.