Unit 1: Using Objects and Methods

Quick lookGoals onlyWith examples

1.1

Introduction to Algorithms, Programming, and Compilers

I can describe an everyday process as an algorithm, with clear steps in order.

int score = 10;
score = score * 2;
score = score + 5;
System.out.println(score);

Output

I can explain what happens between writing code and running it.

System.out.println("Line 1");
System.out.println("Line 2")

The compiler says

error: ';' expected

I can tell a syntax error, a logic error, and a run-time error apart.

Syntax error

System.out.println("Hi);

The compiler says

error: unclosed string literal

Logic error

int a = 90;
int b = 80;
double average = a + b / 2;
System.out.println(average);

Output

130.0

Run-time error

int slices = 8;
int people = 0;
System.out.println(slices / people);

Output

Crashes with ArithmeticException

1.2

Variables and Data Types

I can choose whether a value needs a primitive type or a reference type.

int count = 3;          // primitive: a whole number
double gpa = 3.7;       // primitive: a decimal number
boolean done = true;    // primitive: true or false
String name = "Ana";    // reference: an object

I can declare variables that store numbers and true/false values.

The three primitive types
```
int age = 16;
double price = 4.99;
boolean isRaining = false;
```
Not on the AP exam: The other five primitive types: long, short, byte, float, and char.

Changing a variable

int lives = 3;
lives = 2;
System.out.println(lives);

Output

1.3

Expressions and Output

I can write code that prints output, and predict exactly what it shows.

System.out.print("Hello");
System.out.print("World");
System.out.println("!");
System.out.println("Done");

Output

HelloWorld!
Done

I can use string literals, including escape sequences, and predict what they print.

Literals

System.out.println(42);
System.out.println(2.5);
System.out.println(true);
System.out.println("Hi there");

Output

42
2.5
true
Hi there

Escape sequences

System.out.println("She said \"hi\"");
System.out.println("C:\\Users\\me");
System.out.println("line 1\nline 2");

Output

She said "hi"
C:\Users\me
line 1
line 2

I can work out the value of an arithmetic expression, and whether it is an int or a double.

int or double?

System.out.println(7 + 2);
System.out.println(7 + 2.0);
System.out.println(3 * 1.5);

Output

9
9.0
4.5

Not on the AP exam: Math that produces special double values, like infinity or NaN.

Integer division

System.out.println(7 / 2);
System.out.println(7 / 2.0);
System.out.println(2 / 5);

Output

3
3.5
0

Remainder with %
```
System.out.println(17 % 5);
System.out.println(10 % 2);
System.out.println(3 % 8);
```
Output
```
2
0
3
```
Not on the AP exam: A negative number on the left of %, or zero or a negative number on the right.

Order of operations

System.out.println(2 + 3 * 4);
System.out.println((2 + 3) * 4);
System.out.println(20 / 4 * 2);
System.out.println(10 - 7 % 3);

Output

Dividing by zero
```
int total = 50;
int count = 0;
System.out.println(total / count);
```
Output

Crashes with ArithmeticException
Not on the AP exam: Dividing by zero when a double is involved.

1.4

Assignment Statements and Input

I can trace assignment statements and say what each variable holds.

Using a variable before it has a value

int total;
System.out.println(total);

The compiler says

error: variable total might not have been initialized

Assigning and reassigning
```
int x = 5;
int y = x + 1;
x = x * 3;
System.out.println(x);
System.out.println(y);
```
Output
```
15
6
```
Watch out: = is not "equals" like in math. It means "store this value in that variable."

Not on the AP exam: An assignment inside another expression, like a = b = 4;.

An expression's type

double result = 5 / 2;
System.out.println(result);

Output

2.0

I can write code that reads input from the keyboard.

import java.util.Scanner;

Scanner input = new Scanner(System.in);
System.out.print("Your name? ");
String name = input.nextLine();
System.out.println("Hi " + name);

If you type Sam

Your name? Sam
Hi Sam

Not on the AP exam: Reading input in any particular way. You will use Scanner in your own programs, but exam questions do not depend on it.

1.5

Casting and Range of Variables

I can cast between int and double, and predict the value that comes out.

Casting

double d = 7.9;
int n = (int) d;
System.out.println(n);
System.out.println((double) 3);

Output

7
3.0

Casting cuts off the decimal
```
System.out.println((int) 3.99);
System.out.println((int) 7.5 * 2);
System.out.println((int) (7.5 * 2));
```
Output
```
3
14
15
```
Watch out: A cast applies only to the value right after it. To cast a whole expression, put the expression in parentheses.

Putting an int into a double

int count = 10;
double total = count;
System.out.println(total);

Output

10.0

Rounding

double a = 4.6;
double b = -4.6;
System.out.println((int) (a + 0.5));
System.out.println((int) (b - 0.5));

Output

5
-5

I can explain when an int calculation goes out of range.

