Java, change variable value

Question

public class gas_station {
    int start = 0;

    public int commonMethod (int[] gas, int[] cost) {
        int len = gas.length;

        while (this.start < len) {
            if (canComplete(this.start, gas, cost)) {
                return this.start;
            }
        }

        return -1;
    }

    public boolean canComplete (int index, int[] gas, int[] cost) {
        int len = gas.length, sum = 0;
        for (int i = index; i < len + index; i++) {
            sum += gas[i % len] - cost[i % len];
            if (sum < 0) {
                this.start = i + 1;
                return false;
            }
        }

        return true;
    }
}

As you can see, in the canComplete function, I make changes to class member start, and in the commonMethod function, I use it as the loop variable, my question is, is there a way I can make change in the canComplete function and influence the variable in the caller function (here, commonMethod function) instead of making a variable (start) a class member, I know in C++, I can pass a pointer to a function, but what should I do in Java? Thanks!

Nope, Java is purely pass by value. You can modify a member variable, you can return a value, or you can pass in a modifiable object, but you cannot modify the variable in the calling code. — azurefrog
– azurefrog, Commented Oct 9, 2017 at 21:49
OK, I know if I pass parameters other than 8 primitive types such as a array, the changes would reflect in the calling function, so there is no way about primitive types such as int, what if I pass a Integer type of parameter other than an int type? — Haobo_X
– Haobo_X, Commented Oct 9, 2017 at 21:58
Integer is just a wrapper, and is immutable, but you could pass an array or an AtomicInteger — azurefrog
– azurefrog, Commented Oct 9, 2017 at 22:00

Joop Eggen · Accepted Answer · 2017-10-09 21:58:05Z

2

A typical java pattern is to have a class searching or calculating something, maintaining a state like that start field. On a boolean success one can provide something like getStart().

The reason for not having variables passed by reference to a method, was a design decision to make code quality better. Wherever you see f(x) you know x is not assigned to. The compiler can take advantage of that, the garbage collector.

Java SE examples: Matcher, PreparedStatement, ResultSet

Pattern pat = Pattern.compile("(\\w+)\\s*=\\s*[1-9X]+;");
Matcher m = pat.matcher(s);
while (m.find()) {
    System.out.println(m.group(1) + "=" + m.group(2));
}

answered Oct 9, 2017 at 21:58

Joop Eggen

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user3437460 · Accepted Answer · 2017-10-09 22:30:15Z

1

is there a way I can make change in the canComplete function and influence the variable in the caller function (here, commonMethod function) instead of making a variable (start) a class member, I know in C++, I can pass a pointer to a function

You can't pass a pointer or by reference in Java. All arguments are passed by value. However if you want the int variable to change in the method. There are a few work around solutions:

Return the value to be changed:

public int myMethod(){
    //your code
    return val;
}
int val = myMethod();

Wrap the primitive variable into a class:

class MyClass{
    private int val;
}

public void myMethod(MyClass val){
    //changing val here change its origin as well
}

Create a size 1 array:

int[] val = new int[1];
public void myMethod(int[] val){
    //changing val here change its origin as well
}

Important point to note: Java had this design for a reason. What I have provided is just to show you some possibilities. Personally, I don't recommend the above mentioned approach, but instead, try a different design in your implementation to achieve what you need.

answered Oct 9, 2017 at 22:30

user3437460

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3 Comments

Haobo_X Over a year ago

Thanks! I just wanna make sure. got it.

Haobo_X Over a year ago

I did, but don't know why can't be displayed.

user3437460 Over a year ago

@Haobo_X What about now?

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