Varargs method parameter -

Varargs method parameter

Figure 397. Expecting one int argument

Code	static void main() { printInfo("Winner", 34); } public static void printInfo(final String msg, final int value) { IO.println(msg + ": " + value); }
Result	"Winner": 34

Figure 398. One, two or three int arguments by overloading

Code	printInfo("Winner", 34); // One int value printInfo("Winner", 31, 7); // Two int values printInfo("Winner", 2, 8, 5); // Three int values ----------------------------------------------------------------------------- public static void printInfo(String msg, int value) { ...} public static void printInfo(String msg, int value1, int value2) { IO.println(msg + ": " + value1 + " " + value2 ); } public static void printInfo(String msg, , int value1, int value2, int value3) { IO.println(msg + ": " + value1 + " " + value2 + " " + value3); }
Result	Winner: 34 Winner: 31 7 Winner: 2 8 5

Code

printInfo("Winner", 34);          // One int value
printInfo("Winner", 31, 7);       // Two int values
printInfo("Winner",  2, 8, 5);    // Three int values
-----------------------------------------------------------------------------
public static void printInfo(String msg, int value) { ...}

public static void printInfo(String msg, int value1, int value2) {
    IO.println(msg + ": " + value1 + " " + value2 );
}
public static void printInfo(String msg, , int value1, int value2, int value3) {
    IO.println(msg + ": " + value1 + " " + value2 + " " + value3);
}

Result

Winner: 34
Winner: 31 7
Winner: 2 8 5

Figure 399. Observations

Similar code in multiply overloaded methods.
Tedious: Each added argument requires adding another method.

Objective: Get rid of boilerplate code!

Figure 400. Arbitrary number of arguments by array

Code	printInfo("Winner", new int[]{ 1, 2, 3, 4, 5, 6, 7, 8, 9}); ------------------------------------------------------------- public static void printInfo(String msg, int[] values) { IO.print(msg + ": " ); for (int v: values) { IO.print(v + " "); } IO.println(); }
Result	Winner: 1 2 3 4 5 6 7 8 9

Code

printInfo("Winner", new int[]{ 1, 2, 3, 4, 5, 6, 7, 8, 9});
-------------------------------------------------------------
public static void printInfo(String msg, int[] values) {
    IO.print(msg + ": " );
    for (int v: values) {
        IO.print(v + " ");
    }
    IO.println();
}

Result

Winner: 1 2 3 4 5 6 7 8 9

Figure 401. Observations

Clumsy, requires

printInfo("Winner", new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 });

rather than

printInfo("Winner", { 1, 2, 3, 4, 5, 6, 7, 8, 9 }); // Syntax error

Objective: Simplify parameter passing

Figure 402. Vararg: Syntactic sugar by ellipsis »...«

Code	final int[] values = { 1, 2, 3, 4, 5, 6, 7, 8, 9}; printInfo("Winner", values); // Array of int printInfo("Winner", 20, 19, 18, 17, 16, 15); // Separate values --------------------------------------------------------------- public static void printInfo(String msg, int ... values) { IO.print(msg + ": " ); for (int v: values) { IO.print(v + " "); } IO.println(); }
Result	Winner: 1 2 3 4 5 6 7 8 9 Winner: 20 19 18 17 16 15

Code

final int[] values = { 1, 2, 3, 4, 5, 6, 7, 8, 9};

printInfo("Winner", values);                 // Array of int

printInfo("Winner", 20, 19, 18, 17, 16, 15); // Separate values
---------------------------------------------------------------          
public static void printInfo(String msg, int ... values) {
    IO.print(msg + ": " );
    for (int v: values) {
        IO.print(v + " ");
    }
    IO.println();
}

Result

Winner: 1 2 3 4 5 6 7 8 9 
Winner: 20 19 18 17 16 15

Figure 403. Varargs: Generalizing arrays

Replace
```
printInfo(String msg, int[] values)
```
By
```
printInfo(String msg, int ... values)
```

Figure 404. Using varargs

Only one varargs parameter per method.
Must be the method's last argument.

Figure 405. Further reading on varargs

Video

Java Tutorial: Method VarArgs

Text

exercise No. 147

Creating a flexible max(...) method.

Get inspired by Math.max() and extend it to a method of your own allowing for an arbitrary number of arguments to be used like e.g:

Code	`IO.println(max(1, 6, 0, -3, 20));`
Result	20

The varargs mechanism allows for defining:

/**
 * <p>Get the maximum from a list of values.</p>
 * @param values At least one value.
 *
 * @return The largest of all arguments.
 */
public static int max(int ... values) {
   int max = values[0];
   for (int i = 1; i < values.length; i++) {
      max = Math.max(max, values[i]);
   }
   return max;
}

exercise No. 148

Enforcing mandatory arguments

As being stated in the max(...) method's documentation the previous solution requires at least one argument. Violating this contract is a recipe for disaster:

Code

IO.println(max()); // No argument

Result

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
    at de.hdm_stuttgart.App.max(App.java:18)
    at de.hdm_stuttgart.App.main(App.java:9)

Unfortunately this error does not show up until run-time and thus represents a hidden flaw.

Tasks:

Why can't we solve this issue by just changing the implementation?
Solve the issue by modifying the method's signature requiring at least one value optionally being followed by additional values.

Due to its signature the max(...) method must return a value. If no argument is being passed there is no plausible way defining an empty list's maximum value.

Changing its signature solves the issue:

/**
 * <p>Get the maximum from a list of values.</p>
 *
 * @param value The first value.
 * @param values Optional additional values.
 *
 * @return The largest of all arguments.
 */
public static int max(int value,  int ... values) {
    for (int v:  values) {
        value = Math.max(value, v);
    }
    return value;
}

Now at least one argument is being required. Violation yields a compile-time rather than just a run-time error:

Code	IO.println(max()); // No argument
Result	Cannot resolve method 'max()'

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