Programming Refresher

1Variables are named places for values

String name = "Ana";
int score = 92;
double average = 85.0;
boolean passed = true;

Read each line as type name = value. The type says what kind of value fits: int for whole numbers, double for decimals, boolean for true or false, String for text. A clear name (score, not s) tells the reader what the value means.

2Assignment stores; expressions calculate

int x = 5;          // x starts at 5
x = x + 2;          // x becomes 7
int total = x * 3;  // total becomes 21

The = sign is assignment: "work out the right side, then store the result in the variable on the left." It does not mean "is equal to." That is why x = x + 2 makes sense: take the old x (5), add 2, and store 7 back in x.

These operators build expressions:

3Conditions let a program choose a path

if (score >= 75) {
  passed = true;
} else {
  passed = false;
}

If the score is 75 or higher, one path runs; otherwise, the other. Comparisons use == (equal), != (not equal), <, >, <=, >=. Each comparison gives true or false.

Good to know: to compare two Strings, use name.equals("Ana"), not ==. With text, == can say false even when the letters match, because it checks whether both are the very same thing in memory. (You will meet this idea, "the same object", in the Linked lists lesson.)

More than two paths: else if. When there are several cases, chain the checks. Java checks from the top and runs only the first part whose check is true; the rest are skipped:

int score = 82;
if (score >= 90) {
  System.out.println("Excellent");
} else if (score >= 75) {
  System.out.println("Passed");      // this one runs
} else {
  System.out.println("Failed");
}

Two checks at once: &&, ||, !. Sometimes one check is not enough. To enter the exam room, you need your ID and your permit. To pay at the canteen, cash or GCash will do. Java has three words for this:

boolean hasID = true;
boolean hasPermit = false;
System.out.println(hasID && hasPermit);  // false: both are needed
System.out.println(hasID || hasPermit);  // true: one is enough
System.out.println(!hasPermit);          // true: "not false"

You will see these in later lessons, for example while (!queue.isEmpty()): "while the queue is not empty."

4Loops repeat a block of work

for (int i = 0; i < 4; i++) {
  System.out.println(i);   // prints 0, 1, 2, 3
}

A loop runs the same lines again and again, with a new value of i each round. A for loop has three controls: where to start (i = 0), when to keep going (i < 4), and how to update after each round (i++).

The while loop. A for loop is best when you know how many rounds you need. A while loop keeps going as long as a check is true, like playing an arcade game while you still have coins:

int coins = 3;
while (coins > 0) {                  // check first
  System.out.println("Play one round");
  coins--;                           // use one coin
}
System.out.println("No coins left");

It prints "Play one round" 3 times, then "No coins left". The check comes before every round, so if coins started at 0, the block would not run at all.

5A method performs one reusable task

static int larger(int a, int b) {
  if (a > b) return a;
  return b;
}

Think of a method as a small machine: larger(3, 8) puts 3 into a and 8 into b, and the machine gives back 8.

Notice that if (a > b) return a; has no braces. When an if controls only one line, Java lets you leave the braces out. And return ends the method right away, so return b; runs only when a > b is false. That is why no else is needed.

Two more words in the first line. int before the name is the type of the value the method gives back. A method that gives nothing back (it only prints, for example) says void there instead, like static void greet(String name). And static lets main (block 8) call the method directly. You will learn about methods that are not static in the Linked lists lesson.

Methods matter in this course because every operation on a data structure is a method, such as push(), pop(), and insert(), and we can test each one on its own.

6An array is a row of numbered slots

Counting starts at 0, so arr[2] is the third slot: 42. The array is the first data structure of this course: you will see it again in Introduction to DSA, and it gets a whole lesson in Arrays.

Making an array in Java. There are two ways: list the values, or ask for empty slots and fill them later.

int[] arr = {19, 3, 42, 7, 56};     // 5 slots with values
int[] scores = new int[5];          // 5 empty slots (all 0)
scores[0] = 92;                     // store 92 in slot 0
System.out.println(arr[2]);         // 42
System.out.println(scores.length);  // 5

Here is how to read each line aloud:

7Trace code one line at a time

 1int total = 0;
 2for (int i = 1; i <= 3; i++) {
 3  total = total + i;
 4}
 5System.out.println(total);

A trace table shows how each variable changes, step by step. Output: 6. You will use trace tables in every lesson of this course.

8Putting it together: a complete program

So far you have seen pieces of code. To actually run them, Java needs them inside a complete program. Think of a school building: the building has a name, it has rooms, and everyone comes in through the front door.

Here is a complete program that uses the larger method from block 5:

 1public class Main {
 2  static int larger(int a, int b) {
 3    if (a > b) return a;
 4    return b;
 5  }
 6
 7  public static void main(String[] args) {
 8    int big = larger(4, 9);
 9    System.out.print("Larger: ");
10    System.out.println(big);
11    System.out.println("Done!");
12  }
13}

▶ This program runs as is. Try it in the Programiz online compiler: tap Copy, paste it there, and press Run.

Here is how to read the new lines:

That is why the output has Larger: and 9 on the same line: print did not move to a new line, and the next println continued there.

OutputLarger: 9
Done!

Good to know: in later lessons, some files hold a second class, like class Node, written outside the braces of Main. Only the class that holds main is public, and the file is named after it.