Review 1

CS 1331

Hello, Java!

import java.util.Random;
public class Hello {
    private static final String[] GREETINGS = {"Hello!", "Hi!", "W'sup!"};
    private String greeting;

    private Hello() {
        Random rand = new Random();
        int greetingsIndex = rand.nextInt(GREETINGS.length);
        greeting = GREETINGS[greetingsIndex];
    }
    public Hello(int anIndex) {
        int greetingsIndex = anIndex % GREETINGS.length;
        greeting = GREETINGS[greetingsIndex];
    }
    public String getGreeting() {return greeting;}
    public static void main(String[] args) {
        Hello h = new Hello();
        System.out.println(h.getGreeting());
    }
}

The `import` Statement

The statement

import java.util.Random;

allows us to use the java.util.Random class in our program. Without the import, the line

  Random rand = new Random(1);

would prodiuce the following error:

Hello.java:11: error: cannot find symbol
        Random rand = new Random(1);
        ^
  symbol:   class Random
  location: class Hello

No-arg Constructors

public Hello() {
    Random rand = new Random(10);
    int greetingsIndex = rand.nextInt(greetings.length);
    greeting = greetings[greetingsIndex];
}

Constructors initialize an object of a class. In this case, the constructor initializes the greeting instance variable
Constructors are called with operator new
After Hello h = new Hello();, h holds the address of a Hello object which has some randomly assigned greeting instance variable
h is a reference to a Hello object

What does Random rand = new Random(10); do?

Constructors With Parameters

public Hello(int anIndex) {
    int greetingsIndex = anIndex % greetings.length;
    greeting = greetings[greetingsIndex];
}

To call this constructor, provide an argument in the call to new, as in Hello h = new Hello(1);
Providing the argument in the new call selects a particular constructor, in this case the constructor that takes one int parameter

Static versus Non-Static

Consider Doberman.java:

public class Doberman {
    private static int dobieCount = 0;
    private String name;

    public Doberman(String name) {
        this.name = name;
        dobieCount++;
    }
    public String reportDobieCount() {
        return name + " says there are " + dobieCount + " dobies.";
    }
}

dobieCount is shared between all instances of the Doberman class.
Each instance of Doberman has its own distinct copy of name.

Reference Counting

Given DaringDobermans.java:

public class DaringDobermans {
    public static void main(String[] args) {
        Doberman fido = new Doberman("Fido");
        Doberman prince = new Doberman("Prince");
        Doberman chloe = new Doberman("Chloe");
        System.out.println(fido.reportDobieCount());
        System.out.println(prince.reportDobieCount());
        System.out.println(chloe.reportDobieCount());
    }
}

and our definition of Doberman, what will java DaringDobermans print?

$ java DaringDobermans
???
???
???

Reference Counting

Given DaringDobermans.java:

public class DaringDobermans {
    public static void main(String[] args) {
        Doberman fido = new Doberman("Fido");
        Doberman prince = new Doberman("Prince");
        Doberman chloe = new Doberman("Chloe");
        System.out.println(fido.reportDobieCount());
        System.out.println(prince.reportDobieCount());
        System.out.println(chloe.reportDobieCount());
    }
}

and our definition of Doberman, what will java DaringDobermans print?

$ java DaringDobermans
Fido says there are 3 dobies.
Prince says there are 3 dobies.
Chloe says there are 3 dobies.

Not Reference Counting

Now remove static from the definition of dobieCount:

    private int dobieCount = 0;

Now when we run DaringDobermans we get

$ java DaringDobermans
Fido says there are 1 dobies.
Prince says there are 1 dobies.
Chloe says there are 1 dobies.

The difference is that now each Doberman instance has its own copy of dobieCount, not a class-wide, or static dobieCount

Review Question 1

public class Doberman {
    private static int dobieCount = 0;
    private String name;

    public Doberman(String name) {
        this.name = name;
        dobieCount++;
    }
    public String reportDobieCount() {
        return name+" says there are "+dobieCount+" dobies.";
    }
}

What does this code print?

