Core Java Cheat Sheet – Basics Of Java Programming

Last updated on Jun 14,2019 18.2K Views
Swatee Chand
Research Analyst at Edureka. A techno freak who likes to explore different... Research Analyst at Edureka. A techno freak who likes to explore different technologies. Likes to follow the technology trends in market and write about...

Are you an aspiring Java developer? Well, if you are, then I bet you can make use of this Java Cheat Sheet. Java is known for its pre-built classes and libraries and sometimes, keeping a track of them becomes a little tricky. So, here I bring you the Core Java Cheat Sheet.

This cheat sheet will act as a crash course for Java beginners and help you with various fundamentals of Java. 

Core Java Cheat Sheet

Java is an open source programming language that has been changing the face of the IT market since ages. It is widely preferred by the programmers as the code written in Java can be executed securely on any platform, irrespective of the operating system or architecture of the device. The only requirement is, Java Runtime Environment (JRE) installed on the system.

Primitive Data Types

Let’s start off by learning the primitive data types that Java offers:

Data TypeSizeRange
byte 8
-128..127
short 16-32,768..32,767
int 32-2,147,483,648.. 2,147,483,647
long64-9,223,372,036,854,775,808.. 9,223,372,036,854,775,807
float 323.4e-0.38.. 3.4e+0.38
double 641.7e-308.. 1.7e+308
char16Complete Unicode Character Set
Boolean1True, False

Java Operators

There are mainly 8 different types of operators available in Java:

Operator TypeOperators
Arithmetic+, – , *, ? , %
Assignment=, +=, -=, *=, /=, %=, &=, ^=, |=, <<=, >>=, >>>=
Bitwise^, &, |
Logical &&, ||
Relational<, >, <=, >=,==, !=
Shift<<, >>, >>>
Ternary?:
Unary++x, –x, x++, x–, +x, –x, !, ~

Java Variables

Variables in Java refers to the name of the reserved memory area. You need variables to store any value for the computational or reference purpose.

 There are 3 types of variable in Java:

  1. Local Variables
  2. Instance Variables
  3. Static Variables
{public | private} [static] type name [= expression | value];

Java Methods

A method is a set of code that is grouped together to perform a specific operation. A method is completed in two steps:

  1. Method Initialization
  2. Method Invocation

A method can be invoked either by calling it by reference or by value.

{public | private} [static] {type | void} name(arg1, ..., argN ){statements} 

Data Conversion

The process of changing a value from one data type to another type is known as data type conversion. Data Type conversion is of two types:

  1. Widening: The lower size datatype is converted into a higher size data type without loss of information.
  2. Narrowing: The higher size datatype is converted into a lower size data type with a loss of information.
// Widening (byte<short<int<long<float<double)
int i = 10; //int--> long
long l = i; //automatic type conversion
// Narrowing 
double d = 10.02;
long l = (long)d; //explicit type casting
// Numeric values to String
String str = String.valueOf(value);
// String to Numeric values
int i = Integer.parseInt(str);
double d = Double.parseDouble(str);

User Input

Java provides three ways to take an input from the user/ console:

  1. Using BufferReader class
  2. Using Scanner class
  3. Using Console class
// Using BufferReader
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String name = reader.readLine();

// Using Scanner
Scanner in = new Scanner(System.in);
String s = in.nextLine();
int a = in.nextInt();

// Using Console
String name = System.console().readLine();

Basic Java Program

A basic program in Java will consist of at least the following components:

  1. Classes & Objects
  2. Methods
  3. Variables
public class Demo{ 
  public static void main(String[] args) 
   { System.out.println("Hello from edureka!");}
} 

Compile a Java Program

You need to save your Java Program by the name of the class containing main() method along with .java extension.

className.java

Call the compiler using javac command.

javac className

Finally, execute the program using below code:

java className

Flow Of Control

Iterative Statements

Iterative statements are used when you need to repeat a set of statements until the condition for termination is not met.

