Reading 2: Basic Java

In the Java Tutor in Eclipse, complete the first two categories: Basic Java and Numbers & Strings.

After you have done these categories, your Java Tutor pane in Eclipse should show the categories fully checked off with a green checkmark:

✓ Basic Java 2/2

✓ Numbers & Strings 2/2

Then check your understanding by answering some questions about how the basics of Java compare to the basics of Python:

int a = 5;     // (1)
if (a > 10) {  // (2)
    int b = 2; // (3)
} else {       // (4)
    b = 4;     // (5)
}              // (6)
b *= 3;        // (7)

fahrenheit = 212.0
celsius = (fahrenheit - 32) * 5 / 9

It will be useful for us to draw pictures of what’s happening at runtime, in order to understand subtle questions. Snapshot diagrams represent the internal state of a program at runtime – its stack (methods in progress and their local variables) and its heap (objects that currently exist).

Here’s why we use snapshot diagrams in 6.031:

• To talk to each other through pictures (in class and in team meetings)
• To illustrate concepts like primitive types vs. object types, immutable values vs. unreassignable references, pointer aliasing, stack vs. heap, abstractions vs. concrete representations.
• To pave the way for richer design notations in subsequent courses. For example, snapshot diagrams generalize into object models in 6.170.

Although the diagrams in this course use examples from Java, the notation can be applied to any modern programming language, e.g., Python, JavaScript, C++, Ruby.

The simplest kind of snapshot diagram shows a variable name with an arrow pointing to the variable’s value:

int n = 1;
double x = 3.5;

When the value is an object value (as opposed to a primitive value), it is denoted by a circle labeled by its type:

BigInteger val = new BigInteger("1234567890");

Snapshot diagram syntax is intended to be flexible, not necessarily showing all the details all the time, so that we can draw simple diagrams that focus on particular aspects of the program state that we want to discuss.

For example, the diagrams at the right are all reasonable ways to display a string variable in a snapshot diagram.

String s = "hello";

We may use different diagrams in different contexts. Sometimes we care about the particular value of s, and sometimes we don’t. Sometimes we want to emphasize that a String is an object value (as opposed to a primitive), and sometimes that isn’t relevant.

When we want to show more detail about an object value, we will write field names inside the object circle, with arrows pointing out to their values.

Point pt = new Point(5, -3);

For still more detail, variables can include their declared types. Some people prefer to write x:int instead of int x, but both are fine.

Snapshot diagrams give us a way to visualize the distinction between changing a variable and changing a value:

• When you assign to a variable or a field, you’re changing where the variable’s arrow points. You can point it to a different value.

• When you change the contents of a mutable object – such as an array or list – you’re changing references inside that value.

Reassignment and immutable values

For example, if we have a String variable s, we can reassign it from a value of "a" to "ab".

String s = "a";
s = s + "b";

String is an example of an immutable type, a type whose values can never change once they have been created. Immutability is a major design principle in this course, and we’ll talk much more about it in future readings.

In a snapshot diagram, when we want to emphasize the immutability of an object like String, we draw it with a double border, as shown in the diagram here.

Mutable values

By contrast, StringBuilder (another built-in Java class) is a mutable object that represents a string of characters, and it has methods that change the value of the object:

StringBuilder sb = new StringBuilder("a");
sb.append("b");

When we want to emphasize the mutability of an object like StringBuilder, we draw it with a dashed border, as shown here.

These two snapshot diagrams look very different, which is good: the difference between mutability and immutability will play an important role in making our code safe from bugs.

Unreassignable references

Java also gives us immutability for references: variables that are assigned once and never reassigned. To make a reference unreassignable, declare it with the keyword final:

final int n = 5;

In this code, n can never be reassigned; it will refer to the value 5 for its entire lifetime. If the Java compiler isn’t convinced that your final variable will only be assigned once at runtime, then it will produce a compiler error. So final gives you static checking for unreassignable references.

In a snapshot diagram, when we want to focus on reassignability, we will use a double-arrow for unreassignable (final) references, and a dashed arrow for reassignable references. The diagram at the right shows an object whose id never changes (it can’t be reassigned to a different number), but whose age can change.

