Introduction

This lecture covers:

• Static vs. dynamic typing
• Variables
• Primitive vs. reference types
• Arrays
• Default values
• Control flow
  • Conditionals
  • Looping

Data types in Java

Since many of you are likely coming from a Python background, some of this discussion will refer back to commonalities and differences between Java and Python. If you have no Python experiences, that is fine. We do not assume Python knowledge.

We’ll work through the JavaSample.java file in this lecture. Due to some language features in Java, I will sometimes ask you to ignore certain keywords for the time being. I only do this because I will explain what those mean later when we get to those topics. By the end of the quarter, there won’t be keywords we see in class that I’ll ask you to ignore.

Static typing

Java is a statically typed language. What does this mean? It means that before you can run Java code, a compiler checks that variables are used in ways that are consistent with their declared types.

For example, consider the following expression:

"foobar" / 10

It attempts to divide the text "foobar" by the number 10, which is a meaningless operation. The above expression would result in errors in statically-typed languages (like Java) as well as in dynamically-typed languages (like Python). But exactly when those errors occur is different between the two languages.

In a dynamically-typed language like Python, the program would begin to run, encounter the expression above, and then crash with an error message. The program would execute right up until it encounters the erroneous line, and then crash. That is, the error appears at run time.

In a statically-typed language like Java, before the program runs, the compiler checks the program for erroneous uses of values like we see above. If the compiler finds any errors, you must fix them before you’re able to run the program.

If the code has type errors like this, the code will not execute at all.

A statically-typed language like Java helps us catch these errors at compile time instead of at run time. There is ongoing debate about whether statically-typed languages are better than dynamically-typed languages, and also what “better” means in this context.

Proponents of statically-typed languages argue that programming is less error-prone with static typing, particularly in large codebases or in codebases where the developer is unfamiliar with the code (i.e., they didn’t write it themselves).

Proponents of dynamically-typed languages argue that programming becomes less verbose and more flexible, and that a good test suite can catch most of the type-related errors that a compiler would flag.¹

Declaring and initialising variables

You may already know what it means to declare and initialize variables using whatever language you’re coming from, but we will revisit this vocabulary briefly.

When you declare a variable, you are creating it, i.e., you are saying that there is now a variable with the given name. When you initialize a variable, you are giving that variable an initial value. When you assign a value to a variable, you are giving it a value, potentially replacing its previous value.

When you declare a variable in Java, you must also say what its data type is. For example:

int z;

The variable z above can now only hold integer values. Its type also dictates that we can only perform “integer-friendly” operations using its value, like math operations or printing. We can’t do things like turn it to “upper case”, because that doesn’t make sense for an integer.

z = "this isn't a number!"; // this code will not compile
z = 10; // this will be fine

You can also declare and initialize a variable in the same line (and will usually do this).

int z = 10; 
int y = -12;

z = 12; // you can update z's value

Primitive types and reference types

There are eight basic or primitive data types in Java:

There are also reference types, for example the String type. Reference types are abstractions created for programming convenience. They are built using primitive types or other reference types as building blocks.

The most commonly used reference type in Java is the String type. It stores text. The String type “strings together” a bunch of chars to make a longer piece of text.

You can declare and initialise a String variable like below. Notice the double-quotes! Single-quotes are reserved for the char type. You must use double-quotes for Strings.

String course = "CSC 203"; 

The “value” of a variable that is declared as a reference type is a reference to the data stored somewhere in memory. Hence the name. You can have multiple references to the same data.

Reference types and equality

There are implications to this.

Consider the == operator (“double equals”).

For the 8 primitive types mentioned above, the behaviour of the == operator is pretty much what you’d expect.

int a = 10;
int b = 12;

System.out.println(a == b); // This will print false.
System.out.println(a == 10); // This will print true.

char theLetterA = 'a';
chat theLetterAAgain = 'a'; 

System.out.println(theLetterA == theLetterAAgain); // This will print true.

