Question

New to Java here, please help. How arguments are passed in java? Why am I unable to change argument value in the calling method from within called method?

Code

public class PassTest {
    public static void changeInt(int value)
    {
         value=55;
    }

    int val;
    val=11;
    changeInt(val);
    System.out.println("Int value is:" + val);// calling modifier changeInt 
}

Output

Int value is: 11

why it is not 55..?

Answer 1

Java passes ByValue, meaning the value of the object you put as a parameter is passed, but not the object itself, therefore

val=11;
changeInt(val);

does the exact same thing as

int val=11;
int val2=val
changeInt(val2);

int is a primitive, primitives don’t “wrap” a value, you could try to use an Integer class, or make your own class that stores an integer, and then change that classes integer value. Instances of an object are sometimes passed ByReference if setup right. here is an example

MyStringClass.java

public class MyStringClass{

    private String string = null;

    public MyStringClass(String s){
        string = s;
    }
    public String getValue(){
        return string;
    }
    public void setValue(String s){
        string = s;
    }
}

and then the workings

public static void addTo(String s){
    s += " world";
}
public static void addTo(MyStringClass s){
    s.setValue(s.getValue() + " world");
}
public static void main(String[] args){
    String s = "hello";
    MyStringClass s1 = new MyStringClass("hello");
    addTo(s);
    addTo(s1);
    System.out.println(s);//hello
    System.out.println(s1);//hello world
}

I would wonder why you need to change the value instead of just returning it? isn’t it easier?