Java Data Types

Primitive Data Types

byte
short
int
long
float: ~6-7 decimal digits
double: 15 decimal digits
char: surrounded by single quotes
boolean

Notes:

End the value with an "f" for floats and "d" for doubles

Non-Primitive Data Types

String (object): surrounded by double quotes

Methods

length(), charAt(), indexOf(), split(), toCharArray(), valueOf(), repeat(), toUpperCase(), toLowerCase(), indexOf(), concat() or +,
see more here.

Notes:

Non-primitive types can be used to call methods to perform certain operations, while primitive types cannot.
A primitive type has always a value, while non-primitive types can be null.
If you add a number and a string, the result will be a string concatenation.

Java Array

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (int i = 0; i < cars.length; i++) {
  System.out.println(cars[i]);
}

or,

int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };

Java Class

Math

min(), max(), abs(), random()
see more here.

ArrayList

The ArrayList class is a resizable array.

Methods

add(), get(), set(), remove(), clear(), size()
Use Collections for sorting lists alphabetically or numerically.

import java.util.ArrayList;
import java.util.Collections;  // Import the Collections class

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> myNumbers = new ArrayList<Integer>();
    myNumbers.add(33);
    myNumbers.add(15);
    myNumbers.add(20);
    myNumbers.add(34);
    myNumbers.add(8);
    myNumbers.add(12);

    Collections.sort(myNumbers);  // Sort myNumbers

    for (int i : myNumbers) {
      System.out.println(i);
    }
  }
}

Notes

Elements in an ArrayList are actually objects. In the examples above, we created elements (objects) of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, Float for float, etc.

LinkedList

The LinkedList class has all of the same methods as the ArrayList class because they both implement the List interface.

Difference

The ArrayList class has a regular array inside it. When an element is added, it is placed into the array. If the array is not big enough, a new, larger array is created to replace the old one and the old one is removed.
The LinkedList stores its items in "containers." The list has a link to the first container and each container has a link to the next container in the list. To add an element to the list, the element is placed into a new container and that container is linked to one of the other containers in the list.
Use an ArrayList for storing and accessing data, and LinkedList to manipulate data.

Methods

addFirst(), addLast(), removeFirst(), removeLast(), getFirst(), getLast()

HashMap

A HashMap however, store items in "key/value" pairs, and you can access them by an index of another type.

Methods

put(), get(), remove(), clear(), size(), keySet(), values()

// Import the HashMap class
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap object called capitalCities
    HashMap<String, String> capitalCities = new HashMap<String, String>();

    // Add keys and values (Country, City)
    capitalCities.put("England", "London");
    capitalCities.put("Germany", "Berlin");
    capitalCities.get("England");
    capitalCities.remove("England");
    for (String i : capitalCities.keySet()) {
      System.out.println(i);
    }
  }
}

HashSet

A HashSet is a collection of items where every item is unique.

Methods

add(), contains(), remove(), clear(), size()

// Import the HashSet class
import java.util.HashSet;

public class Main {
  public static void main(String[] args) {
    HashSet<String> cars = new HashSet<String>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    for (String i : cars) {
      System.out.println(i);
    }
  }
}

Iterator

An Iterator is an object that can be used to loop through collections, like ArrayList and HashSet.

Methods

hasNext(), next(), remove()

import java.util.ArrayList;
import java.util.Iterator;

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<Integer>();
    numbers.add(12);
    numbers.add(8);
    numbers.add(2);
    numbers.add(23);
    Iterator<Integer> it = numbers.iterator();
    while(it.hasNext()) {
      Integer i = it.next();
      if(i < 10) {
        it.remove();
      }
    }
    System.out.println(numbers);
  }
}

Enums

An enum is a special class, cannot be used to create objects.

enum Level {
  LOW,
  MEDIUM,
  HIGH
}
Level myVar = Level.MEDIUM;

for (Level myVar : Level.values()) {
  System.out.println(myVar);
}

Class Inheritance

To inherit from a class, use the extends keyword.

Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma.

interface FirstInterface {
  public void myMethod(); // interface method
}

interface SecondInterface {
  public void myOtherMethod(); // interface method
}

class DemoClass implements FirstInterface, SecondInterface {
  public void myMethod() {
 System.out.println("Some text..");
  }
  public void myOtherMethod() {
 System.out.println("Some other text...");
  }
}

Java If ... Else

Syntax:

if (condition1) {
  // block of code to be executed if condition1 is true
} else if (condition2) {
  // block of code to be executed if the condition1 is false and condition2 is true
} else {
  // block of code to be executed if the condition1 is false and condition2 is false
}

Java Switch

Syntax:

int day = 4;
switch (day) {
  case 6:
    System.out.println("Today is Saturday");
    break;
  case 7:
    System.out.println("Today is Sunday");
    break;
  default:
    System.out.println("Looking forward to the Weekend");
}
// Outputs "Looking forward to the Weekend"

Java For Each Loop

Used exclusively to loop through elements in an array

Syntax:

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
  System.out.println(i);
}

Java Lambda Expressions

A lambda expression is a short block of code which takes in parameters and returns a value. Lambda expressions are similar to methods, but they do not need a name and they can be implemented right in the body of a method.

Syntax

(parameter1, parameter2) -> { code block }

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<Integer>();
    numbers.add(5);
    numbers.add(9);
    numbers.add(8);
    numbers.add(1);
    numbers.forEach( (n) -> { System.out.println(n); } );
  }
}

Java Regular Expressions

Use java.util.regex package to work with regular expressions. The package includes the following classes:

Pattern Class - Defines a pattern (to be used in a search)
Matcher Class - Used to search for the pattern
PatternSyntaxException Class - Indicates syntax error in a regular expression pattern

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    Pattern pattern = Pattern.compile("w3schools", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("Visit W3Schools!");
    boolean matchFound = matcher.find();
    if(matchFound) {
      System.out.println("Match found");
    } else {
      System.out.println("Match not found");
    }
  }
}

Java Exception

Use try, catch, finally:

public class Main {
  public static void main(String[] args) {
    try {
      int[] myNumbers = {1, 2, 3};
      System.out.println(myNumbers[10]);
    } catch (Exception e) {
      System.out.println("Something went wrong.");
    } finally {
      System.out.println("The 'try catch' is finished.");
    }
  }
}

Java Packages

A package in Java is used to group related classes.

Import a class

import java.util.Scanner;

Import a whole package

import java.util.*;

Java Modifiers

Access Modifiers

Class

Modifier	Description
`public`	The class is accessible by any other class
default	The class is only accessible by classes in the same package. This is used when you don't specify a modifier

Function

Modifier	Description
`public`	The class is accessible by any other class
default	The code is only accessible in the same package. This is used when you don't specify a modifier
`private`	The code is only accessible within the declared class
`protected`	The code is accessible in the same package and subclasses

Non-Access Modifiers

Class

final, abstract

Function

final, static, abstract, transient, synchronized, volatile
details here.

Java Quick Cheatsheet

Java Data Types

Primitive Data Types

Notes:

Non-Primitive Data Types

Methods

Notes:

Java Array

Java Class

Math

ArrayList

Methods

Notes

LinkedList

Difference

Methods

HashMap

Methods

HashSet

Methods

Iterator

Methods

Enums

Class Inheritance

Java If ... Else

Syntax:

Java Switch

Syntax:

Java For Each Loop

Used exclusively to loop through elements in an array

Syntax:

Java Lambda Expressions

Syntax

Java Regular Expressions

Java Exception

Java Packages

Import a class

Import a whole package

Java Modifiers

Access Modifiers

Class

Function

Non-Access Modifiers

Class

Function

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