However, for reference types, this operator’s behaviour differs between Java and Python.

For reference types, the == operator checks whether the two operands are the same (as in, physically the same item in memory), and not whether they are equal according to some logical rule for equality.

Example. Suppose we have two Point objects, representing points in 2-dimensional space.

(We will talk more about Points later—those details are less relevant for this example.)

In the example below, we use the new keyword to create two new Point objects.

Point p1 = new Point(1, 2); // A point at coordinates (1, 2).
Point p2 = new Point(1, 2); // Another point at coordinates (1, 2).

What would the expression p1 == p2 result in? true or false?²

Because p1 and p2 are reference types, each of those variables is pointing to a different location in memory. Now, you and me and everyone else understands that if two points have the same x and y coordinates, they should probably be considered equal.

That is why all reference types provide an equals function, which checks equality (using some logical definition of equality) rather than sameness.

We can use it as follows:

Point p1 = new Point(1, 2);
Point p2 = new Point(1, 2);

Point p1Again = p1; // p1Again and p1 are both pointing to the same Point.

// This will print false, because p1 and p2 are not the same object.
System.out.println(p1 == p2);

// This will print true, because p1 and p1Again are the same object.
System.out.println(p1 == p1Again);

// This will print true, because p1 and p2 are logically equal objects.
System.out.println(p1.equals(p2));

String equality

Strings are special type of reference type. They are so commonly used that Java provides a specialized syntax for creating a new String without using the new keyword.

String make = "Santa Cruz";
String model = "Bronson";

Since they are reference types, you should compare Strings for equality using the equals function, and not the == operator.

However, the Java compiler is smart enough to SOMETIMES recognise when multiple String variables hold the same String value. It will therefore intern or cache the String value. Each additional String variable with the same value will point to the same String in memory.

This means that, sometimes, the equals function and the == operator will have similar behaviour for Strings, even though String is a reference type.

String csc203 = "CSC 203";
String csc203Again = "CSC 203";

System.out.println(csc203.equals(csc203Again)); // This will print true, as expected.
System.out.println(csc203 == csc203Again); // This will also print true.

However, this is a compiler optimisation, and you should not rely upon it. You can’t predict when == and equals will behave the same for Strings, so you should still use equals to compare Strings for equality.

Arrays

To store a collection of items in Java, the simplest collection you can use is an array. Two things are important to know about arrays:

• Their types are fixed. You can’t mix types in an array.
• Their sizes are fixed. An array can’t grow or shrink after its initial creation.

Here is how to declare and initialize an integer array with some data:

int[] scores = {83, 43, 77, 92, 73, 95, 81, 42};

Arrays are 0-indexed, which means that the first item in the array is at position 0, the second is at position 1, etc.

You can use box brackets ([ and ]) to index into an array, i.e., access a particular position in an array. You can use this syntax to read data from an array, or write data to the array.

int[] scores = {83, 43, 77, 92, 73, 95, 81, 42};

// Reading data
System.out.println(scores[0]); // This will print the value 83
System.out.println(scores[7]); // This will print the value 42

// Writing data
System.out.println(scores[1]); // This will print the value 43.
scores[1] = 37;
System.out.println(scores[1]); // This will now print 37.

If you try to look at a position that’s beyond the bounds of the array’s size, your program will crash (i.e., there will be an error at run time).

int[] scores = {83, 42, 77, 92, 73, 95, 81, 42};

System.out.println(scores[11]); // There is no position 11. This program will crash.

Remember that arrays sizes are fixed! You can’t grow the array beyond its initial size.

int[] scores = {83, 42, 77, 92, 73, 95, 81, 42};
scores[8] = 78; // There is no position 8. This program will crash.

Similar to variables, you can also declare an array without initializing it.

int[] scores; // without initialization

You can also declare an array and fix its size, but not specify its contents.

int[] scores = new int[4]; // A new array of size 4

The above line created an array scores of size 4. But we didn’t specify the contents of this array. However, the array is not “empty”—there is no such thing as an empty array in Java.

