Core Java Cheat Sheet – Basics Of Java Programming

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

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];

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} 

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(); }

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;
  }

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;
  }
}

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. 

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