What is going on with this Java array passed to a method?

user_15040769 0
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I've learned that in Java, arrays are passed by reference, meaning that they can be modified within functions. However, I recently came across this piece of code that confused me, because it seems to show inconsistent behavior.

import java.io.*;
import java.util.*;

public class TestProgram {

    public static void fMethod(int[] f){
        f[0] = 9;
        f[1] = 7;
        f = new int[4];
    }

    public static void main(String[] args){
        int[] fParam = new int[3];
        fMethod(fParam);
        System.out.println(Arrays.toString(fParam)); // prints [9, 7, 0]
    }
}

Because the function fMethod() seems to reset f to a new int[4] at the end, I expected to see [0, 0, 0, 0] printed to the console. However, it seems that only the f[0] = 9 and the f[1] = 7 lines were actually executed, while the last line was ignored. I find this strange. Can somebody please point me in the right direction?

arrays java
2 Answers

Remember, that:

In Java, everything is passed by value.

and in case of objects (arrays are also objects), address of the object, a value, is being copied into another variable.

In your code, int[] f is a local variable (scoped in a method), which initially refers to the original array and:

f[0] = 9;
f[1] = 7;

changes original array.

However, then you assign a new array object to f and you, eventually, discard that object entirely (as you don't return anything).

As long as f refers to the first array, it alters that array, and therefore, it only changes first two elements of it. After that, f refers to something else.

In your main method, though, fParam still refers to the previous array object and you print that object, 3rd element of which, is an integer array's default value, 0.

user_15535370

Within fMethod, you get the array passed as variable f. As long as you modify the content of the variable, it will actually change your reference. On line 3 of your method: f = new int[4] you assign a new array to the variable f. This means you overwrite the reference the variable f holds by assigning a new value, so any modifications afterwards to the array happen to the new assigned array. If you would introduce a class variable instead and pass it to the method, your application would act as you'd expect it to.

user_128754610
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