While, If, For

What would be nice is if we could somehow list all the arguments on the command line without having to see those errors.

While.java
1	public class While {
2	public static void main(String[] args) {
3	int i = 0;
4	while(i < args.length) {
5	System.out.println(i+"] "+args[i]);
6	i++; // increment by one
7	}
8	}
9	}
$ javac While.java
$ java While a 0] a
$ java While a b 0] a 1] b
$ java While a b see 0] a 1] b 2] see

The while command executes everything in between the { }'s until it's condition is true. In this case, as long as the variable "i" is less than the length of args. Each iteration of the loop i is incremented by 1. If you comment out line 6 you'll find that the loop will execute forever.

There are two variations on the ++ operator: the prefix and the suffix. This short example shows how that works:

PlusPlus.java
1	public class PlusPlus {
2	public static void main(String[] args) {
3	int a = 1, b = 1;
4	System.out.println("a++="+(a++));
5	System.out.println("++b="+(++b));
6	System.out.println("a="+a+", b="+b);
7	}
8	}
$ javac PlusPlus.java
$ java PlusPlus a++=1 ++b=2 a=2, b=2

The suffix version increments a, but it evaluates to what a was before the increment. The prefix also increments, but it evaluates to what the variable is after the increment.

There are a number of operations that you can use for comparison inside the parenthesis of a while loop.

operator	meaning
a < b	true if a is less than b
a <= b	true if a is less than or equal to b
a > b	true if a is greater than b
a >= b	true if a is greater than or equal to b
a == b	true if a is equal to b
a != b	true if a is not equal to b
!	can be used to mean "not", thus !(a < b) is the same as a >= b

But there is more than one flavor of loop....

For.java
1	public class For {
2	public static void main(String[] args) {
3	for(int i=0;i<args.length;i++) {
4	System.out.println(i+"] "+args[i]);
5	}
6	}
7	}
$ javac For.java
$ java For a 0] a
$ java For a b 0] a 1] b
$ java For a b see 0] a 1] b 2] see

As you can see, the more concise (and more usual) way of writing this code is with a for loop. Really, the for and while loops written here do exactly the same thing and there is no real reason why you have to use one or the other except taste.

The easiest way that I know of to explain what a for loop does is to say that it does the same thing as the while loop above.

DoWhile.java
1	public class DoWhile {
2	public static void main(String[] args) {
3	int i=0;
4	do {
5	System.out.println(i+"] "+args[i]);
6	i++;
7	} while(i<args.length);
8	}
9	}
$ javac DoWhile.java
$ java DoWhile Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0 at DoWhile.main(DoWhile.java:5)
$ java DoWhile a b c 0] a 1] b 2] c

The do { } while(); loop is similar to the regular while loop, except it tests for termination only after it's processed the body. This means that the body of the loop will always execute at least once.

Loops allow us to print out each element of args individually, but what if we want to print a special message when no arguments are supplied?

If.java
1	public class If {
2	public static void main(String[] args) {
3	if(args.length == 0) {
4	System.out.println(">>No arguments given<<");
5	}
6	for(int i=0;i<args.length;i++) {
7	System.out.println(i+"] "+args[i]);
8	}
9	}
10	}
$ javac If.java
$ java If >>No arguments given<<
$ java If b 0] b
$ java If b see 0] b 1] see

Here we have introduced the if statement. It works like the while loop, but it only ever runs a single time.

If2.java
1	public class If2 {
2	public static void main(String[] args) {
3	if(args.length == 0)
4	System.out.println(">>No arguments given<<");
5	for(int i=0;i<args.length;i++)
6	System.out.println(i+"] "+args[i]);
7	}
8	}
$ javac If2.java
$ java If2 >>No arguments given<<
$ java If2 b 0] b
$ java If2 b see 0] b 1] see

If there is only a single statement (one semi-colon) in the loop or if block, then the {}'s are not necessary. Things look a little more concise this way.

If3.java
1	public class If3 {
2	public static void main(String[] args) {
3	if(args.length == 0)
4	System.out.println(">>No arguments given<<");
5	else
6	System.out.println("Number of args = "+args.length);
7	for(int i=0;i<args.length;i++)
8	System.out.println(i+"] "+args[i]);
9	}
10	}
$ javac If3.java
$ java If3 >>No arguments given<<
$ java If3 b Number of args = 1 0] b
$ java If3 b see Number of args = 2 0] b 1] see

This demonstrates the "else." It executes if the if fails.

IfElse.java
1	public class IfElse {
2	public static void main(String[] args) {
3	if(args.length == 0) {
4	System.out.println("No args given.");
5	} else if(args.length == 1) {
6	System.out.println("One arg given.");
7	} else if(args.length == 2) {
8	System.out.println("Two args given.");
9	} else {
10	System.out.println("Many args given.");
11	}
12	}
13	}
$ javac IfElse.java
$ java IfElse No args given.
$ java IfElse a One arg given.
$ java IfElse a b Two args given.
$ java IfElse a b c Many args given.

It is common to chain if / else's together.

ContBreak.java
1	public class ContBreak {
2	public static void main(String[] args) {
3	for(int i=0;i<10;i++) {
4	if(i == 4)
5	continue;
6	System.out.println("i="+i);
7	if(i == 7)
8	break;
9	}
10	}
11	}
$ javac ContBreak.java
$ java ContBreak i=0 i=1 i=2 i=3 i=5 i=6 i=7

The keyword "continue" means to skip over the remainder of the current loop body and begin the next iteration.

Keyword "break" means to exit the loop altogether.

ContWhile.java
1	public class ContWhile {
2	public static void main(String[] args) {
3	int i = 0;
4	while(i<10) {
5	if(i == 4)
6	continue;
7	System.out.println("i="+i);
8	if(i == 7)
9	break;
10	i++;
11	}
12	}
13	}
$ javac ContWhile.java

This code would run forever if we tried to execute it. Can you see why?

ArrayInit.java
1	public class ArrayInit {
2	public static void main(String[] args) {
3	int[] numbers = new int[6];
4	numbers[0] = 1;
5	numbers[1] = 3;
6	numbers[2] = 8;
7	numbers[3] = 2;
8	numbers[4] = 9;
9	numbers[5] = 3;
10	for(int i=0;i<numbers.length;i++)
11	System.out.println(i+"] "+numbers[i]);
12	}
13	}
$ javac ArrayInit.java
$ java ArrayInit 0] 1 1] 3 2] 8 3] 2 4] 9 5] 3

The above code shows a way to create and initialize an array. It is somewhat awkward and verbose. Fortunately, there's a nice shorthand that allows us to do this much more simply:

ArrayInit2.java
1	public class ArrayInit2 {
2	public static void main(String[] args) {
3	int[] numbers = new int[]{1,3,8,2,9,3};
4	for(int i=0;i<numbers.length;i++)
5	System.out.println(i+"] "+numbers[i]);
6	}
7	}
$ javac ArrayInit2.java
$ java ArrayInit2 0] 1 1] 3 2] 8 3] 2 4] 9 5] 3