VarArgs

Index

Overview
Simple Varargs Example
AddElementsWithVarargs
UsageOfAddWithVarargs
Full Source (Model)
Full Source (Demo)
'heap pollution' Warning -- Varargs with Generics

Varargs means variable # of arguments. The best way to learn this is by example.

Left: Coding for varargs (param nums comes into method as array)
Right: Usage for varargs. Please note how we can send zero, one, two, three, etc parameters to the method (variable length args/param list)

private void addEmUp(int... nums)  {
	int sum = 0;
	for (int each: nums)
		sum += each;
	out.println("");
	out.println("Summing Array: " + Arrays.toString(nums));
	out.printf("Sum = %d%n", sum);
}

private void useAddEmUp()  {
	addEmUp();
	addEmUp(10);
	addEmUp(10, 20);
	addEmUp(10, 20, 30);
	addEmUp(10, 20, 30, 40);
}

AddElementsWithVarargs

Left: Traditional "add" method (param "newFirstName" is simple String type)
Right: "add" method with varargs (param "names" is varargs type meaning it can be zero to many). See next example.

public void add(String newFirstName) {
	this.firstNames.add(newFirstName);
}

public void add(String... names) {
	//varargs -- variable number of arguments
	//names is a String[] array
	for (String newFirstName: names)
		add(newFirstName);
}

UsageOfAddWithVarargs

Left: Traditional usage with four lines of code to add four elements.
Right: varargs usage with one line of code to add any number of elements.

private void demo_notUsingVarargs()  {
	Family fam = new Family();
	fam.add("Kofi");
	fam.add("Sally");
	fam.add("Siddhartha");
	fam.add("Bao");
	out.println("Without varargs -- " + fam.toString());
}

private void demo_usingVarargs()  {
	Family fam = new Family();
	fam.add("Kofi", "Sally", "Siddhartha", "Bao");
	out.println("With varargs -- " + fam.toString());
}

Full Source (Model)

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Family {

	public List<String> firstNames;

	public Family() {
		super();
		this.firstNames = new ArrayList<>();
	}

	public void add(String newFirstName) {
		this.firstNames.add(newFirstName);
	}

	public void add(String... names) {
		//varargs -- variable number of arguments
		//names is a String[] array
		for (String newFirstName: names)
			add(newFirstName);
	}

	public String toString() {
		return "Family: " + Arrays.toString(this.firstNames.toArray());
	}

}

Full Source (Demo)

import static java.lang.System.out;

import java.util.Arrays;

public class FamilyDemo {

	public static void main(String[] args) {
		out.println("");
		FamilyDemo demo = new FamilyDemo();
		demo.demo_notUsingVarargs();
		demo.demo_usingVarargs();
		demo.useAddEmUp();
		out.println("");
	}

	private void demo_notUsingVarargs() {
		Family fam = new Family();
		fam.add("Kofi");
		fam.add("Sally");
		fam.add("Siddhartha");
		fam.add("Bao");
		out.println("Without varargs -- " + fam.toString());
	}

	private void demo_usingVarargs() {
		Family fam = new Family();
		fam.add("Kofi", "Sally", "Siddhartha", "Bao");
		out.println("With varargs -- " + fam.toString());
	}

	private void addEmUp(int... nums) {
		int sum = 0;
		for (int each: nums)
			sum += each;
		out.println("");
		out.println("Summing Array: " + Arrays.toString(nums));
		out.printf("Sum = %d%n", sum);
	}

	private void useAddEmUp() {
		addEmUp();
		addEmUp(10);
		addEmUp(10, 20);
		addEmUp(10, 20, 30);
		addEmUp(10, 20, 30, 40);
	}

}

'heap pollution' Warning -- Varargs with Generics

When using varargs with generics, you may get a warning similar to this:

Type safety: Potential heap pollution via varargs parameter

Java gives this conservative warning because a coder may be doing some things like improper typecasting to make it work. After validating that your method is doing simple understandable things (you should not even need typecasting), then you can suppress the warning like this:

@SafeVarargs
public Tuple(T... newArray) {
//...

Java Fundamentals

Chapter 101 - Common Examples VarArgs

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