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
loopsdo-while
loopsfor
loopsfor-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. Oncebreak
is executed, the loop will stop iterating.continue
can be placed inside of a loop if we want to skip an iteration. Ifcontinue
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 theString
.concat()
, which concatenates twoString
s together.equals()
, which checks forString
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