The largest and smallest int

System.out.println(Integer.MAX_VALUE);
System.out.println(Integer.MIN_VALUE);

Output

2147483647
-2147483648

Overflow

int big = Integer.MAX_VALUE;
big = big + 1;
System.out.println(big);

Output

-2147483648

I can explain why math with double values can come out slightly wrong.

```
System.out.println(0.1 + 0.2);
```
Output
```
0.30000000000000004
```
Not on the AP exam: Special decimal types, like BigDecimal, that avoid round-off errors.

1.6

Compound Assignment Operators

I can trace compound assignment operators and ++/--, and say what the variable ends up holding.

+=, -=, *=, /=, %=

int points = 10;
points += 5;
points *= 2;
points %= 7;
System.out.println(points);

Output

++ and --
```
int count = 0;
count++;
count++;
count--;
System.out.println(count);
```
Output
```
1
```
Not on the AP exam: The prefix form (++x), and using ++ or -- inside another expression, like arr[x++].

1.7

Application Program Interface (API) and Libraries

I can read documentation to find what a class stores and what it can do.

// From the Java Quick Reference: class String
int length()                        // how many characters
String substring(int from, int to)  // part of the string
int indexOf(String str)             // where str first appears

1.8

Documentation with Comments

I can use comments to explain code, and describe a method using its precondition and postcondition.

Comments

// a one-line comment
/* a block comment
   can cover several lines */
/** a Javadoc comment */
System.out.println("Comments never run");

Output

Comments never run

Preconditions and postconditions

/**
 * Precondition: count > 0
 * Postcondition: returns the average, total / count
 */
public static double average(int total, int count)
{
    return (double) total / count;
}

System.out.println(average(170, 2));

Output

85.0

1.9

Method Signatures

I can use a method's signature to pick the right method to call.

public static void sayHi()                     // sayHi()
public static void printStars(int n)           // printStars(int)
public static double area(double w, double h)  // area(double, double)

I can call methods correctly, and trace what happens when they run.

void methods

int x = System.out.println("hi");

The compiler says

error: incompatible types: void cannot be converted to int

Methods that return a value

int d = Math.abs(-7);                  // stored
System.out.println(Math.abs(-3) + d);  // used in an expression
Math.abs(-5);                          // runs, but the 5 is lost

Output

Arguments are copies

public static void addTen(int n)
{
    n = n + 10;
    System.out.println("inside: " + n);
}

int x = 5;
addTen(x);
System.out.println("outside: " + x);

Output

inside: 15
outside: 5

Overloaded methods

public static void show(int n)
{
    System.out.println("int " + n);
}

public static void show(double d)
{
    System.out.println("double " + d);
}

public static void show(String s, int n)
{
    System.out.println(s + n);
}

show(4);
show(4.0);
show("Lesson ", 3);

Output

int 4
double 4.0
Lesson 3

The order things run in

public static void middle()
{
    System.out.println("B");
}

System.out.println("A");
middle();
System.out.println("C");

Output

A
B
C

1.10

Calling Class Methods

I can call class methods and figure out what they return.

public class Temperature
{
    public static double toFahrenheit(double celsius)
    {
        return celsius * 9 / 5 + 32;
    }
}

double f = Temperature.toFahrenheit(100.0);
System.out.println(f);

Output

212.0

1.11

Math Class

I can use Math methods in expressions, including random numbers in a range, and predict the value produced.

abs, pow, and sqrt

System.out.println(Math.abs(-8));
System.out.println(Math.abs(-2.5));
System.out.println(Math.pow(2, 3));
System.out.println(Math.sqrt(49));

Output

8
2.5
8.0
7.0

Watch out: Math.pow and Math.sqrt always return a double, even when the answer is a whole number.

Random numbers in a range

int die = (int) (Math.random() * 6) + 1;   // 1 to 6
int coin = (int) (Math.random() * 2);      // 0 or 1
double temp = Math.random() * 10 + 20;     // 20.0 up to, not including, 30.0

1.12

Objects: Instances of Classes

I can explain the relationship between a class and an object.

Two objects, one class

String a = "cat";
String b = "horse";
System.out.println(a.length());
System.out.println(b.length());

Output

3
5

Superclasses and subclasses

// Object
// └── Vehicle          has a speed, can move()
//     ├── Car          gets speed and move() from Vehicle
//     └── Bike         gets speed and move() from Vehicle

Not on the AP exam: Designing and writing your own superclasses and subclasses. You only need to know what the words mean.

I can declare variables that hold objects.

String a = "hi";   // a holds the address of the "hi" object
String b = a;      // b gets a copy of that address: same object, not a new one

1.13

Object Creation and Storage (Instantiation)

public class Book
{
    private String title;
    private int pages;

    public Book(String t)
    {
        title = t;
        pages = 0;
    }

    public Book(String t, int p)
    {
        title = t;
        pages = p;
    }

    public int getPages()
    {
        return pages;
    }
}

I can create an object by calling a constructor.