Note that we can have an unreassignable reference to a mutable value whose value can change even though we’re pointing to the same object:

final StringBuilder sb = new StringBuilder("a");
sb.append("b");

We can also have a reassignable reference to an immutable value where the value of the variable can change because it can be re-pointed to a different object:

String s = "a";
s = "ab";

Double lines and dashed lines help to emphasize the changeability or unchangeability of parts of a snapshot diagram. But when reassignability or mutability is obvious, or not relevant to the discussion, we will keep the diagram simple, and just use single-line arrows and object borders.

void f(String s, StringBuilder sb) {
  s.concat("b");
  s += "c";
  sb.append("d");
}

String t = "a";
StringBuilder tb = new StringBuilder(t);
f(t, tb);

Java has two different ways to test equality of values, depending on whether the values are primitives or objects:

• The == operator compares the values of primitives. For example, 5 == 5 returns true, as does 'a' == 'a'.
• The .equals() method compares the values of objects. For example, "abc".equals("abc") returns true.

In Python, which lacks a distinction between primitive and object values, you use == for both of these purposes. This can lead to confusion and mistakes in Java – using the wrong kind of equality for the type you’re trying to compare.

Using equals() on primitive types is fortunately easy to catch. 5.equals(5) produces a static error because Java doesn’t allow calling any method on a primitive type.

But the other mistake, using == to compare object values for equality, is much more painful, because == is overloaded in Java. When used on object types, == tests whether the two expressions refer to the same object in memory. In terms of the snapshot diagrams we’ve been drawing, two references are == if their arrows point to the same object bubble. In Python, this operator is called is.

So if the state of the program looks like the figure on the right, then:

As a general rule:

• Use == for comparing primitive values, like ints, chars, and doubles.
• Use equals() for comparing object values, like lists, arrays, strings, and other objects.

And remember that, in Java:

• char values are primitives, representing exactly one character. A char literal is always single-quoted, like 'a'.
• String values are objects, represent a string of zero or more characters. A String literal is double-quoted, like "abc" and "".

void f(String s1, String s2, int i1, int i2, char c1, char c2)

String s = "foobar";
String t = "foobar";
if (s == t) { ... }

Lists, Sets, and Maps

Literals

lst = [ "a", "b", "c" ]

map = { "apple": 5, "banana": 7 }

String[] arr = { "a", "b", "c" };

List.of("a", "b", "c");

Set.of("a", "b", "c");
Map.of("apple", 5, "banana", 7);

Generics: declaring List, Set, and Map variables

List<String> cities;        // a List of Strings
Set<Integer> numbers;       // a Set of Integers
Map<String,Turtle> turtles; // a Map with String keys and Turtle values

sequence.add(5);              // wrap 5 as an Integer object, and append it to the sequence
int second = sequence.get(1); // get the second Integer element, and unwrap it into an int

ArrayLists and LinkedLists: creating Lists

List<String> firstNames = new ArrayList<String>();
List<String> lastNames = new LinkedList<String>();

List<String> firstNames = new ArrayList<>();
List<String> lastNames = new LinkedList<>();

List<String> firstNames = new ArrayList<>(List.of("Huey", "Dewey", "Louie"));
List<String> lastNames = new ArrayList<>(Set.of("Duck"));

HashSets and HashMaps: creating Sets and Maps

Set<Integer> numbers = new HashSet<>();

Map<String,Turtle> turtles = new HashMap<>();

Iteration

List<String> cities        = new ArrayList<>();
Set<Integer> numbers       = new HashSet<>();
Map<String,Turtle> turtles = new HashMap<>();

for city in cities:
    print(city)

for num in numbers:
    print(num)

for key in turtles:
    print("%s: %s" % (key, turtles[key]))

for (String city : cities) {
    System.out.println(city);
}

for (int num : numbers) { 
    System.out.println(num);
}

for (String key : turtles.keySet()) {
    System.out.println(key + ": " + turtles.get(key));
}

numbers = [100,200,300]
for num in numbers:
    numbers.remove(num) # danger!!! mutates the list we're iterating over
print(numbers) # list should be empty here -- is it?

for (int ii = 0; ii < cities.size(); ii++) {
    System.out.println(cities.get(ii));
}

Map<String, Double> treasures = new HashMap<>();
String x = "palm";
treasures.put("beach", 25.);
treasures.put("palm", 50.);
treasures.put("cove", 75.);
treasures.put("x", 100.);
treasures.put("palm", treasures.get("palm") + treasures.size());
treasures.remove("beach");
double found = 0;
for (double treasure : treasures.values()) {
    found += treasure;
}

