Java Methods - Parameter Passing And Scope

Methods - Importance >
Siva Nookala - 14 Mar 2016

It is very important to understand how the parameters are passed from the calling method to the called method and how the return value is sent back to the calling method. As discussed in Basic Java Methods, the main method is the calling method and convertToCelsius is the called method.

Lets look at another example to understand the parameters passing better.

public static void main(String s[])
{
    int a = 5;
    int b = 6;
    int c = sum(a, b);
}

public static int sum(int a, int b)
{
    int c = a + b;
    return c;
}

Here we have created a main method and a sum method. main method is calling method and sum is the called method. Please note the following points about these methods.

The parameter names in the calling method and the called method need not be same as shown above. In the method sum, the parameters could be called x and y instead of a and b. Only the data types should be same.

The name of the return variable also need not be same as the calling method. The return value c could have been z.

// Valid
public static int sum(int x, int y)
{
    int z = x + y;
    return z;
}

It is not necessary that we create a variable for returning, we can directly return with out creating the variable.

// Valid
public static int sum(int x, int y)
{
    return x + y;
}

Only variables passed as parameters are accessible in the called method. For eg. the variable r created in the main method is not accessible in the sum method.

public static void main(String s[])
{
    int a = 5;
    int b = 6;
    int r = 21;
    int c = sum(a, b);
}

public static int sum(int a1, int b1)
{
    int c1 = a1 + b1;
    System.out.println(" r = " + r); // Compilation Error.
    return c1;
}

Similarly, any new variables created in the called method are not accessible in the calling method. For eg. the variable m created in the sum method is not accessible in the main method.

public static void main(String s[])
{
    int a = 5;
    int b = 6;
    int c = sum(a, b);
    System.out.println(" m = " + m); // Compilation Error.
}

public static int sum(int a, int b)
{
    int c = a + b;
    int m = 78;
    return c;
}

Even if the variables have same name in both the calling method and the called method, since their scope is different, the changes done on one variable will not impact the other. Here the variable k in the main method and the k in the sum method are different, changes made to one will not impact the other.

public static void main(String s[])
{
    int a = 5;
    int b = 6;
    int k = 78;
    int c = sum(a, b);
    // k's value will be 78
}

public static int sum(int a, int b)
{
    int c = a + b;
    int k = 88; // Declare k again and initialize to 88, this will not impact the k in the main method.
    return c;
}

Even if the parameters have the same name, if the data types are different then it throws a compilation error. As shown below, the data types in the main method are int where as they are boolean in the called method sum, hence it throws a compilation error.

public static void main(String s[])
{
    int a = 5;
    int b = 6;
    int c = sum(a, b); // Compilation error
}

public static int sum(boolean a, boolean b)
{
    return a && b ? 25 : 45;
}

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