A201/A597 Introduction to Programming I (4/3 cr.)

Spring Semester 2002

(Getting Back In Shape After Spring Break)

"Now, Andy did you hear about this one? Tell me, [...] are we losing touch?"

Consider this: Will name be changed?

String name = "Alice"; 
name = name.toUpperCase(); 
System.out.println("Hello " + name +"!");

Very good question.

And I know the answer.

You look like you know the next question too.

Yes: what if the part in blue is taken away?

Things change then. And name remains unchanged.

Now, consider this:

Oh, boy.

String vehicle = "snowmobile";

What's vehicle? A String variable.

What's "vehicle"? A String constant.

What's vehicle.substring(0, 4)? A (new) String gets created: "snow".

What's vehicle.length()? A number.

What's vehicle + vehicle.length()? "snowmobile10"

Consider this:

(x <= y)

I could write it this way:

(y >= x)

How about this:

(x == y) || (x < y)

Same thing (only more verbose).

How about this, then:

(x - y) <= 0

Same thing, only more algebraic.

And this?

(x <= y) && (x == x)

That's

(x <= y) && true

Which is...

(x <= y)

How about this, then?

(x < y) && (x == y)

That's just false.

True. I think I heard you laughing...

I think you heard me try. Consider this:

(x > 3) || (x < 5)

What is x? An int.

Then this is always true. I think I've said too much.

Consider this:

Oh, no, curly braces, again!

if (x > 3) {
  if (x < 5)       y = 1; 
  else if (x != 6) y = 2;
  else             y = 3; 
} else             y = 4;

Assume that x and y are integers. Let's just imagine that.

And y is 4 at the end. Then x must have been 3 or less.

Assume y ends up being 1. Then x must have been exactly 4.

What if y ends up being 3. Then x had a value of 6.

For what x does y end up with a value of 2? I can see that all branches modify y.

Indeed they do. Then x was 7 or bigger for sure.

You're very good.

I think I heard you saying that.

Well, I haven't said enough.

Consider this:

(x > 3) && (x < 5)

And what is x? An int.

Does it matter?

This time it does.

Then this is the same as saying

x == 4

For any value of x the two expressions end up either both true or both false.

Which makes them equivalent.

Consider this:

'5' - '0'

That's just 53 - 48.

characters have codes. In a number context their codes take over.

So this evaluates to (the number) 5

What's this:

(4 * 5 + 3) / 4

23/4

The result is an int. I'm losing my composure.

The result is 5.

What's this:

("01" + "0" + "12").length()

"01012".length() Which is also, ... 5.

And 8 - 1 - 2, is it also 5? No question about that.

What is '5', then? That is a char, and its code is 53.

If you print it it looks like a 5. But does it act as 5?

Does it act like it? Add 3 to it.

5 + 3 is 8. '5' + 3 is the code for '8'.

That's 56. Try it.

Well, then consider this: Prints as a + b = 01

class One {
  public static void main(String[] args) {
    int a = 0; 
    int b = 1; 
    System.out.println("a + b = " + a + b); 
  } 
}

The blueprint is empty. There's only one static member, main.

It has two local variables inside it.

But they're not part of the blueprint.

Not part of the factory either. They're just that: local to main.

Consider this:

Looks like a loop, when in fact it's not.

int x = 10; 
while (x == 0) {
  x = x + 1; 
  System.out.println(x); 
  x = x % x; 
}

Then, consider this:

This one never ends.

int x = 10; 
do {
  x = x + 1; 
  System.out.println(x); 
  x = x % x;  
} while (x == 0);

What does it print? An infinite number of 1's.

I don't understand x % x I think I've heard you try.

If x is not 0 (zero) then I can divide x by itself and there is no remainder. I think that's very good.

Consider this:

It won't compile.

class One {
  public static void main(String[] args) {
    greet("Andy"); 
  } 
  void greet(String name) {
    System.out.println("Hello, " + name + "!"); 
  } 
}

Why not? greet is not a static method.

Consider this:

You're calling one constructor from the other.

class BankAccount {
  double balance; 
  BankAccount(double initialBalance) { 
    this.balance = initialBalance; 
  }
  public BankAccount() { 
    this(0); 
  } 
}

Yes, pp. 117-118 in the text.

Consider this:

java java kona java kona java

What would you want it to be?

A command-line invocation.

Line at the prompt?

Yes, what are those words, how can you name them to me?

If you could call your main class java ... then we'd have four arguments.

The first one is kona ... which also appears as args[2].

But can you call a class java. I think I'll let you try.

