Java for DSA Part 1

Introduction to Java:

• Java is general purpose and powerful language. It is fundamentally an object oriented programming language.

• Java is platform independent(portable), you write a program in your system and it can also run on other platform, you just have to take the byte code.

• Java is strongly typed language, whereas python, javascript are not.

• Java does automatic memory allocation.

• There are many other features, keeping in mind that we are preparing for DSA we would focus more on how to write code in java.

What do computers understand?

• computers understand 0s and 1s (binary code), writing programs in binary is tedious and error prone.

• So we came up with high level programming languages(c,c++,java,python), which are more closer to the developer, binary code is closer to machine.

• For running programs written in high level language we have transform them into machine code. For this we have tools like compilers, Interpreters and assemblers.

How does Java code run?

• Before that we have talk about JDK (Java Development Kit), which helps us write and run java Programs. It has different tools and libraries like JRE, JVM etc

• JRE is java runtime environment (provide environment to run) which consists JVM (Java virtual Machine- Executes bytecode), this thing makes java platform independent and special.

• Now how does it run: will illustrate using the picture:

Source code goes through the compiler and we get a .class file which is bytecode. And to execute this JVM produces native code(which is specific to machine/platform).

~java FileName.java

Basic Syntax:

public class Main
{
   public static void main(String args[])
   {

   }
}

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Variables:

• Variables are names for memory locations, where we store data. Using variables we can access, retrieve, and modify stored data.

dataType variableName = value; 

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or
you can declare variable and assign value afterwards.

dataType variableName;
variableName = value;

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Data Types:

• Data types can be divided into two types: Primitive and non primitive.

• Primitive types are: byte, short, char, boolean, int, long, float, double

• Non primitive: String, class, Array, object, Interface

Size of data types:

• 1 byte is equal to 8 bits, a bit can have one of either states 0 or 1

1. byte – 1 byte (-128 to 127)
2. short – 2 bytes
3. char – 2 bytes
4. boolean – 1 byte (true or false)
5. int – 4 bytes
6. long – 8 bytes
7. float – 4 bytes
8. double – 8 bytes

Output:

• Output in java to the console can be done with the following functions.

System.out.println("Hello there!");

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or

System.out.print("Hello there!");

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println() -> goes to a new line
print() -> does not go to a new line
\n – can also be used for printing a new line

Input in Java:

• To take input in java we have different methods, here we will talk about Scanner class.

Step 1: We have to import Scanner class (this should be done at the top of the program)

import java.util.Scanner;

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or

import java.util.*;

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imports all the utility package.

Step 2: Now we have to create instance/ object of Scanner class

Scanner in = new Scanner(System.in);

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• We have to note something, there are different functions for different data types through which we can take input.

1. next() – this is for string and it only (captures) takes input till the first space.
2. nextLine() – this is for string and it take (captures) complete line
3. nextInt() – this is for int data type.
4. nextFloat() – this is for float data type.
5. nextBoolean() – this is for boolean values.
6. nextDouble() – this is for double values
7. nextLong() – this is for long values.
8. nextByte() – this is for byte values.
9. nextShort() – this is for short values.
10. next.charAt(0) – this is for char values, 0 inside () tells about the place, 0 is the starting point of the text.

Type Conversion:

• Type conversion, by that I mean a data type converting to another data type happens in two cases:
  1. When type is compatible eg: int <–> float (but float value changing into int will cause truncation of decimal values)
  2. When destination type is bigger than source type (destination type > source type) byte -> short -> int -> float -> long -> double

Type Casting:

• By type casting, I mean explicitly converting a dataType.

float val = 23.456f;
int num = (int)val;

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Operators:

-Operators are the symbols that tell the compiler to perform some operation.

Type of operators:

1. Arithmetic operators (binary, unary and ternary)
2. Relational operators
3. Logical operators
4. Bitwise operators (Bit manipulation)
5. Assignment operators

Arithmetic Operators:

• These are used to perform arithmetic operations.

Binary:

+ : used for addition 
- : used for subtraction
* : used for multiplication
/ : used for division
% : used to get remainder (modulo)

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Unary:

++ : Increment by 1 (a = a+1)
-- : decrement by 1 (a = a-1)

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• There are two types in each unary operators: 1.Pre increment (++a) -> First Changes the value and second uses the value 2.Post increment (a++) -> First uses the value and second changes the value Same for decrement also. Pre decrement (–a) and post decrement(a–)

Relational Operators:

== : Checks equality
!= : Not equal to (check inequality)
>  : Greater than
<  : Less than
>= : greater than equal to
<= : Less than equal to

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Logical Operators:

&& : logical AND
|| : logical OR
!  : logical Not (Negation)

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Assignment Operators:

= : assigns value of right side operand to left operand 
+= : a = a+b -> a+=b
-= : a = a-b -> a-=b
*= : a = a*b -> a*=b
/= : a = a/b -> a/=b
%= : a = a%b -> a%=b

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Conditional Statements:

• Conditional statements get executed when the condition give is true. The conditional statements are:
• if, else
• else if
• ternary operator
• switch

Syntax of if else: 
if(condition){
   // code goes here --> this code runs when the condition is true
} else{
// code --> this code runs when the condition is false
}

eg: 
int age = 19
if (age>18){
 System.out.println("U are good to drive");
//this code runs because condition is true
}else{
System.out.println("U should not drive now");
//this code does not run because condition is not false
} 

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Syntax of else if:
if (condition1){
//code
}else if(condition2){
//code
}else{
//code}

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Syntax for ternary operator: 
variable = condition ? statement1: statement2;

eg: 
boolean largeNum = (145>89) ? true:false;

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Syntax of switch: 
switch(variable){
case 1: //code
       break;
case 2: //code
       break;
default : //code
         break;

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Loops:

• Loops are used for repetitive tasks. The loops are:
  1. while
  2. for
  3. do while

Syntax of while loop: 

initialisation;
while(condition){
//code
updation; //used to avoid looping infinite times
}

eg: 
//print 1 to 10
int count = 1;
while(count<=10){
System.out.println(count);
count++;
}

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Syntax of For: 

for(initialisation; condition; updation){
//code
}

eg: 
//print 1 to 10

for(int i =1; i<=10; i++){
System.out.println(i);
}

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The only way to learn a language is to code it.
This is my first blog, please forgive me for any errors.

原文链接：Java for DSA Part 1