Java

• Everything is an object yay!!

• Most similar to C++

Running

FirstJavaProgram.java (Same filename as class)

import java.util.Date;

public class Hello {
    public static void main(String[] args) {
        Date now = new Date();
        System.out.println("Hello World: " + now);
    }
}

javac FirstJavaProgram.java
java FirstJavaProgram

Java App Structure

App must have one class

Every file can have any # of classes, but must have one public class that is the filename, public class main is the entry point

import java.util.Arrays;

public class UTXO implements Comparable<UTXO> {
    private byte[] txHash;
    private int index;

    //constructor
    public UTXO(byte[] txHash, int index) {
        this.txHash = Arrays.copyOf(txHash, txHash.length);
        this.index = index;
    }
}

Must compile all classes used with javac individually, but dont need to import as it will link automatically b/c names

Data Types

byte(8) | short(16) | int(32) | long(64) | float(32) | double(64) | char(16) | Boolean(1)

Type Conversions

String str = String.valueOf(value);
int i = Integer.parseInt(str);
double d = Double.parseDouble(str);
int i = (int) numeric expres­sion;

Blocks

C++ if, else if, else, while, do while, for, switch

for ( var : collection ) {
    //statements
}

Data Structures

Builtin Array:

• [] syntax, fixed size, store obj & primitatives(int), .length

ArrayList:

• only store objects(Integers), access with functions(.get())

• dynamic size, .size(), generics

import java.util.*;//all
import java.util.ArrayList;
ArrayList<String> arr = new ArrayList<String>();
for (int i = 0; i < arr.size(); i++) {
  System.out.println("Index " + i + " : "+ arr[i]); 
}

import java.util.HashMap; 
HashMap<String, Integer> map = new HashMap<>(); 
Set<String> hash_Set = new HashSet<String>(); //HashSet, LInkedHashSet, or TreeSet

.add(item)

.get(i)

.size()

.remove(i)

.set(i, val)

Object Methods

Equals

Default will check object reference is equal, can override equal to be different, essential for Java data structures like HashMaps which will use this equals

Comparator & Comparable

import java.io.*; 
import java.util.*; 

class Movie implements Comparable<Movie> 
{ 
    private String name; 
    private int year; 

    public int compareTo(Movie m) 
    { 
        return this.year - m.year; 
    } 

    public Movie(String nm, int yr) 
    { 
        this.name = nm; 
        this.year = yr; 
    } 

    public String getName()   {  return name; } 
    public int getYear()      {  return year;  } 
} 

// Class to compare Movies by name 
class NameCompare implements Comparator<Movie> 
{ 
    public int compare(Movie m1, Movie m2) 
    { 
        return m1.getName().compareTo(m2.getName()); 
    } 
} 

class Main 
{ 
    public static void main(String[] args) 
    { 
        ArrayList<Movie> list = new ArrayList<Movie>(); 
        //add elements
        NameCompare nameCompare = new NameCompare(); 
        Collections.sort(list, nameCompare); //by name
        Collections.sort(list); // by default year
    } 
}