// for loop
for (condition) {expression}
// for each loop
for (int i: someArray) {}  
// while loop
while (condition) {expression} 
// do while loop
do {expression} while(condition)

Generating a Fibonacci series.

for (i = 1; i <= n; ++i)
{System.out.print(t1 + " + ");
int sum = t1 + t2;
t1 = t2;
t2 = sum;}

Creating a pyramid pattern.

k = 2*n - 2;
for(i=0; i<n; i++)
{ for(j=0; j<k; j++){System.out.print(" ");}
k = k - 1;
for(j=0; j<=i; j++ ){System.out.print("* ");}
System.out.println(); }

Decisive Statements

Selection statements used when you need to choose between alternative actions during execution of the program.

//if statement
if (condition) {expression} 
//if-else statement
if (condition) {expression} else {expression} 
//switch statement
switch (var) 
{ case 1: expression; break; default: expression; break; } 

Checking the given number is prime or not.

if (n < 2) { return false; } 
for (int i=2; i <= n/i; i++) 
{if (n%i == 0) return false;}
return true;

Finding the factorial using recursion function.

int factorial(int n)
 { 
   if (n == 0)
       {return 1;}       
   else 
       {return(n * factorial(n-1));}       
 }    

Java Arrays

Single Dimensional (1-D)

Single Dimensional or 1-D array is a type of linear array in which elements are stored in a continuous row.

// Initializing
type[] varName= new type[size];

// Declaring
type[] varName= new type[]{values1, value2,...};

Creating an array with random values.

double[] arr = new double[n];
for (int i=0; i<n; i++)
{a[i] = Math.random();}

Searching the max value in the array.

double max = 0;
for(int i=0; i<arr.length(); i++)
 { if(a[i] > max) max = a[i]; }

Reversing an array.

for(int i=0; i<(arr.length())/2; i++)
 { double temp = a[i];
   a[i] = a[n-1-i]; 
   a[n-1-i] = temp;
  }

Multi Dimensional (2-D)

Two Dimensional or 2-D array is an array of an array where elements are stored in rows and columns.

// Initializing
datatype[][] varName  =  new dataType[row][col];

// Declaring
datatype[][] varName  =  {{value1, value2....},{value1, value2....}..};

Transposing a matrix.

for(i = 0; i < row; i++)
{ for(j = 0; j < column; j++)
  { System.out.print(array[i][j]+" "); }
  System.out.println(" ");
}

Multiplying two matrices.

for (i = 0; i < row1; i++)
{ for (j = 0; j < col2; j++)
  { for (k = 0; k < row2; k++)
    { sum = sum + first[i][k]*second[k][j]; } 
   multiply[i][j] = sum;
   sum = 0;
  }
}

Java Strings

Creating a String

String in Java is an object that represents a sequence of char values. A String can be created in two ways:

  1. Using a literal
  2. Using ‘new’ keyword
String str1 = “Welcome”; // Using literal

String str2 = new String(”Edureka”); // Using new keyword

The java.lang.String class implements Serializable, Comparable and CharSequence interfaces. Since the String object is immutable in nature Java provides two utility classes:

  1. StringBuffer: It is a mutable class that is thread-safe and synchronized.
  2. StringBuilder: It is a mutable class that is not thread-safe but is faster and is used in a single threaded environment. 

String Methods

Few of the most important and frequently used String methods are listed below:

str1==str2 //compares address;
String newStr = str1.equals(str2); //compares the values
String newStr = str1.equalsIgnoreCase() //compares the values ignoring the case
newStr = str1.length() //calculates length
newStr = str1.charAt(i) //extract i'th character
newStr = str1.toUpperCase() //returns string in ALL CAPS
newStr = str1.toLowerCase() //returns string in ALL LOWERvCASE
newStr = str1.replace(oldVal, newVal) //search and replace
newStr = str1.trim() //trims surrounding whitespace
newStr = str1.contains("value"); //check for the values
newStr = str1.toCharArray(); // convert String to character type array
newStr = str1.IsEmpty(); //Check for empty String
newStr = str1.endsWith(); //Checks if string ends with the given suffix
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