Creating an object with new

Book b = new Book("Wonder", 310);
System.out.println(b.getPages());

Output

Arguments in the wrong order

Book c = new Book(310, "Wonder");

The compiler says

error: incompatible types: int cannot be converted to String

The order things run in

public class Greeter
{
    public Greeter()
    {
        System.out.println("B");
    }
}

System.out.println("A");
Greeter g = new Greeter();
System.out.println("C");

Output

A
B
C

I can declare a variable of the right type to hold an object.

Book next = null;   // no Book yet
next = new Book("Holes");

I can use its signature to identify which constructor is being called.

Book a = new Book("Holes");
Book b = new Book("Wonder", 310);
System.out.println(a.getPages());
System.out.println(b.getPages());

Output

0
310

1.14

Calling Instance Methods

public class Counter
{
    private int count;

    public Counter()
    {
        count = 0;
    }

    public void click()
    {
        count++;
    }

    public int getCount()
    {
        return count;
    }
}

I can call methods on an object, and figure out what those calls do.

Each object has its own values

Counter a = new Counter();
Counter b = new Counter();
a.click();
a.click();
b.click();
System.out.println(a.getCount());
System.out.println(b.getCount());

Output

2
1

Calling a method on null
```
Counter c = null;
c.click();
```
Output

Crashes with NullPointerException

1.15

String Manipulation

I can create and combine strings, and predict the result.

Two ways to make a String

String a = "hello";
String b = new String("hello");

Strings are immutable

String a = "Hi";
String b = a;
a = a + "!";
System.out.println(a);
System.out.println(b);

Output

Hi!
Hi

Joining strings

String msg = "Score: " + 7;
msg += "!";
System.out.println(msg);
System.out.println("1" + 2 + 3);
System.out.println(1 + 2 + "3");

Output

Score: 7!
123
33

Watch out: Java works left to right. Numbers before the first string are added as numbers. Everything after a string is joined as text.

Joining an object to a string

public class Point
{
    private int x;
    private int y;

    public Point(int x0, int y0)
    {
        x = x0;
        y = y0;
    }

    public String toString()
    {
        return "(" + x + ", " + y + ")";
    }
}

Point p = new Point(3, 4);
System.out.println("At " + p);

Output

At (3, 4)

Not on the AP exam: Writing your own toString method.

I can call String methods and predict what they return.

Indexes and out of bounds

String s = "code";
System.out.println(s.substring(3, 4));
System.out.println(s.substring(4, 5));

Output

Crashes with StringIndexOutOfBoundsException

length

String s = "banana";
System.out.println(s.length());

Output

substring
```
String s = "banana";
System.out.println(s.substring(1, 4));
System.out.println(s.substring(2));
```
Output
```
ana
nana
```
Watch out: substring(from, to) stops before to, so the result is always to - from characters long.

indexOf

String s = "banana";
System.out.println(s.indexOf("an"));
System.out.println(s.indexOf("x"));

Output

1
-1

equals
```
String s = "banana";
System.out.println(s.equals("Banana"));
System.out.println(s.equals("banana"));
```
Output
```
false
true
```
Watch out: Compare strings with equals, not ==.

Not on the AP exam: Using equals to compare a String with something that is not a String.

compareTo

String s = "banana";
System.out.println("apple".compareTo(s));
System.out.println("banana".compareTo(s));
System.out.println("cherry".compareTo(s));

Output

-1
0
1

One character at a time

String s = "java";
System.out.println(s.substring(0, 1));
System.out.println(s.substring(3, 4));

Output

j
a

Introduction to Algorithms, Programming, and Compilers

Syntax error

Logic error

Run-time error

Variables and Data Types

The three primitive types

Changing a variable

Expressions and Output

Literals

Escape sequences

int or double?

Integer division

Remainder with %

Order of operations

Dividing by zero

Assignment Statements and Input

Using a variable before it has a value

Assigning and reassigning

An expression's type

Casting and Range of Variables

Casting

Casting cuts off the decimal

Putting an int into a double

Rounding

The largest and smallest int

Overflow

Compound Assignment Operators

+=, -=, *=, /=, %=

++ and --

Application Program Interface (API) and Libraries

Documentation with Comments

Comments

Preconditions and postconditions

Method Signatures

void methods

Methods that return a value

Arguments are copies

Overloaded methods

The order things run in

Calling Class Methods

Math Class

abs, pow, and sqrt

Random numbers in a range

Objects: Instances of Classes

Two objects, one class

Superclasses and subclasses

Object Creation and Storage (Instantiation)

Creating an object with new

Arguments in the wrong order

The order things run in

Calling Instance Methods

Each object has its own values

Calling a method on null

String Manipulation

Two ways to make a String

Strings are immutable

Joining strings

Joining an object to a string

Indexes and out of bounds

length

substring

indexOf

equals

compareTo

One character at a time