        Doberman fido = new Doberman("Fido");
        System.out.println(fido.reportDobieCount());
        Doberman prince = new Doberman("Prince");
        System.out.println(prince.reportDobieCount());
        Doberman chloe = new Doberman("Chloe");
        System.out.println(chloe.reportDobieCount());

Review Question 1

Given the definition of Doberman from the previous slide,

        Doberman fido = new Doberman("Fido");
        System.out.println(fido.reportDobieCount());
        Doberman prince = new Doberman("Prince");
        System.out.println(prince.reportDobieCount());
        Doberman chloe = new Doberman("Chloe");
        System.out.println(chloe.reportDobieCount());

prints

Fido says there are 1 dobies.
Prince says there are 2 dobies.
Chloe says there are 3 dobies.

Review Question 2

Is int n = 2.2; legal?
What's the value of the expression 17 % 4?
What's the value of n after int n = (int) 2.2;?
After the line above and n++;, what's the value of n?
After the line above and n += 2;, what's the value of n?
After the line above and String s = "Answer: " + n;, what's the value of s?

Review Question 2

int n = 2.2; is not legal -- implicit narrowing conversion
17 % 4 is 1
After int n = (int) 2.2; n is 2
After the line above and n++;, n is 3
After the line above and n += 2;, n is 5
After the line above and String s = "Answer: " + n;, s is "Answer: 5"

Review Question 3

Will this code compile?

String condition = "true";
if (condition) {
    System.out.println("The true path.");
} else {
    System.out.println("The false path.");
}

Review Question 3

Will this code compile?

String condition = "true";
if (condition) {
    System.out.println("The true path.");
} else {
    System.out.println("The false path.");
}

No. condition must be a boolean expression.

Review Question 4

What will this code print?

boolean a = true;
boolean b = false;
if (a && b ) {
    System.out.println("the true path");
} else {
    System.out.println("the false path");
}

Review Question 4

boolean a = true;
boolean b = false;
if (a && b ) {
    System.out.println("the true path");
} else {
    System.out.println("the false path");
}

prints

the false path

&& is logical and.

Review Question 5

What will this code print?

public class ShortCircuit {

    private static int counter = 0;

    private static boolean inc() {
        counter++;
        return true;
    }
    public static void main(String args[]) {
        boolean a = false;
        if (a || inc()) {
            System.out.println("Meow");
        }
        System.out.println("counter: " + counter);
        if (a && inc()) {
            System.out.println("Woof");
        }
        System.out.println("counter: " + counter);
    }
}

Review Question 5

Substitute values, track counter and output:

Code                                       counter  Output

boolean a = false;                         0
if (a || inc()) {                          1
    System.out.println("Meow");            1        Meow
}                                          1
System.out.println("counter: " + counter); 1        counter: 1
if (a && inc()) {                          1
    System.out.println("Woof");            1
}                                          1
System.out.println("counter: " + counter); 1        counter: 1

Key points:

inc() always returns true
Due to short-curcuit evaluation, inc() not always evaluated

Review Qustion 6

How would you write this while loop as a for loop?

int n = 0;
while (n < 5) {
    System.out.println("Hip, hip, hooray!");
    n++;
}

Answer:

???
???
???

Review Qustion 6

How would you write this while loop as a for loop?

int n = 0;
while (n < 5) {
    System.out.println("Hip, hip, hooray!");
    n++;
}

Answer:

for (int n = 0; n < 5; n++) {
    System.out.println("Hip, hip, hooray!");
}

Review Question 7

What will this code print?

public class Foo {
    private String bar;
    public Foo(String bar) {
        this.bar = bar;
    }
    public boolean equals(Object other) {
        return this.bar.equals(((Foo) other).bar);
    }
    public static void main(String[] args) {
        Foo foo1 = new Foo("bar");
        Foo foo2 = new Foo("bar");
        Foo foo3 = foo1;
        System.out.println("foo1.equals(foo2): " + foo1.equals(foo2));
        System.out.println("foo1 == foo2: " + (foo1 == foo2));
        System.out.println("foo1 == foo3: " + (foo1 == foo3));
    }
}

Review Question 7

Given the definition of Foo on previous slide and:

        Foo foo1 = new Foo("bar");
        Foo foo2 = new Foo("bar");
        Foo foo3 = foo1;

You have:

foo1.equals(foo2): true
foo1 == foo2: false
foo1 == foo3: true

Review Question 8

public class Kitten {
    private String name;
    public Kitten(String name) {
        name = name;
    }
    public String toString() {
        return "Kitten: " + name;
    }
}