Basic Syntax in Java

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Comments

There are two types of comments in Java: inline, and block.

// This is an inline comment. It only includes this line.

/*
 This is a block comment. Anything between the asterisks is part of the comment.
*/

Printing

In Java, you can print statements using System.out.print() and System.out.println(). The latter ends with a new line.

System.out.print("I'm first!");
System.out.println("I'm second!");
System.out.print("I'm last!");

/* Prints:
I'm first!I'm second!
I'm last!
*/

Variable Types

Variables are used to name, store, and reference different types of data.

Primitive data types are predefined types of data and include int, double, boolean, and char.

Reference data types contain references to an object. An example reference data type is String.

// int - stores whole numbers:
int num = 10;

// double - stores decimal numbers:
double dec = 4.99;

// boolean - stores true or false values:
boolean isTrue = true;

// char - stores a single character value:
char firstLetter = 'A';

// String - stores multiple characters:
String message = "hello there";

Manipulating Number Variables

Different math operations can be applied to int, double, and float data types.

int a = 3;
int b = 5;

int num1;
num1 = a + a; // num1 now equals 6
num1 = a - b; // num1 now equals -2
num1 = a * b; // num1 now equals 15
num1 = 9 / a; // num1 now equals 3
num1 = 10 % a; // num1 now equals 1

int num2 = 10;
num2 -= a; // num2 now equals 8
num2 += b; // num2 now equals 13
num2 %= 6; // num2 now equals 1
num2 *= 4; // num2 now equals 4
num2 /= 2; // num2 now equals 2

int num3 = 3;
num3++; // num3 now equals 4
num3--; // num3 now equals 3

Conditional Statements

In Java, conditional statements execute code based on the truth value of given boolean expressions.

boolean expression1 = false;
boolean expression2 = false;
boolean expression3 = true;

if (expression1) {
  System.out.println("The first expression is true");
} else if (expression2) {
  System.out.println("The second expression is true");
} else if (expression3) {
  System.out.println("The third expression is true");
} else {
  System.out.println("All other expressions were false");
}
// Prints: The third expression is true

Comparison and Logical Operators

Conditional operators and logical operators evaluate the relationship between values in order to determine a true or false value.

// Comparison Operators:
int a = 1;
int b = 5;
System.out.println(a > b); // Prints: false
System.out.println(a < b); // Prints: true
System.out.println(a >= 1); // Prints: true
System.out.println(a + 4 <= b); // Prints: true
System.out.println(a == 1); // Prints: true
System.out.println(b != 5); // Prints: false

// Logical Operators:
System.out.println(!true); // Prints: false
System.out.println(!false); // Prints: true

System.out.println(true && true); // Prints: true
System.out.println(true && false); // Prints: false
System.out.println(false && true); // Prints: false
System.out.println(false && false); // Prints: false

System.out.println(true || true); // Prints: true
System.out.println(true || false); // Prints: true
System.out.println(false || true); // Prints: true
System.out.println(false || false); // Prints: false

Loops

Java has four kinds of loops that rely on a boolean condition and continue to iterate until the condition is no longer true:

while loops
do-while loops
for loops
for-each loops

// An example of a while loop:
int x = 0;
while (x < 2) {
  System.out.println(x);
  x++;
} // Prints: 0 and 1


// An example of a do-while loop:
do {
  System.out.println("Impossible!");
} while (2 == 4); // Prints: Impossible!

  
// An example of a for loop:
for (int i = 0; i < 10; i++) {
	System.out.println(i);
} // Prints: 0 to 9, inclusive


// An example of a for-each loop:
String[] colors = {"Red", "Blue", "Yellow"};
for (String c : colors) {
	System.out.println(c);
} // Prints: Red, Blue, and Yellow

break and continue

Java has two keywords that help further control the number of iterations in a loop:

break is used to exit, or break, a loop. Once break is executed, the loop will stop iterating.
continue can be placed inside of a loop if we want to skip an iteration. If continue is executed, the current loop iteration will immediately end, and the next iteration will begin.

// An example of a break statement:
for (int i = 0; i < 10; i++) {
  System.out.println(i);
  if (i == 4) {
    break;
  }
} // Prints: 0 to 4, inclusive


// An example of a continue statement:
int[] numbers = {1, 2, 3, 4, 5};
for (int i = 0; i < numbers.length; i++) {
  if (numbers[i] % 2 == 0) {
    continue;
  }
  System.out.println(numbers[i]);
} // Prints 1, 3, and 5

Methods

A method is a modular, reusable block of code that can be called throughout a program to complete a certain task.

/*
The following method is a public method called findSum. The method takes in two int parameters called int1 and int2. This method returns an int value.
*/
public static int findSum(int num1, int num2) {
  return num1 + num2;
}

public static void main(String[] args) {
  // Call the method with the arguments 3 and 4
  int sum = findSum(3,4);
  System.out.println(sum); // Prints: 7
}

String Methods

Java’s String class has many useful methods including:

.length(), which returns the length of the String
.concat(), which concatenates two Strings together
.equals(), which checks for String equality
.indexOf(), which returns the index of the first occurrence of a specified character
.charAt(), which returns the character at a specified index
.substring(), which extracts a substring

// Using the .length() method:
String str = "Hello World!";
System.out.println(str.length()); // Prints: 12


// Using the .concat() method:
String name = "Code";
name = name.concat("cademy");
System.out.println(name); // Prints: Codecademy


// Using the .equals() method:
String flavor1 = "Mango";
String flavor2 = "Matcha";
System.out.println(flavor1.equals(flavor2)); // Prints: false 


// Using the .indexOf() method:
String letters = "ABCDEFGHIJKLMN";
System.out.println(letters.indexOf("C")); // Prints: 2


// Using the .charAt() method:
String currency = "Yen";
System.out.println(currency.charAt(2)); // Prints: n


// Using the .substring() method
String line = "It was the best of times, it was the worst of times.";
System.out.println(line.substring(26)); // Prints: it was the worst of times.
System.out.println(line.substring(7, 24)); // Prints: the best of times