When you create an array, you’re allocating that amount of contiguous space for its contents. So, whether or not you declare the contents of the array, that space is allocated.

What’s sitting in that space?

Default values

Java has sort of “placeholder” values for all data types. They are referred to as their default values. So if you create an array but don’t declare its contents, Java places the default values for that type in the array.

Here are the default values for all the primitive types:

All reference types like String or Point or others we learn about have the same default value. It is a special value in Java called null. When a variable’s “value” is null, it means that the variable is “pointing to nothing”.

Control flow

Conditionals

Java uses if, else if, and else for conditional logic.

Unlike Python, Java does not use indentation to denote scope. We use curly braces to denote what happens inside each clause of the “if-else ladder” below.

However, you should still use appropriate indentation to improve readability.

Point p1 = new Point(1, 2);
Point p2 = new Point(1, 2);

if (p1 == p2) { 
    // The above condition is false, so this line will not execute.
    System.out.println("Condition 1");
} else if (p1 == new Point(1, 2)) {
    // The above condition is also false, so this line will not execute, either.
    System.out.println("Condition 2");
} else {
    // This is the "otherwise" clause.
    System.out.println("Condition 3");
}

Repetition (“looping”)

There are 4 looping constructs in Java.

In most cases, any looping construct can be used to perform any task involving repetition. But the different types of loops are provided as “syntactic sugar”—each loop type is “nicer” (more intuitive, less error-prone, etc.) to use for some tasks than others.

The for loop

Use this when you want to do something repeatedly a certain number of times.

The for loop has 3 main pieces:

• The counter variable: int i = 0 in the example below. This statement runs once before the entire loop.
• The looping condition: i < 10 in the example below. The loop will keep repeating as long as this condition is true.
• The counting step: i++ in the example below. This is shorthand for i = i + 1. This statement runs after each iteration of the loop.

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

The code above will print

0
1
2
3
4

You can use the for loop to step through (or iterate over) an array.

int[] scores = {83, 43, 77, 92, 73, 95, 81, 42};

for (int i = 0; i < scores.length; i++) {
    System.out.println(scores[i]);
}

Because i runs through the values 0 through 7, we can use i to access elements from the scores array. The code above would print

83
43
77
92
73
95
81
42

There is a lot of “surface area” for potential errors in these kinds of loops. For example, you could mess up the starting value of i (should it be 0 or 1?) or the looping condition (should it be i < scores.length, i <= scores.length?).

So for iteration, we use another looping construct: the for-each loop.

The for-each loop

The for-each loop takes care of the details of stepping through a collection for us.

int[] scores = {83, 43, 77, 92, 73, 95, 81, 42};

for (int item : scores) {
    System.out.println(item);
}

In the loop above, the item variable steps through the scores array and updates in each step, stopping when you run out of items.

You can read the loop above as “for each item in scores”.

It’ll print:

83
43
77
92
73
95
81
42

The while loop

The “simplest” kind of loop. It simply keeps on running as long as the given condition is true.

You typically use this loop when you don’t know how many times the loop is going to run beforehand. For example, if you are reading lines from a file, and you want to keep reading it line-by-line as long as there are more lines to be read.

while (fileStillHasLines) {
    // Get the next line. Assume this function exists for this example.
    String line = nextLine(); 
    System.out.println(line);
}

It’s important to know that the looping condition will eventually be false, otherwise your code will go into an infinite loop and the program will never progress or end.

This is useful when you want to perform an action 0-to-many times, depending on some condition.

The do-while loop

This loop functions just like the while loop, except for one difference. The while loop checks the looping condition before each iteration. The do-while loop checks the looping condition after each iteration.

do {
    String line = nextLine(); // Suppose, for example, our file is guaranteed to have at least one line.
    System.out.println(line);
} while (fileStillHasLines);

This can be a useful loop to use in cases where you want to perform an action at least once, and then repeat based on some condition.