Sometimes a type has a small, finite set of immutable values, such as:

• months of the year: January, February, …, November, December
• days of the week: Monday, Tuesday, …, Saturday, Sunday
• compass points: north, south, east, west
• available colors: black, gray, red, …

When the set of values is small and finite, it makes sense to define all the values as named constants, which Java calls an enumeration and expresses with the enum construct.

public enum Month { 
    JANUARY, FEBRUARY, MARCH, APRIL, 
    MAY, JUNE, JULY, AUGUST, 
    SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER;
}

public enum PenColor { 
    BLACK, GRAY, RED, PINK, ORANGE, 
    YELLOW, GREEN, CYAN, BLUE, MAGENTA;
}

You can use an enumeration type name like PenColor in a variable or method declaration:

PenColor drawingColor;

Refer to the values of the enumeration as if they were named static constants:

drawingColor = PenColor.RED;

Note that an enumeration is a distinct new type. Older languages, like Python 2 and early versions of Java, tend to use numeric constants or string literals to represent a finite set of values like this. But an enumeration is more typesafe, because it can catch mistakes like type mismatches:

int month = TUESDAY;  //  no error if integers are used
Month month = DayOfWeek.TUESDAY; // static error if enumerations are used 

or misspellings:

String color = "REd";        // no error, misspelling isn't caught
PenColor drawingColor = PenColor.REd; // static error when enumeration value is misspelled

Python 3 has enumerations, similar to Java’s, though not statically type-checked.

Previous sections in this reading have a number of links to documentation for classes that are part of the Java platform API.

API stands for application programming interface. If you want to program an app that talks to Facebook, Facebook publishes an API (more than one, in fact, for different languages and frameworks) you can program against. The Java API is a large set of generally useful tools for programming pretty much anything.

• java.lang.String is the full name for String. We can create objects of type String just by using "double quotes".

• java.lang.Integer and the other primitive wrapper classes. Java automagically converts between primitive and wrapped (or “boxed”) types in most situations.

• java.util.List is like a Python list, but in Python, lists are part of the language. In Java, Lists are implemented in… Java!

• java.util.Map is like a Python dictionary.

• java.io.File represents a file on disk. Take a look at the methods provided by File: we can test whether the file is readable, delete the file, see when it was last modified…

• java.io.FileReader lets us read text files.

• java.io.BufferedReader lets us read in text efficiently, and it also provides a very useful feature: reading an entire line at a time.

Let’s take a closer look at the documentation for BufferedReader. There are many things here that relate to features of Java we haven’t discussed! Keep your head, and focus on the things in bold below.

At the top right corner is the search box. You can use it to jump to a class or interface, or straight to a particular method.

Under the Class BufferedReader heading is the class hierarchy for BufferedReader and a list of implemented interfaces. A BufferedReader object has all of the methods of all those types (plus its own methods) available to use.

Next we see direct subclasses, and for an interface, implementing classes. This can help us find, for example, that HashMap is an implementation of Map.

Next up: a description of the class. Sometimes these descriptions are a little obtuse, but this is the first place you should go to understand a class.

If you want to make a new BufferedReader the constructor summary is the first place to look. Constructors aren’t the only way to get a new object in Java, but they are the most common.

Next: the method summary lists all the methods we can call on a BufferedReader object.

Below the summary are detailed descriptions of each method and constructor. Click a constructor or method to see the detailed description. This is the first place you should go to understand what a method does.

Each detailed description includes:

Specifications

Map<String, TreasureChest> treasures = new HashMap<>();
treasures.put("beach", new TreasureChest(25));
TreasureChest result = treasures.putIfAbsent("beach", new TreasureChest(75));

Map<String, String> translations = new HashMap<>();
translations.put("green", "verde");
result = translations.replace("green", "verde", "ahdar");

At this point you should have completed all the reading exercises above. Completing the reading exercises prepares you for the nanoquiz at the beginning of each class meeting. To check your reading exercise status, see classes/02-basic-java on Omnivore. Submitting the exercises is required by 10:00pm MIT time the evening before class.

At this point you should have also completed the Java Tutor levels shown below. Java Tutor exercises are also due 10:00pm MIT time the evening before class.

✓ Basic Java 2/2

✓ Numbers & Strings 2/2