If you believe there's nothing up my sleeve

... then nothing is cool.

Consider this: Prints 0 1

public class A {
  public static void main(String[] args) {
    int x = 1; 
    System.out.print(fun(x) + " "); 
    System.out.print(x + " "); 
  }
  public static int fun(int x) {
    x = x - 1; 
    return x; 
  } 
}

Indeed.

Now consider this:

public class A {
  public static void main(String[] args) {
    int[] x = {1, 2, 3, 4, 5}; 
    System.out.print(fun(x) + " "); 
    System.out.print(x[2] + " "); 
  } 
  public static int fun(int[] x) {
    x[2] = x[2] - 1; 
    return x[2]; 
  } 
}

This prints the same value twice.

Precisely.

Trying to keep up with you.

And I don't know if I can do it better.

And you are not me, so

... consider this:

public class A {
  public static void main(String[] args) {
    System.out.print(fun(5) + " "); 
  }
  public static int fun(int x) {
    System.out.print(x + " "); 
    return x + 1;
  } 
}

Tricky. Yes, 5 is printed first.

So, consider this: What's mix("aoteon", "mnnhmo")?

public static String mix(String s, String t) {
  String ans = ""; 
  int slen = s.length(); 
  int tlen = t.length(); 
  for (int i = 0; i < slen && i < tlen; i++) {
    ans = ans + t.charAt(i) + s.charAt(i); 
  }   
  return ans; 
}

I'm losing my forbearance.

I think I said it first.

But listen have you heard about this one:

public class A {
  public static void main(String[] args) {
    int yo = 12; 
    System.out.println(yo(yo)); 
  }
  public static boolean yo (int yo) {
    if (yo == 12) return true;
    return false; 
  }  
}

Yo, are we losing touch?

Functions and variables have different namespaces.

So the code above compiles.

That's all you need to know. true

Consider this:

public class A {
  public static void main(String[] args) {
    System.out.println(fun(1, fun(fun(2, fun(3, 4)), 5)));
  }
  public static int fun(int a, int b) {
    return a + b; 
  }
}

What if all these fantasies come flailing around?

I think you know the answer.

(1 + ((2 + (3 + 4)) + 5))

Exactly.

I think I've seen enough.

No.	Well, then, consider this: a[a[a[a[0]]]]

I think I need an a.

int[] a = { 1, 2, 3, 4, 5};

Then, it's a[a[a[1]]] ... which is a[a[2]]

... that is, a[3] ... and finally 4

I think I've seen the light.

a[1 + a[1 + a[0]]]

That's 5

OK, then look at this:

String[] name = { "One", "Two", "Three", "Four", "Five"};
boolean[] mark = { false, false, false, false, false};

... and the following definition:

public static String fun(String[] a, boolean[] b) {
  for (int i = 0; i < b.length; i++)
    if (b[i]) return a[i];
  return "One"; 
}

Trying to keep up with you.

I haven't said enough.

I know. What's fun(name, mark)?

This "One" is simple.

And what if mark is:

boolean[] mark = { false, true, false, false, false};

I think I know this, "Two". I think you're right.

Consider this: Now a is { 10, 9, 8, 7, 6}.

int[] a = new int[5];
a[0] = 10; 
for (int i = 1; i < a.length; i++)
  a[i] = a[i-1]-1;

int value = a[0];

A value of 10.

Which gets smaller and smaller. Until it gets to 6.

for (int i = 0; i < a.length; i++) 
  if (value > a[i]) 
    value = a[i];
System.out.println(value);

Oh, no, you've said too much.

I set it up.

So maybe we should do a final one.

Are we losing touch?

I thought that I heard you laughing.

I thought that I heard you sing.

int[][] a = { { 1, 2, 3, 4},
              { 2, 3, 4}, 
              { 3, 4},
              { 4}};

I thought that you heard me try. This will print 4321.

int sum = 0; 
for (int i = 0; i < a.length; i++) 
  System.out.print(a[i].length);

What if you change this last one to: This prints 30.

int sum = 0; 
for (int i = 0; i < a.length; i++) 
  for (int j = 0; j < a[i].length; j++) 
    sum += a[i][j];
System.out.println(sum);

And if you change it to: It prints the number of even numbers in a

int sum = 0; 
for (int i = 0; i < a.length; i++) 
  for (int j = 0; j < a[i].length; j++) 
    if (a[i][j] % 2 == 0) 
      sum += 1;
System.out.println(sum);

Well, then, that's me in the corner.

No, it's just a man on the moon.

Last updated: Mar 7, 2002 by Adrian German for A201